Checklist: Pressure Transmitter 4-20mA Common Faults and Error

Pressure Transmitter 4-20mA is the most commonly used one in industrial process control. In industrial process control, the measurement and control of pressure has always been a very important parameter. The 4-20mA output pressure transmitter is widely used due to its own characteristics, such as: good anti-interference performance, less and simple wiring, etc.

Sino-Inst sorted out the corresponding solutions according to the problems that customers often encountered in the process of installing and using this type of pressure transmitter. In particular, it pointed out that this article is aimed at common problems in the use of 4-20mA output pressure transmitters.

Pressure Transmitter 4-20mA Common Faults and Error

Featured 4-20ma Pressure Transmitters for Sale

When checking whether the pressure transmitter can work normally, the tool we use most is the multimeter. After you confirm the output type of the pressure transmitter (common ones are 4-20 mA current type and 0-10V voltage type) and working voltage and other parameters. You can start the operation.

1. Pressure detection.

For current output type (4-20 mA): Set the multimeter to the current measurement function. And connected to the output loop of the pressure transmitter, connected in series.
For voltage output type (0-10V): Set the multimeter to the voltage measurement function. And connected in parallel at both ends of the output of the pressure transmitter.

Turn on the power and record the reading on the multimeter.

For a transmitter with a 4-20 mA output: With no pressure input, a current close to 4 mA should be displayed. At maximum pressure input, a current of approximately 20 mA should be displayed.
For a transmitter with 0-10V output: when there is no pressure input, a voltage close to 0V should be displayed. At maximum pressure input, a voltage close to 10V should be displayed.

Through the above methods, the status of a transmitter can basically be detected. If accurate detection is required, a standard pressure source is required. Apply pressure to the sensor and calibrate the transmitter according to the pressure and the change in the output signal. And when conditions permit, conduct temperature detection of relevant parameters.

2. Zero point detection.

Use the voltage range of the multimeter to detect the zero output of the transmitter without applying pressure. This output is generally a mV level voltage. If it exceeds the technical specifications of the sensor, it means that the zero point deviation of the transmitter is out of range.

3.Detection of bridge roads.

It mainly detects whether the circuit of the transmitter is correct, usually a Wheatstone full-bridge circuit. Use the ohm range of the multimeter to measure the impedance between the input terminals and the impedance between the output terminals. These two impedances are the input and output impedances of the pressure sensor. If the impedance is infinite, the bridge is open. It means there is a problem with the transmitter or the pin definition is not correct.

Troubleshooting for 4-20ma Pressure Transmitter

1. The pressure transmitter has no output and no display

  • Check whether the power supply is reversed. Connect the polarity of the power supply correctly
  • Measure the power supply of the transmitter, whether there is 24V DC voltage. It is necessary to ensure that the power supply voltage supplied to the transmitter is ≥12V (intelligent type) ≥15V (ordinary type). If there is no voltage, check whether the circuit is open or not, and check the secondary instrument/PLC sampling resistance (input impedance should be ≤250Ω)
  • If it has a head, check the head for damage. You can short-circuit the two wires of the meter head first. If it is normal after short-circuiting, it means that the meter head is damaged. Replace the header.
  • Connect the ammeter in series to the 24V power circuit to check whether the current is normal. If it is normal, it means that the transmitter is normal. At this time, you should check whether other instruments in the loop are normal.
  • Whether the wiring is loose. Connect the wires and tighten.
  • Whether the power supply is correctly connected to the power input terminal of the transmitter. Correctly connect to the power terminal according to the power requirements.

Extended Reading: Resistive Pressure Transducer

2. Pressure transmitter output ≥ 20mA

  • Whether the power supply of the transmitter is normal. If it is less than 12VDC, you should check whether there is a large load in the circuit. The input impedance of the transmitter load should comply with RL≤(transmitter power supply voltage-12V)/(0.02A) Ω.
  • Whether the actual pressure exceeds the upper limit of the full scale. Reselect a pressure transmitter with an appropriate range.
  • Whether the sensor is damaged. Severe overloading can sometimes damage the isolating diaphragm. It needs to be returned to the factory for inspection.
  • Whether the power cord is connected correctly. The power cord should be connected to the corresponding terminal.

3. Transmitter output≤4mA

  • Whether the power supply of the transmitter is normal. If it is less than 12VDC, you should check whether there is a large load in the circuit. The load capacity of the transmitter should comply with RL≤(transmitter power supply voltage-12V)/(0.02A)Ω.
  • Whether the wiring is loose causes the contact resistance to increase, which is equivalent to the increase of the load. Connect the wires and tighten them, and the load should meet RL≤(transmitter power supply voltage-12V)/(0.02A) Ω.
  • Whether the actual pressure exceeds the selected range of the pressure transmitter. Reselect a pressure transmitter with an appropriate range.
  • Whether the sensor is damaged, severe overload sometimes damages the isolation diaphragm. It needs to be returned to the factory for inspection.

4. The pressure display/output signal is incorrect and has errors

  • Whether the power supply of the transmitter is normal. If it is less than 12VDC, you should check whether there is a large load in the circuit, and the load capacity of the transmitter should meet RL≤(transmitter power supply voltage-12V)/(0.02A) Ω.
  • Whether the reference pressure value must be correct. Replace with the same model product. If the error is consistent, check the reference pressure gauge. If the accuracy of the instrument is low, another instrument with higher accuracy needs to be replaced.
  • Whether the range of the secondary instrument is consistent with the range of the pressure transmitter. The range of the adjusted secondary instrument must be consistent with the range of the pressure transmitter.
  • The load capacity of the transmitter should comply with RL≤(transmitter power supply voltage-12V)/(0.02A)Ω. If it does not meet the requirements, corresponding measures can be taken according to the difference: such as increasing the supply voltage (but must be lower than 36VDC), reducing the load, etc.
  • Whether the corresponding equipment casing is reliably grounded. The equipment shell is reliably grounded.
  • Whether the on-site strong current and weak current are wired separately. Scientific and reasonable wiring to avoid signal instability/error caused by interference.
  • The sensor diaphragm is easily deformed, and overload, foreign objects, and hard objects sometimes damage the isolation diaphragm. It needs to be returned to the factory for inspection.
  • Impurities in the pipeline will affect the measurement accuracy. Impurities need to be cleaned, and a filter is added before the pressure port.

Extended Reading: Smart pressure transmitter

5. The pressure control system cannot be connected

The input of the pressure indicating instrument is 4~20mA, and the output signal of the transmitter can be directly connected.
If the input of the pressure indicating instrument is 1~5V. Then a resistor with a precision of 1/1000 or above and a resistance of 250Ω must be connected to the input terminal of the pressure indicating instrument. Then connect to the input of the transmitter.

No matter which pressure transmitter you choose. The signal output by the pressure transmitter, such as 4-20mA. The pressure signal can be interfaced to a paperless recorder. Multi-channel pressure signal monitoring.

6. recorder no record

When the multipoint paper recorder does not record. If the input is open, then:

  1. No other loads can be carried;
  2. Use another recorder with input impedance ≤250Ω when there is no record.

7. The temperature of the pipeline is too high

The temperature of the pipeline is too high, add a buffer tube to dissipate heat. It is best to add some cold water to the buffer tube before use. In order to prevent the superheated steam from directly impacting the sensor. Thereby damaging the sensor or reducing the service life.

Read more about: Pressure Sensor Applications-Featured Industry Applications

8. Pressure Transmitter 4-20mA installation problem

In steam flow measurement, steam mainly involves two kinds, one is external steam, and the other is superheated steam of boiler.

The externally supplied steam is the steam after decompression and decompression. The temperature is not high, and it is mixed with a lot of water. It needs to be used and not used. The steam flow rate can be changed according to the user’s requirements.

In the actual flow measurement process, sometimes the flow rate is too large and sometimes the flow rate is too small. It is very unstable. Blowdown is often required. The measurement of the transmitter is accurate after each blowdown. All joints on the pressure guiding pipe leak steam.

In the use of measuring superheated steam, the biggest problem found is that sometimes the machine shuts down, and the flow rate will deviate after restarting, resulting in inaccuracy. Sometimes there is still a little flow display after stopping.

Generally, the installation position of the transmitter is lower than the measuring pipe. But in the actual installation, the condensing tank and the transmitter for external steam flow are both higher than the measuring pipe, and the pressure guiding pipe that is laid down at least 1 meter from the throttling device is too short.

The superheated steam flow of the boiler also has the problem of inconsistent heights between the condensing tank and the measuring pipe. This leads to an imbalance in the height of the condensed water, which causes a static pressure difference.

9. Pressure guide pipe blockage problem

In pressure measurement, sometimes the indicated pressure does not vary with operating conditions. After opening the drain valve, there is only a small amount of sewage and no water flows out. This is because there will be a small amount of floating dust in the water quality or compressed air, which will enter the pressure guiding pipe for sedimentation along with the water flow. After a long period of operation, the wall of the pressure guiding pipe will be corroded and fouled, and blockage will occur.

10. Pressure Transmitter 4-20mA equipment itself fault problem

In the measurement of lubricating oil pressure, since the lubricating oil pressure signal participates in the shutdown interlock control. The signal measured by the lubricating oil pressure transmitter is transmitted to the computer. On the one hand, it is displayed. On the other hand, the signal is compared by the program. When the pressure is lower than 0.06MPa, it will send a signal to stop due to lack of oil.

From the trend graph of the lubricating oil pressure signal, it can be seen that the pressure is an instantaneous drop in a straight line, resulting in a trip. After checking the transmitter, it was found that the internal module of the transmitter was damaged. Although the transmitter is regularly calibrated every year, it is used for production after passing the test.

After several years of operation of the transmitter, its accuracy, sensitivity, stability and other performance indicators will gradually decrease. The internal diaphragm and integrated block will also be damaged and malfunction.

11. There is an interference problem

In the measurement of the exhaust pressure of the air compressor, the fluctuation of the exhaust pressure signal is large. After the transmitter is verified, it meets the accuracy requirements, and the fault of the transmitter itself is eliminated.

Check that the pressure guide tube and connector are not damaged or leaked. The connection of the signal cable is in good contact.
However, the direction of the cables is introduced into the control room through the cable bridge next to the high-end distribution room. There is a lot of electromagnetic interference around.

Learn More About Pressure Transmitter 4-20mA

pressure transducer often called a pressure transmitter, is a transducer that converts pressure into an analog electrical signal.

Although there are various types of pressure transducers, one of the most common is the strain-gage base transducer.

The conversion of pressure into an electrical signal, is achieved by the physical deformation of strain gauges, which are bonded into the diaphragm of the pressure transducer, and wired into a Wheatstone bridge configuration.

Pressure applied to the pressure transducer produces a deflection of the diaphragm, which introduces strain to the gages.

The strain will produce an electrical resistance change proportional to the pressure.

A pressure transmitter is a device that converts pressure into a pneumatic signal or an electric signal for control and remote transmission.

It can convert the physical pressure parameters such as gas and liquid felt by the load cell sensor into a standard electrical signal (such as 4~20mADC, etc.). It can be used to supply secondary instruments such as indicating alarms, recorders, and regulators for measurement and indication. and process regulation.

4-20mA means that the minimum current is 4mA and the maximum current is 20mA.

The industry generally needs to measure various non-electrical physical quantities, such as temperature, pressure, speed, angle, etc. They all need to be converted into analog electrical signals before they can be transmitted to the control room or displays red meters of equipment away. This device that converts physical quantities into electrical signals is called a transmitter. The most widely used in the industry is to use a 4-20mA current to transmit analog quantities.

Because 4-20mA signals are least affected by electrical noise and resistance in the signal line, these sensors are best used when signals must be transmitted over long distances. These sensors are typically used in applications where the lead must be 1000 feet or more.

A pressure sensor is a device or device that can sense a pressure signal and convert the pressure signal into a usable output electrical signal according to a certain rule.

A pressure sensor usually consists of a pressure-sensitive element and a signal processing unit. According to different test pressure types, pressure sensors can be divided into gauge pressure sensors, differential pressure sensors and absolute pressure sensors. A pressure sensor is the core part of pressure transmitter.

In a pressure transducer, a thin-film or piezo-resistive pressure sensor is mounted on a process connection. The transducer converts pressure into an analog electronic output signal, typically as a millivolt per volt output. These signals are not linearized or temperature compensated.

pressure transmitter has additional circuitry that linearizes, compensates, and amplifies the signal from a transducer. The different signal types are typically voltage signals (eg, 0 to 5 or 0 to 10 volts), milliamp (eg, 4 to 20 milliamp), or digital. The instrument then can transmit the signal to a remote receiver.

Two-wire transmission has specific requirements for its minimum supply voltage, output current, and transmitter power consumption.

Only when these three conditions are satisfied in the design can the two-wire transmission be realized. The four-wire system requirements are not as high as the two-wire system.

Therefore, the two-wire system and the four-wire system have different requirements for the electronic components of the whole circuit.

For the two-wire instrument, the output signal is mostly 4-20mA (there are also frequency signal output).

But the four-wire instrument has more output signals, such as current signal 4-20mA, 0-10mA, voltage (millivolt) signal, frequency signal, etc.

Read more about: Pressure Transducer Wiring: 2 Wire-3 Wire-4 Wire

If you cannot find an answer to your question in our Pressure Transmitter 4-20m, you can always contact us and we will be with you shortly.

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Sino-Inst is a manufacturer of Pressure Transmitter 4-20mA. We offer more than 50 types of Transmitters. Main products include compact pressure transmitters, industrial pressure transmitters, differential pressure transmitters, single crystal silicon pressure transmitters, diffused silicon pressure transmitters , Diaphragm pressure transmitter, high temperature pressure transmitter, explosion-proof pressure transmitter, pressure transmitter, custom pressure transmitter, with remote pressure transmitter, 316 diaphragm pressure transmitter.

The Pressure Transmitter 4-20mA is mainly used to measure the pressure and liquid level of the medium. In industrial production, it is widely used. Sino-Inst pressure transmitter adopts high-quality diffused silicon type, and the pressure transmitter adopts a dedicated integrated module. Fine temperature, zero point, full scale and nonlinear compensation. Realize accurate measurement and transmission of pressure changes in liquid, gas, steam and other media. Realize the ideal monitoring of changes in pressure and vacuum media in various places for enterprises, scientific research institutes and other departments.

Sino-Inst’s Pressure Transmitters 4-20mA are the most commonly used sensors in industrial applications. Widely used in water conservancy and hydropower, railway transportation, intelligent building, production control, aerospace, military industry, petrochemical industry, oil well, electric power, ship, machine tool, pipeline and other industries.

Sino-Inst’s entire team is well trained, so we can ensure that each customer’s needs are met. If you need any help with your product requirements, whether it is a Pressure Transmitter 4-20mA, level sensors, or other equipment, please give us a call.

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About KimGuo11

Wu Peng, born in 1980, is a highly respected and accomplished male engineer with extensive experience in the field of automation. With over 20 years of industry experience, Wu has made significant contributions to both academia and engineering projects. Throughout his career, Wu Peng has participated in numerous national and international engineering projects. Some of his most notable projects include the development of an intelligent control system for oil refineries, the design of a cutting-edge distributed control system for petrochemical plants, and the optimization of control algorithms for natural gas pipelines.