What Is Thermal Resistance?

Thermal resistance is one of the most commonly used temperature detectors in medium and low temperature regions.

Thermal resistance thermometry is based on the characteristic that the resistance value of metal conductors increases with temperature. Its main features are high measurement accuracy and stable performance.

Among them, the measurement accuracy of platinum thermal resistance is the highest. It is not only widely used in industrial temperature measurement, but also made into a standard benchmark.

Thermal resistances are mostly made of pure metal materials. The most widely used are platinum and copper. In addition, materials such as nickel, manganese and rhodium have been used to manufacture thermal resistances.

There are many types of temperature-sensing materials commonly used in metal thermal resistors, and platinum wire is the most commonly used.

In addition to platinum wire, metal thermal resistance materials for industrial measurement include copper, nickel, iron, iron-nickel, etc.

Features of Thermal Resistance

  • Stable physical and chemical properties, high measurement accuracy, corrosion resistance and long service life.
  • The temperature coefficient of resistance should be large, that is, the sensitivity should be high.
  • The resistivity should be high to make the thermal resistance smaller and reduce the time constant of temperature measurement.
  • The heat capacity should be small, so that the thermal inertia of the resistor body is small and the response is more sensitive.
  • Good linearity, that is, the relationship between resistance and temperature is linear or a smooth curve.
  • It is easy to process, inexpensive, and reduces manufacturing costs.
  • Good reproducibility, easy for mass production and parts interchange.

Measuring Principle of Thermal Resistance

The temperature measurement principle of thermal resistance is different from that of thermocouple in that thermal resistance measures temperature based on the thermal effect of resistance. That is, the resistance of the resistor body changes with temperature.

Therefore, as long as the resistance change of the temperature-sensing thermal resistance is measured. temperature can be measured. There are mainly two types of metal thermistors and semiconductor thermistors.

The resistance value and temperature of a metal thermal resistor can generally be expressed by the following approximate relationship.
That is, Rt=Rt0[1+α(t-t0)]

where:
Rt is the resistance value at temperature t;
Rt0 is the corresponding resistance value at temperature t0 (usually t0=0℃);
α is the temperature coefficient.

The relationship between the resistance value and temperature of the semiconductor thermistor is: Rt=AeB/t

where Rt is the resistance value when the temperature is t;
A and B are constants that depend on the structure of the semiconductor material.

In comparison, the thermistor has a larger temperature coefficient. The resistance value at room temperature is higher (usually in the thousands of ohms). But the interchangeability is poor and the nonlinearity is serious. The temperature measurement range is only about -50~300℃. It is widely used in temperature detection and control for home appliances and automobiles.

Learn more: Thermal resistance From Wikipedia

Common Types of Thermal Resistance

Platinum thermal resistance is characterized by high measurement accuracy. Good stability and reliable performance.

However, in reducing media, especially at high temperature, it is easily stained and brittle by the steam reduced from the oxide. And change the relationship between resistance and temperature.

In order to overcome the above shortcomings, the thermal resistance core should be installed in a protective sleeve.

Copper thermal resistance is commonly used in industry to measure the temperature in the range of -5O℃~+15O℃.

Copper is easy to purify and is much cheaper than platinum. The temperature coefficient of resistance is large and the relationship is linear. Therefore, when making a thermal resistance with a certain resistance value, compared with platinum, if the length of the resistance wire is the same, the copper resistance wire is very thin, and the mechanical strength is reduced. If the wire diameter is the same, the length will increase many times and the volume will increase.

In addition, copper is easily oxidized above 100°C and has poor corrosion resistance, so the working temperature does not exceed 150°C.

Nickel thermal resistance has a large temperature coefficient and a higher sensitivity than platinum and copper. It is often used to measure temperatures in the range of -60°C to +180°C.

Due to the complicated manufacturing process of nickel thermal resistance, it is difficult to obtain nickel wires with the same α. Therefore, its measurement accuracy is lower than that of platinum thermal resistance. The graduation numbers of standardized thermal resistances currently stipulated in my country are Ni100, Ni300, and Ni500.

The semiconductor spot thermometer is a thermistor made of metal oxides such as manganese, nickel, copper and iron as the temperature measuring element. Its shapes are bead-shaped, round, washer-shaped and sheet-shaped. Commonly used are 61 type bead and miniature bead semiconductor thermistors.

The difference from general thermal resistance is that it has a negative temperature coefficient of resistance. As the temperature increases, the resistance decreases. The range of change is also large, and the temperature coefficient of resistance α reaches -2 to -7%. It is 10 to 100 times larger than metal thermal resistance. Therefore, display instruments with lower accuracy can be used.

Thermal Resistance vs Thermal Conductivity

Thermal conductivity, also known as “thermal conductivity”. [1] is a measure of the thermal conductivity of a material. The symbol is λ or K.
It refers to the heat transferred through the unit horizontal cross-sectional area per unit time when the vertical downward gradient of the temperature is 1°C/m.

Its specific definition is: take two parallel planes with a distance of 1 meter and an area of 1 square meter inside the object perpendicular to the direction of heat conduction. If the temperature difference between the two planes is 1K, it will be conducted from one plane to another within 1 second. The amount of heat is specified as the thermal conductivity of the substance. Its unit is: watt · m-1 · Kai-1 (W · m-1 · K-1).

If there is no heat loss, for a block material with parallel opposite sides, there is
E/t=λA(θ2-θ1)/ι

In the formula, E is the energy transferred in the time t. A is the cross-sectional area. ι is the length. θ2 and θ1 are the temperatures of the two sections, respectively.

In general there are:
dE/dt=-λAdθ/dι

Thermal Resistance Example Problems

TroubleshootingReasonMethod of exclusion
The displayed value of the instrument is lower than the actual value or the displayed value is unstableThere are metal shavings, dust in the protection tube, dirt between the terminals and short circuit of the thermal resistance (water droplets, etc.)Remove metal chips, clean dust, water droplets, etc., find short-circuit points, strengthen insulation, etc.
Display value infinityThe thermal resistance or the lead wire is disconnected and the connection terminal is loosened, etc.Replace the resistor body, or weld and tighten the wiring screws, etc.
There is a change in the relationship between resistance and temperatureThe platinum resistance wire material is corroded and deterioratedReplace the thermal resistance
The display value of the meter is zero or has a negative valueThe thermal resistance wiring is wrong, the thermal resistance is short-circuited, or the cable is short-circuitedCorrect wiring, find short circuit, strengthen insulation, replace resistor body or cable

Temperature Range And Tolerance Level

GraduationTolerance levelWire wound element effective temperaturerangeMembrane element effective temperaturerangeRange of temperature measurement
Pt100AA-50~2500~150±(0.1+0.0017丨t丨 )
Pt100A-100~450-30~300±(0.15+0.002丨t丨 )
Pt100B-196~600-50~500±(0.3+0.005丨t丨 )
Pt100C-196~600-50~600±(0.6+0.01丨t丨 )
Pt100-50~150±(0.3+0.006丨t丨 )
t = temperature Absolute value,units for C

Thermal Resistance With No-fixed Device

Thermal resistance with no-fixed device  1

WZP-120 WZP -120 2 WZP-120G WZP -120G 2 WZP-121 WZP -121 2 WZP- 121G WZP -121G 2 WZP- 130 WZP -130 2 WZP- 130G WZP -130G 2 WZP- 131 WZP -131 2 WZP- 131G WZP -131G 2 WZC-120G WZC-120 WZC-130 WZC-130G WZC-121 WZC-121G WZC-131 WZC-131G

1) Model 120 and 121 are anti-spring type with the protective grade being IP65; Model 130 and 131 are waterproof type with the protective grade grade being IP55. 2) Protective tube material is 1Cr18Ni9Ti and the rest of the material is ordered according to the agreement.

Fixed Screw Type Thermal Resistance

Fixed screw type thermal resistance  1

WZP-220 WZP -220 2 WZP-220G WZP -220G 2 WZP-221 WZP -221 2 WZP- 221G WZP -221G 2 WZP- 230 WZP -230 2 WZP- 230G WZP -230G 2 WZP- 231 WZP -231 2 WZP- 231G WZP -231G 2 WZC-220 WZC-220G WZC-230 WZC-230G WZC-221 WZC-221G WZC-231 WZC-231G

1) Model 220 and 221 are anti-spraying type with the waterproof grade being IP65. model 230 and 231 are waterproof type with the protective grade being IP55. 2) Protective tube material is 1Cr18Ni9Ti and the rest of the material is ordered according to the agreement. 3) Nominal pressure is ≤4.0MPa.

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