An RTD (resistance temperature detector) measures temperature by tracking how the resistance of a platinum element rises with temperature. A thermocouple measures temperature by reading the millivolt signal produced when two dissimilar metal wires join at a hot end. Same goal, very different physics — and that difference is what makes one right for your tank, oven, or pipeline and the other a poor fit.

This guide gives you a 5-application decision matrix, the wiring rules that trip up most installers, and a 5-year cost comparison so you can make a defensible choice the next time procurement asks why you specified one over the other.

Contents

• RTD and Thermocouple: Core Difference in One Sentence
• Working Principle: Resistance vs Voltage
• Accuracy and Stability Over Temperature
• Temperature Range and Sensor Limits
• Response Time and Self-Heating
• Wiring: 2/3/4-Wire RTD vs Type K/J/T Thermocouple
• 5-Year Cost of Ownership
• Decision Matrix: 5 Common Industrial Applications
• Common Selection Mistakes
• Featured Sensors from Sino-Inst
• FAQ

RTD and Thermocouple: Core Difference in One Sentence

An RTD changes its electrical resistance with temperature; a thermocouple generates a small voltage when its hot junction sees a different temperature than its cold junction. That single sentence drives every spec-sheet number that follows — accuracy, range, wiring complexity, drift, and cost.

The reference standards are IEC 60751 for platinum RTDs (Pt100 at 0 °C resistance of 100 Ω, α = 0.00385 Ω/Ω/°C) and IEC 60584 for thermocouples (covering Types K, J, T, N, S, R, B). Anyone selling you a sensor outside those standards is selling you trouble.

Working Principle: Resistance vs Voltage

A Pt100 RTD is a small platinum film or wire-wound coil whose resistance follows the Callendar−Van Dusen equation. Above 0 °C: R(T) = R₀[1 + AT + BT²], where R₀ = 100 Ω, A = 3.9083 × 10⁻³, B = −5.775 × 10⁻⁷. Drive a small constant current (typically 1 mA) through the platinum and you measure the resulting voltage; resistance follows by Ohm’s law.

A thermocouple is built from two dissimilar metal wires welded at one end (the measuring junction). When that junction is at a different temperature than the open end (cold junction), a millivolt-level Seebeck voltage appears. The relationship is non-linear, so the converter or transmitter uses a polynomial reference table from NIST ITS-90 to translate millivolts into temperature.

Accuracy and Stability Over Temperature

An IEC 60751 Class A Pt100 is rated to ±(0.15 + 0.002·|T|) °C. At 100 °C, that’s ±0.35 °C. A Class B Pt100 is ±(0.30 + 0.005·|T|) °C — about three times looser. RTDs drift by less than 0.05 °C per year if treated gently.

A Class 1 Type K thermocouple is rated to ±1.5 °C below 375 °C, then ±0.4% of reading above. At 1000 °C, that’s ±4 °C. Type T is the most accurate thermocouple, ±0.5 °C below 125 °C, but it only goes to 400 °C. Type S (platinum/platinum-rhodium) is more stable than K above 1000 °C but ten times the price.

Bottom line: below 600 °C, RTDs win on accuracy by a factor of five to ten. Above 600 °C, thermocouples are the only practical choice — RTDs cannot survive there long.

Temperature Range and Sensor Limits

Sensor	Operating Range	Continuous Use Limit
Pt100 RTD (thin film)	−50 to +500 °C	+400 °C
Pt100 RTD (wire-wound)	−200 to +650 °C	+600 °C
Type T thermocouple	−200 to +400 °C	+350 °C
Type J thermocouple	−40 to +750 °C	+700 °C
Type K thermocouple	−200 to +1260 °C	+1100 °C
Type N thermocouple	−270 to +1300 °C	+1200 °C
Type S / R / B	0 to +1700 °C	+1600 °C

Beyond the continuous limit, the platinum migrates in an RTD and the thermocouple wire alloys degrade. A K-type pushed past 1100 °C in air drifts at roughly 1–2 °C per 100 hours of operation. We’ve seen factory floors blame “controller drift” when the real problem was an exhausted K-type that nobody documented.

Response Time and Self-Heating

Thermocouples are faster. A 1.5 mm sheathed Type K reaches 63% of a step change in about 0.5 second in water, 5–7 seconds in air. A 6 mm sheathed Pt100 takes 3–4 seconds in water, 30+ seconds in still air. If you’re tuning a fast loop or chasing a transient, that gap matters.

RTDs also suffer self-heating: the 1 mA excitation current dissipates I²R in the element. A typical 100 Ω element with 1 mA gives 0.1 mW. In still air that’s enough to raise the reading by 0.05–0.1 °C, which is more than the Class A error budget. Thermocouples have no excitation current and no self-heating.

Wiring: 2/3/4-Wire RTD vs Type K/J/T Thermocouple

An RTD reading is just a resistance, so lead wire resistance adds directly to the measurement. A 2-wire RTD adds the full lead resistance — about 0.4 Ω per 10 m of 22 AWG copper, which is roughly 1 °C error. A 3-wire RTD subtracts one lead from the resistance bridge and cuts the lead error by about 90%. A 4-wire RTD passes current through two leads and measures voltage on the other two; lead resistance disappears from the math entirely. Use 4-wire for laboratory accuracy, 3-wire for everything else.

Thermocouples use extension wire of the same alloy as the thermocouple itself — never plain copper. Substituting copper extension on a Type K creates a second junction at the head, and that junction reads room temperature into your loop as a 4–6 °C offset. Color codes are international (IEC 60584): Type K is green positive / white negative; Type J is black/white; Type T is brown/white. For background on extension wiring conventions and shielded cables, see our note on shielded twisted-pair cables for industrial instrumentation.

For full Type K reference values, see the Type K thermocouple chart with mV reference and tolerance bands. For Pt100 element construction, see our WZP Pt100 series assembled thermal resistance page.

5-Year Cost of Ownership

Cost Item	Pt100 RTD (Class A, 3-wire)	Type K Thermocouple (Class 1)
Initial sensor + thermowell	$140	$70
Transmitter (4-20 mA)	$110	$95
3-wire cable (50 m)	$95	$210 (Type K extension)
Annual calibration (5 yrs)	$60 × 5 = $300	$40 × 5 = $200
Replacement during 5 yrs	0 (typical)	1 (high-temp service)
5-yr total	$645	$645 + $70 = $715

RTD wins on TCO in service below 600 °C even though the sensor itself costs twice as much. Two reasons: copper cable is much cheaper than Type K extension cable, and RTDs rarely need mid-service replacement. The math flips above 800 °C — RTDs cannot survive there, so the comparison ends.

Decision Matrix: 5 Common Industrial Applications

Application	Typical Range	Recommended Sensor	Why
HVAC chiller water	4 to 12 °C	Pt100 Class A	Tight accuracy needed for BTU calc
Boiler feedwater	80 to 150 °C	Pt100 Class B	Stable, easy 3-wire run to PLC
Plastic injection mold	180 to 280 °C	Type J or Pt100 wire-wound	Either works; J is cheaper if many sensors
Heat-treat furnace	700 to 1100 °C	Type K Class 1	RTD cannot survive
Glass / kiln / cement	1200 to 1600 °C	Type S or B	Only platinum-alloy TCs handle this

Common Selection Mistakes

• Specifying Pt100 for a 1200 °C kiln. The platinum will diffuse into the sheath in weeks. Use Type S.
• Running 2-wire RTD for a 30 m cable run. Lead-wire error swamps Class A accuracy. Use 3-wire minimum, 4-wire for lab work.
• Reusing copper cable as Type K extension. You just put a 4–6 °C junction error at the head.
• Mixing Type K and Type J how a pressure transmitter works. A Type J transmitter reading a Type K sensor under-reads by 30–40 °C at 500 °C.
• Ignoring sheath material. Inconel 600 is fine in oxidizing air; in sulfur-bearing flue gas it pits in months. Spec Hastelloy or ceramic.

Featured Sensors from Sino-Inst

Need help picking? Send your service conditions (medium, temperature range, pipe size, connection) to our engineers using the form below — we typically reply within one working day with a sized quote.

FAQ

Which is more accurate, RTD or thermocouple?

Below 600 °C, an IEC Class A Pt100 RTD beats any thermocouple by a factor of 5 to 10. Above 600 °C, RTDs degrade and thermocouples become the only practical choice.

Can I replace a thermocouple with an RTD?

Only if the service temperature stays under the RTD’s continuous limit (typically 600 °C wire-wound or 400 °C thin-film) and the transmitter accepts RTD input. You’ll also need to swap from thermocouple extension wire to plain copper.

What is the difference between Pt100 and Pt1000?

Both are platinum RTDs following IEC 60751. Pt100 has 100 Ω at 0 °C and is the industrial standard. Pt1000 has 1000 Ω at 0 °C; the higher base resistance makes lead-wire error 10x less important, so it’s popular in HVAC and 2-wire installations.

What is RTD full form?

RTD stands for Resistance Temperature Detector. It is a temperature sensor whose electrical resistance changes predictably with temperature. The most common type is the platinum Pt100 defined by IEC 60751.

Which thermocouple type should I use for general industrial work?

Type K is the default workhorse for 0–1100 °C — wide range, cheap, readily available. Use Type T if you need accuracy below 200 °C, Type J for older European installations, Type N for long-term stability above 800 °C, and Type S/R/B above 1100 °C.

How long does an RTD last in service?

Properly specified Pt100 RTDs in non-cycling service routinely last 10+ years with drift under 0.05 °C/year. Thermal cycling, vibration, and sheath corrosion shorten that life. We recommend a 12-month calibration check for any sensor in custody-transfer or food/pharma service.

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A Type K thermocouple chart converts the millivolt output of a chromel/alumel junction into a temperature reading at 0 °C cold-junction reference. The chart is the IEC 60584-1 reference table — the same numbers used inside every Type K transmitter and recorder for cold-junction compensation. To use the chart correctly you need the formula T_hot = chart⁻¹(V_measured + V_{cold-junction}), the IEC tolerance class for your wire, and the regional color code so you do not reverse polarity.

Contents

• How to Read the Chart with Cold-Junction Compensation
• Type K mV Reference Table (−200 to +1372 °C)
• Eight Thermocouple Types Compared at a Glance
• IEC 60584 and ASTM E230 Tolerance Classes
• Color Codes: ANSI, IEC, JIS, BS
• Five Common Mistakes Reading a Thermocouple Chart
• Featured Thermocouples and Temperature Transmitters
• FAQ

How to Read the Chart with Cold-Junction Compensation

A Type K thermocouple does not directly measure the hot junction temperature — it measures the difference between the hot junction and the cold (reference) junction. The chart assumes the cold junction is at exactly 0 °C. In practice it is at room temperature, so the measurement procedure has three steps.

1. Measure the thermocouple voltage V_tc with a high-impedance meter (>1 MΩ).
2. Measure the cold-junction (terminal-block) temperature T_cj with a separate sensor — typically an internal RTD on the recorder or transmitter.
3. Look up V_cj from the chart at T_cj, add to V_tc, then look up the temperature for the corrected mV. T_hot = chart⁻¹(V_tc + V_cj).

Worked example. The wire reads V_tc = 8.000 mV. The terminal block is at T_cj = 25 °C, which on the chart is V_cj = 1.000 mV. Total = 9.000 mV. From the Type K chart, 9.000 mV corresponds to about 221 °C. That is the hot-junction temperature. Skipping the cold-junction step in this example would have given 196 °C — 25 °C low, exactly the terminal-block error. Modern transmitters and recorders perform this correction automatically; if you ever read a Type K with a bench multimeter, do it manually.

For the broader comparison between resistance and voltage-based temperature sensors, see when to choose RTD over a Type K thermocouple.

Type K mV Reference Table (−200 to +1372 °C)

The full IEC 60584-1 chart for Type K covers 1572 °C of range in 1 °C steps. The condensed reference points below cover most engineering reads.

Temp (°C)	EMF (mV)	Temp (°C)	EMF (mV)	Temp (°C)	EMF (mV)
−200	−5.891	200	8.138	800	33.275
−100	−3.554	250	10.153	900	37.326
−50	−1.889	300	12.209	1000	41.276
0	0.000	400	16.397	1100	45.119
25	1.000	500	20.644	1200	48.838
50	2.023	600	24.905	1300	52.410
100	4.096	700	29.129	1372	54.886

The Seebeck coefficient (slope of the curve, dV/dT) is approximately 41 µV/°C across the working range — the highest among the base-metal types. That is why Type K is the workhorse of industrial temperature measurement: high signal-to-noise and a 41 µV resolution per 1 °C, comfortably above the ~10 µV input noise of any decent transmitter. For long-term recording on a multi-channel system see our paperless recorder selection guide.

Eight Thermocouple Types Compared at a Glance

The IEC standard defines eight letter-coded thermocouple types. Type K covers most general industry; the others fill out the high-temperature, vacuum, and precision corners.

Type	Conductors (+ / −)	Range (°C)	Sensitivity at 25 °C (µV/°C)	Best fit
K	Chromel / Alumel	−200 to +1372	41	General industry; oxidising atmosphere up to 1100 °C
J	Iron / Constantan	−210 to +1200	52	Vacuum, inert, reducing atmospheres; sensitive to oxidation above 540 °C
T	Copper / Constantan	−270 to +400	43	Cryogenic and food-process service; resists moisture corrosion
E	Chromel / Constantan	−270 to +1000	68	Highest sensitivity of the base-metal types; cryogenic precision
N	Nicrosil / Nisil	−270 to +1300	39	Drift-resistant alternative to K above 800 °C; aerospace and metallurgy
S	Pt-10%Rh / Pt	0 to +1768	10	Calibration standard; clean oxidising atmospheres up to 1450 °C
R	Pt-13%Rh / Pt	−50 to +1768	11	Industrial high-temperature reference; petrochemical, glass
B	Pt-30%Rh / Pt-6%Rh	0 to +1820	10 (above 600 °C)	Steel-mill, glass-melt; insensitive below 50 °C — no cold-junction compensation needed

Read the comparison this way: K is default, T for sub-zero, E for highest sensitivity, N when K drifts, S/R/B for very high temperatures. For the deeper trade-off vs platinum RTDs see our RTD vs thermocouple comparison.

IEC 60584 and ASTM E230 Tolerance Classes

Tolerance is the maximum permitted deviation between the actual emf and the standard table emf. IEC 60584-2 and ASTM E230 define three classes; the figure quoted on a thermocouple datasheet is the worst-case error before any in-house calibration.

Class	Type K tolerance	Application
Class 1 (IEC) / Special (ASTM)	±1.5 °C up to +375 °C, then ±0.4 % of reading	Lab, calibration, precision process
Class 2 (IEC) / Standard (ASTM)	±2.5 °C up to +333 °C, then ±0.75 % of reading	Default industrial spec, most commercial wire
Class 3 (IEC, sub-zero only)	±2.5 °C up to −167 °C, then ±1.5 % of reading	Cryogenic service; not a US-recognised class

A 1000 °C process measured with a Class 2 K thermocouple has up to ±7.5 °C tolerance from the wire alone. Add the transmitter’s ±0.1 % FS, the cold-junction compensation error of ±0.5 °C, and the cable termination drift, and the loop accuracy is around ±10 °C in the worst case. Class 1 wire halves the wire-side error to about ±4 °C and is worth the price premium for furnace control loops where a 5 °C swing changes the metallurgical result.

Color Codes: ANSI, IEC, JIS, BS

Color code is the most common cause of installation error. Type K wire is yellow under ANSI MC96.1 (US) but green under IEC 60584-3 (Europe). The negative leg is red under ANSI but white under IEC. Crossing the standards results in a polarity reversal and an apparent negative reading.

Standard	Region	Type K positive	Type K negative	Outer jacket
ANSI MC96.1	USA	Yellow	Red	Yellow
IEC 60584-3	Europe, IEC countries	Green	White	Green
BS 1843	UK (legacy)	Brown	Blue	Red
JIS C 1610	Japan	Red	White	Blue
DIN 43710	Germany (legacy, replaced by IEC)	Red	Green	Green

Two practical rules. First, always check the printed standard on the cable jacket before terminating; assume nothing from color alone. Second, the negative leg of every magnetic-iron type (K, J) is the magnetic conductor — alumel and constantan are weakly attracted to a small magnet, while chromel and iron-positive K are not. A field magnet test resolves polarity confusion in seconds.

Five Common Mistakes Reading a Thermocouple Chart

1. Skipping cold-junction compensation. Reading the chart with only the field mV ignores the terminal-block temperature and produces an error equal to the room temperature in °C. Always add V_cj before the lookup.
2. Using the wrong thermocouple type’s chart. Confusing K with J or N looks identical on a multimeter. Identifying by color or jacket print is mandatory; “looked like K” loses 5–10 % accuracy.
3. Reversed polarity on extension wire. Connecting K-positive (yellow/green) to the transmitter’s negative input swings the reading symmetrically — a 200 °C source reads as if at the cold-junction temperature. Drift suddenly looks like the process is cold.
4. Using copper extension wire on a Type K loop. Copper introduces a parasitic junction at the terminal block. The reading is right at room temperature and wrong everywhere else. Always use matching K extension wire (KX) up to the cold-junction reference point.
5. Ignoring the upper limit. Type K above 1100 °C in oxidizing atmosphere drifts +1 to +2 °C per 100 hours from “green-rot” of the chromel leg. The chart is mathematically correct; the wire is not. For continuous service above 1100 °C use Type N or platinum-rhodium types.

Most of these errors are hidden by a transmitter’s burnout-protection and CJC logic; on a bench multimeter they are exposed. If a junior engineer is building a calibration rig from a multimeter and a thermocouple, walk through these five with them on day one. For the wiring side see our 4–20 mA transmitter wiring types guide.

Featured Thermocouples and Temperature Transmitters

FAQ

How do you read a Type K thermocouple table?

Measure the thermocouple millivolts with a high-impedance meter, then add the cold-junction compensation millivolts read from the same chart at the terminal-block temperature. Look up the corrected total in the table to get the hot-junction temperature. Modern transmitters do this automatically; for a bench multimeter you do it by hand.

What is the temperature range of a Type K thermocouple?

−200 to +1372 °C per IEC 60584-1. Continuous service in oxidising atmosphere is rated to 1100 °C; intermittent service to 1300 °C. In reducing or sulfurous atmospheres the upper limit drops to about 800 °C because of green-rot drift in the chromel leg.

What is the millivolt output of a Type K at 100 °C?

4.096 mV at 100 °C with a 0 °C cold-junction reference, per the IEC 60584-1 table. Sensitivity is approximately 41 µV/°C across the working range, so each 1 °C change moves the output 41 µV — easily resolvable by a 16-bit transmitter.

Why is my Type K reading negative when the process is hot?

Polarity is reversed at the terminal block. The Type K positive leg is yellow under ANSI MC96.1 (US) and green under IEC 60584-3 (Europe). Connecting the wrong leg to the positive input swings the reading symmetrically. Swap the leads and verify with a small magnet — the alumel (negative) leg is magnetic.

What is the difference between Type K and Type J thermocouples?

Type K is chromel/alumel, range −200 to +1372 °C, sensitivity 41 µV/°C, default for general industry. Type J is iron/constantan, range −210 to +1200 °C, sensitivity 52 µV/°C, but the iron leg oxidises rapidly above 540 °C and so is restricted to vacuum, reducing, or inert atmospheres.

Do I need extension wire matching the thermocouple type?

Yes. Use Type KX extension wire on Type K loops, JX on Type J, and so on. Copper extension wire introduces a parasitic junction at the terminal block; the reading is right at room temperature and wrong everywhere else. The KX wire has lower-grade alloys than KP/KN element wire but matches the Seebeck curve over 0–200 °C.

What is “green-rot” in a Type K thermocouple?

Selective oxidation of the chromium in the chromel (positive) leg above 800–1100 °C. The leg turns greenish, the Seebeck coefficient drops, and the reading drifts low. Use Type N (nicrosil/nisil) above 1100 °C continuous service or platinum-rhodium types (S/R/B) for atmospheres rich in chromium-attacking species.

Need a thermocouple, transmitter, or paperless recorder configured for your temperature range and accuracy class? Send the temperature range, atmosphere, output requirement, and certification target — our engineering team will quote within 24 hours.

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What is RTD?

RTDs are made of metal wires, usually copper or platinum, that offer resistance to the flow of electricity. The RTD’s resistance changes when its temperature changes, allowing it to be used as a gauge for measuring heat. RTDs are considered to be more accurate than thermocouples as they have a linear relationship between resistance and temperature. RTDs are also less affected by electromagnetic fields than thermocouples.

RTD Working Principle

The full English name of RTD is “Resistance Temperature Detector”, so to be precise, it should be translated as “Resistance Temperature Detector”.

For a 5-application decision matrix and a 5-year cost-of-ownership comparison, see our decision matrix on RTD vs Thermocouple selection.

RTD is a special kind of resistor whose resistance value increases as the temperature increases and decreases as the temperature decreases. In industry, this feature is used for temperature measurement, so RTD is also commonly known as “thermal resistance”.

Not all metals are suitable for making RTDs. Materials that meet this characteristic need to meet the following requirements:

• The resistance value of the metal has a linear relationship with the temperature change energy;
• The metal is more sensitive to temperature changes, that is, the resistance change (temperature coefficient) caused by unit temperature changes is relatively large;
• The metal can resist fatigue caused by temperature changes and has good durability;

There are not many metals that meet this requirement. Common RTD materials are: platinum (Pt), nickel (Ni), and copper (Cu).

Take platinum thermal resistance as an example. According to the different resistance values, it can be divided into Pt50, Pt100, Pt200, Pt500 and Pt1000.

The numerical value in the name indicates the resistance value of the thermal resistance at 0°C.

For example: Pt100, indicating that the resistance value of the sensor at 0°C is 100Ω.
And Pt1000, it means that the resistance value of the sensor at 0 ℃ is 1000Ω.
The resistance value of RTD thermal resistance at different temperatures can be approximated by the formula: R=R0(1+αT).

in:
1) R0 represents the resistance value of RTD at 0℃;
2) a is called the temperature coefficient, which represents the change value of the resistance at unit temperature;
3) T represents the measurement temperature, the unit is °C;

According to the number of lead wires of RTD thermal resistance, RTD can be divided into two-wire, three-wire and four-wire.

The lead of the two-wire RTD is to directly lead out two wires at both ends of the resistor to the temperature measurement module. The temperature measurement module adopts the principle of bridge balance, and RTD is used as one arm of the bridge to measure.

A three-wire RTD can largely eliminate the influence of the sensor leads themselves on the measurement results. The detection accuracy is greatly improved compared to the two-wire system.

Resistance Temperature Detector advantages

No compensation line is required, and the price is cheap;
It can transmit electrical signals over long distances;
High sensitivity and strong stability;
Good interchangeability and high precision.

Disadvantages of thermal resistance:

Although thermal resistance is widely used in industry. But it requires power excitation.
Temperature changes cannot be measured instantaneously.
The temperature measurement range is limited and the application is limited.

What is Thermocouple?

Thermocouples, on the other hand, are made of two different types of metals that are joined together at the sensor end. The junction between these two metals produces a voltage that is proportional to the temperature difference between the junction and the measuring point. Thermocouples are less expensive than RTDs and can measure a wider range of temperatures. They are also faster at responding to changes in temperature.

Thermocouple Working Principle

A thermocouple is a temperature sensing element. It converts the temperature signal into a thermoelectromotive force signal and converts it into the temperature of the measured medium through an electrical instrument.

The basic principle of thermocouple temperature measurement is that two homogeneous conductors of different compositions form a closed loop. When there is a temperature gradient at both ends, a current will flow through the loop. At this time, there is a seebeck electromotive force – thermal electromotive force between the two ends. This is called the Seebeck effect.

The two homogeneous conductors with different compositions are the hot electrodes, and the end with the higher temperature is the working end. The end with the lower temperature is the free end. The free end is usually at some constant temperature.

According to the functional relationship between thermoelectromotive force and temperature, a thermocouple indexing table is made. The index table is obtained under the condition that the free end temperature is at 0°C. Different thermocouples have different scales.

When a third metal material is inserted into the thermocouple loop. As long as the temperature of both junctions of the material is the same. The thermoelectric potential generated by the thermocouple will remain constant. That is, it is not affected by the access of the third metal into the loop.

Therefore, when measuring the temperature of the thermocouple, the measuring instrument can be connected. After measuring the thermoelectromotive force, the temperature of the measured medium can be known.

Thermocouple Advantages:

1. High measurement accuracy: The thermocouple is in direct contact with the measured object and is not affected by the intermediate medium.
2. Fast thermal response time: Thermocouples are sensitive to temperature changes.
3. Large measurement range: thermocouples can measure temperature continuously from -40 to +1600 °C.
4. Reliable performance and good mechanical strength.
5. Long service life and easy installation.

Types and structures of thermocouples

Types of thermocouples Thermocouples include k type (nickel-chromium-nickel-silicon), n-type (nickel-chromium-silicon-nickel-silicon-magnesium), e-type (nickel-chromium-copper-nickel), j-type (iron-copper-nickel) , t-type (copper-copper-nickel), s-type (platinum-rhodium 10-platinum), r-type (platinum-rhodium 13-platinum), b-type (platinum-rhodium 30-platinum-rhodium 6) and so on.

Structural form of thermocouple: The basic structure of a thermocouple is a thermal electrode, an insulating material and a protective tube. Display instrument, recording instrument or computer and other supporting use. In field use, thermocouples suitable for various environments are developed according to various factors such as the environment and the measured medium.

Conclusions, which one should you use?

It really depends on the specific application and what is more important: accuracy or speed. If you need to measure very high or very low temperatures, a thermocouple is the better choice. If you need more accuracy, then an RTD is the way to go.

Read more about: What Is 0-10V Signal Output?

Temperature Measurement Solutions

Sino-Inst is Manufacturer of RTD & Thermocouples for temperature measurement. We supply more than 20 kinds of RTD & Thermocouples. 40% RTD, 60% Thermocouples.

RTD & Thermocouples for diesel fuel measurement are mainly used for temperature measurement of various meadium.

RTD & Thermocouples enable stable temperature measurement. This greatly meets the measurement needs of many applications.

Sino-Inst’s RTD & Thermocouples for temperature measurement, made in China. Having good Quality, With better price. Our temperature measurement instruments are widely used in China, India, Pakistan, the US, and other countries.

The entire team at Sino-Inst’s has received excellent training, so we can ensure that every client’s needs are met. For assistance with your product requirements, whether it’s a RTD & Thermocouples for temperature measurement, flow sensor, or other device, give us a call.

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The standard platinum rhodium 10-platinum thermocouple is used to realize the thermocouple temperature measurement value transfer and precise temperature measurement in the temperature range of 419.527~1084.62 ℃. The accuracy grades are first-class and second-class standards.

The standard platinum rhodium 30-platinum rhodium 6 thermocouple is used for the value transfer and precision temperature measurement of the current thermocouple in the temperature range of 1100~1500℃. The accuracy grade has the second-class standard.

First-Class Standard Thermocouple WRPB-1

Second-Class Standard Thermocouple WRPB-2

Second-Class Standard Platinum Rhodium 30-Platinum Rhodium 6 Thermocouple

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Sino-Inst’s Temperature Transmitters, made in China, Having good Quality, With better price. Our Temperature measurement instruments are widely used in China, India, Pakistan, US, and other countries.

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Armored thermocouples are usually used in conjunction with display instruments, paperless recorders and regulators. It can measure the temperature of liquid, vapor and gaseous media and solid surfaces in the range of 0-1600°C during various production processes.

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The gasifier is the most representative device of the coal chemical industry. Sino-Inst is based on the special conditions of the TEXACO gasifier and the Shell gasifier. It shares the special structure and material selection of the gasifier thermocouple and other professional knowledge. Help users solve special problems The problem of temperature measurement in the occasion.

Accurate measurement and strict control of temperature are the key factors to ensure the normal operation of the process. It will directly affect whether the process is carried out under ideal conditions. It will even affect the service life, safety production and environmental protection of the overall equipment.

The temperature measurement conditions of TEXACO gasifiers and Shell gasifiers in the coal chemical industry are extremely specific due to the gasifier body structure, internal medium, reaction process, and operating conditions. Therefore, the choice of the corresponding Furnace thermocouple also highlights its special importance and complexity.

The author relies on many years of rich experience. The representative and typical working conditions of TEXACO gasifier and Shell gasifier. As well as the selection principles and precautions of the corresponding thermocouples, which I will discuss with you. It aims to provide reference guidance and suggestions for coal chemical users.

1.TEXACO gasifier

① Operating conditions of TEXACO gasifier

Representative sites of TEXACO gasifiers include Ningxia Coal Group’s 250kt/a methanol plant, Shenhua Baotou and Inner Mongolia Jiutai Energy’s coal-to-olefin project.

The above site adopts the TEXACO rapid cooling process and the whole waste boiler process, which is the most advanced energy-saving process in the world.

TEXACO gasifier has special working conditions such as high temperature, high pressure, coexistence of oxidizing and reducing atmospheres, strong scouring, and sudden changes in temperature and pressure.

The operating temperature of the combustion chamber is 1350-1500℃, and the operating pressure is 2.5-8.7 MPa.

The flame gas contains CO, H2, CO2, H2O, CH4, Ar, N2 and H2S and other components.

The melting point of coal ash is 1310-1370°C.

The shell of the combustion chamber and the lining refractory bricks are likely to shear each other due to the different coefficients of thermal expansion.

The most important thing is that the scouring and abrasion in the gasifier is extremely serious, so the measuring point of the thermometer must be located 15-25mm away from the inner wall of the furnace wall.

However, it is difficult to ensure uniformity in the furnace wall during the masonry process. Coupled with the existence of scouring and wear in the furnace. The furnace wall will be inevitably thinned gradually during the production process.

②The performance requirements of TEXACO gasifier for Furnace thermocouple:

Due to the special working conditions of the above-mentioned TEXACO furnace, the temperature sensor must be able to withstand a high temperature of 1600°C and a high pressure of 8.7MPa.

It can resist the double corrosion of oxidizing and reducing gas at the same time.

It has an adjustable insertion length and a follow-up structure. When the inner lining changes in thickness and torsion and shear, corresponding structural changes can be made.

③Temperature measurement solution for TEXACO gasifier

In response to these requirements, the Furnace thermocouple is designed with special materials and structure. It satisfies the harsh requirements of the gasifier operating conditions for the temperature sensor. It is a kind of thermocouple special for gasifiers with adjustable anti-vibration, erosion resistance, anti-oxidation, and multi-stage leakage resistance.

First of all, the main structure of the Furnace thermocouple adopts the design concept of adjustable anti-vibration.

When the thickness of the gasifier wall lining changes, the Furnace thermocouple can be adjusted within the range of ±120mm through the shrinkable threaded sleeve. In order to achieve the measurement, the element is always maintained in the best position.

When the furnace shell and the inner lining are sheared, the universal rotating ball can be used to make the probe rotate with the inclination. And under the action of the limit guide tube and the correction damping spring.

Keep the probe always in the correct position in the center of the wall bushing. It prevents the thermocouple of the gasification furnace from being broken when the furnace wall is deformed.

Multi-stage leak-proof structure design. When the thermowell is damaged, it can prevent convection between the inside of the furnace and the outside through the sleeve. Safety hazards caused by high temperature conduction or even leakage in the furnace. Reflect safety awareness and environmental protection concepts.

Secondly, the Furnace thermocouple protection tube material adopts imported silicon carbide material made by pressureless sintering of sub-micron silicon carbide powder. The material has the dual characteristics of ceramic and metal and can be used reliably even at high temperatures exceeding 1750°C.

Its thermal conductivity is equivalent to stainless steel, 5 times that of alumina. Hardness is one of the high-performance materials second only to diamond. Ultra-fine crystals smaller than 10μm have been tested by helium gas leak detection.

The porosity is almost 0, which can effectively prevent H2 and CO from damaging the Furnace thermocouple wire through the pores on the tube wall. Ultra-high hardness and density make it have ideal wear resistance and long-term resistance to material erosion. It can work in oxidizing and reducing atmospheres and corrosive environments of strong acids and alkalis even at ultra-high temperatures.

2.Shell gasifier

At present, more than ten sets of Shell gasifiers have been introduced in China. Representative sites are Henan Kaixiang Methanol Project, Yunwei Group’s synthetic ammonia and Guangxi Liuhua coal gasification plants.

①Shell gasifier operating conditions characteristics

Conditions to ensure the normal operation of Shell gasifiers:

Neither can cause damage to the over-temperature of the vaporizer. It is necessary to consume the lowest amount of oxygen as much as possible. To achieve complete carbon conversion, temperature control is essential.

However, the structural characteristics of the Shell gasifier determines that the temperature of the gasifier cannot be directly measured. It can only be controlled by indirect methods.

An “annular space” is formed between the inner membrane wall of the gasification furnace and the outer shell. Hot syngas cannot escape into the “annular space”, causing the shell to overheat. Calculating the temperature in the furnace by measuring the temperature of the water wall in the “annular space” is the characteristic of Shell gasification furnace temperature measurement.

The temperature in the “annular space” is collected at multiple points, and each temperature measurement point is the key to understanding whether the gasifier is operating normally. If there is any abnormality, find and analyze the cause immediately. And deal with it in time, otherwise it will cause major damage to the gasifier.

②Shell gasifier performance requirements for Furnace thermocouples

Because the pulverized coal of Shell gasifier is 4MPa. It is combusted with O2 and steam in a gasifier at around 1500°C. And when it was sent to the coal burner, it was already under a pressure of 2MPa. Therefore, the thermometer is required to have a pressure resistance of not less than 2MPa. It also has emergency measures in case of overpressure, and can withstand a high temperature of 1500°C.

Because the Shell gasifier is composite equipment that integrates dynamic and static equipment, as well as combustion, reaction, heat exchange, and quenching processes. Therefore, it is required that the thermocouple used must be easy to install and remove, and easy to replace. Because of its complex structure and many control points, a multi-point thermocouple must be used.

③Shell gasifier temperature measurement solution

The design solution of the special Furnace thermocouple for the multi-point core-pulling and leak-proof Shell gasifier has solved the on-site requirements.

a. Multi-point thermocouples of different lengths (up to 6m) are used to measure the temperature of different control points in one fell swoop, using 3-point or 4-point thermocouples.

b. According to the characteristics of on-site installation, 4in (1in=25.4mm) No. 600 flange mounting parts are used. This part and the temperature measurement body are separable structures. The flange installation and fixing can be completed first. Then the temperature measurement body Installation with flange.

This solution allows the bulky flange to be installed at one time. In daily maintenance, only the main part needs to be replaced, which is convenient and quick. Closely combined with this structure is the independent replaceability of the core elements. Each measuring element is a detachable individual. If any one fails, it can be taken out and replaced separately. The maintenance cost and the quantity and expense of spare parts are reduced.

c. Due to the large amount of HCl in the medium in the furnace. Incoloy and Inconel alloys are often used in equipment manufacturing. In order to maintain a high resistance to high temperature and corrosion, the selection of Furnace thermocouple protection tube materials must all use Inconel600 alloy. Keep temperature measurement components , The material consistency of the mounting flange and the sealing sleeve.

d. The thermometer must have a multi-stage leak-proof sealing structure design. That is, each element must have a double-stage leak-proof device. When any temperature measuring element is damaged, due to the existence of leak-proof, the “annular space” The high temperature and high pressure airflow will not leak. It also guarantees the safety and environmental protection on-site requirements.

3.Furnace thermocouple installation instructions

①After the gasifier is shut down, consider the decompression effect of refractory materials and pressure vessels and the problems of thermal expansion and contraction. The pressure relief and replacement process of the gasifier is carried out slowly, and the furnace pressure should be basically reduced to atmospheric pressure and completely replaced.

After that, while the molten slag has not completely solidified, the gasifier thermocouple is drawn out of the furnace.

If the slag is solidified, the thermocouple ceramic sleeve cannot be pulled out, only the metal part at the back can be pulled out. The ceramic sleeve is usually driven into the furnace with a special tool.

②When installing a new gasifier thermocouple, put the assembled gasifier thermocouple on a special bracket. The bracket is placed on the wheeled trolley so that the gasifier thermocouple is exactly aligned with the gasifier thermocouple.

The equipment takes over in the middle. Slowly push the gasifier thermocouple into the furnace, about 10mm every 5 minutes. Protect the thermocouple from thermal shock.

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Sino-Inst’s Temperature Transmitters, made in China, Having good Quality, With better price. Our Temperature measurement instruments are widely used in China, India, Pakistan, US, and other countries.

A bimetallic stemmed thermometer is a simple but effective tool used in a variety of industries, including food, HVAC, medical, and automotive. It’s easy to use, requires no external power source, and is highly durable. In this guidebook, we’ll explain how a bimetallic stemmed thermometer works, its advantages and disadvantages, how to use it, and its various applications. So, whether you’re a professional or just curious about thermometers, you’ll find everything you need to know right here. Let’s get started!

Benefits of Bimetallic stemmed thermometer

(1) The thermoelectric potential is not disturbed. The thermoelectric potential of the bimetal thermometer is not disturbed by other factors. It has strong adaptability to different measurement environments.　

(2) The structure is diverse and can meet different measurement requirements. It can be used as electric contact bimetal thermometer, explosion-proof electric contact bimetal thermometer, bimetal thermometer with remote transmission, etc.

(3) The temperature response speed is fast, and there is strong sensitivity. At the same time, temperature changes are intuitive and can be displayed and recorded on site.

(4) Small size, convenient application, and strong temperature measurement reliability. Nominal diameter of dial: 60,100 (4 inch), 150 optional;

(5) It has a long service life, can be used in the temperature detection of chemical production for a long time, and has stable physical and chemical properties.

(6) Stable anti-corrosion performance. The application of bimetallic thermometers in chemical production often needs to be used in liquid and gas environments. It has a good anti-corrosion ability to protect pipes. It will not be affected by measurement conditions and cause corrosion problems that affect measurement quality.

(7) Strong pressure bearing. It has good pressure-bearing capacity, can be measured in different pressure levels, and has strong adaptability to measurement conditions.

(8) The conductivity is high. The temperature coefficient of resistance is small, and the thermoelectric potential has a linear relationship with the temperature, and the linearity is good.

Method 1: Ice water

1. Fill the container with ice cubes, and then top up with cold distilled water to form a watery slurry.

2. Insert the thermometer probe into the container, making sure not to touch the side.

3. The temperature should be 32°F (0°C) after 30 seconds. If not, the thermometer needs to be recalibrated. Record the difference and offset the thermometer as needed.

Method 2: boiling water

1. Boil a clean container of distilled water.

2. Once the water boils, insert the thermometer probe. Make sure again that the probe does not touch the side or bottom of the pot.

3. The temperature should be 212°F (100°C). Record the difference and offset the thermometer as needed. Note that the boiling point of water varies with altitude. Use this handy water boiling point calculator to find the temperature that suits your altitude.

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Temperature sensor transmitter is a combination of temperature sensor and temperature transmitter. It can be used in chemistry and medicine.

Thermocouple, thermal resistance and temperature transmitter of SBW series are the temperature transmitter units of on-site installation type in DDZ series instruments and they are used by matching with industrial thermocouple, thermal resistance. It uses dual-wire transmission mode(two conducting wires are used as the common transmission lines for power input and signal output).

It transforms the industrial thermocouple, thermal resistance signal into 4-20mA and 0-10 mA output signals which are linear with input signal and temperature signal. The transmitter can be directly installed in the junction box of the thermocouple and thermal resistance so as to form an integrated structure. As the temperature measuring instruments of new generation, it is widely used in metallurgy, petroleum, chemical industry, electric power, light industry, textile, food, national defense, scientific research and other industrial sectors.

The Difference Between Temperature Sensor Transmitter

A temperature sensor refers to a sensor that can sense temperature and convert it into a usable output signal. Temperature sensor is the core part of temperature measuring instrument, and there are many types. According to the measurement method, it can be divided into two categories: contact type and non-contact type.

Temperature transmitter uses thermocouple and thermal resistance as temperature measuring elementAfter measurement, the output signal is sent to the transmitter after voltage stability filtering, operational amplification, nonlinear correction, V/I conversion, constant current and reverse

The Function Of The Temperature Transmitter

A temperature transmitter is an instrument that converts a temperature variable into a standard output signal that can be transmitted.Mainly used for the measurement and control of temperature parameters in industrial processes.
The temperature current transmitter converts the signal of the temperature sensor into a current signal, and connects it to an auxiliary instrument to display the corresponding temperature.

Advantage Analysis

• High precision
• The range and zero point can be continuously adjusted from the outside
• Good stability
• Positive mobility can reach 500%, negative mobility can reach 600%
• Two-wire system, three-wire system, four-wire system
• Adjustable damping, overvoltage resistance
• Solid state sensor design
• No mechanical moving parts, less maintenance
• Light weight (2.4kg)
• Full series of unified structure, strong interchangeability
• Miniaturization (total height 166mm)
• The diaphragm material contact medium is optional
• Unilateral overpressure
• Low-pressure casting aluminum alloy shell
• Superior measurement performance, used for pressure, differential pressure, liquid level, flow measurement

Technical Parameter

1. input
Thermocouple: (two wire system), voltage: connect cable:sensor lead, maximum impedance 1.5k
Thermal resistance: (three wire system, four wire system), resistance measurement: connect cable: resistance compensation can reach 50.
2.output Isolated voltage 0.5kV instead of 2.5KV
3.Transmitter input options, accuracy and environmental temperature impact.

How Does A Thermocouple Transmitter Work?

Thermocouple or thermal resistance sensor will be converted into electrical signal by the temperature and then, the signal is sent to the input network of the transmitter which is consisted of zero adjustment circuit, a thermocouple compensation circuit and other related circuits. The signal after zero adjustment will be input to the operational amplifier for signal amplification. The signal after amplification will be output as 4 ~ 20mA DC after being processed by the V/I converter in one way; in another way, it will be displayed in gauge outfit after being processed by A/D converter.

There are two kinds of linear circuit in the transmitter, both of which adopt the feedback mode. The thermal resistance sensor is corrected by the positive feedback mode while the thermocouple sensor is corrected by approximation method of multiple-segment line. There are two kinds of display modes for the temperature transmitter of integrated digital display. The temperature transmitter displayed by LCD is output by the two-wire system while the temperature transmitter displayed by LED is output by the three-wire system.

1. Bimetal sensor：The bimetal consists of two pieces of metal with different expansion coefficients glued together. As temperature changes, material A will expand more than other metals, causing the metal plate to bend. The curvature of the bend can be converted into an output signal.
2. Bimetal rod and metal tube sensor：As the temperature rises, the length of the metal pipe (material A) increases,but the length of the unexpanded steel pipe (metal B) does not increase, so due to the change in position, the linear expansion of the metal pipe can be transmitted. Furthermore, this linear expansion can be converted into an output signal.
3. Sensor designed for liquid and gas deformation curve：When the temperature changes, the volume of liquid and gas will change accordingly.

Various types of structures can convert this expansion change into a position change, thereby generating a position change.

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