Pressure Transmitter 4-20mA Guide: Testing and Troubleshooting

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Multimeter to test Pressure Transmitter 4-20mA

What is a pressure transmitter 4-20mA?

A 4-20mA pressure transmitter is an instrument that measures medium pressure and converts it into a 4-20mA current output signal. In industrial processes, the most common method for transmitting analog signals is using a 4-20mA current loop. The signal is transmitted to a remote control panel for pressure monitoring and control. For example, this can involve audible and visual alarms or initiating certain trip and shutdown procedures.

In a 4-20mA pressure transmitter, the 4-20mA output is linearly proportional to the pressure range. 4mA corresponds to a zero reading, the lower limit of the pressure range. 20mA corresponds to a full-scale reading, the upper limit of the pressure range.

What is the purpose of a pressure transmitter?

Pressure transmitters are used to measure the pressure of liquids, gases, or steam and convert it into a standard electrical signal for output. They are widely used in industrial automation, environmental monitoring, and medical equipment.

Main Applications:

  1. Pressure Measurement and Control.

Pressure transmitters measure and display pressure in real time. The output signal can be connected to a control system for real-time pressure monitoring and regulation.

  1. Flow Measurement.

By detecting the pressure changes exerted by liquid or gas within a pipeline, pressure transmitters can accurately monitor and control flow.

  1. Liquid Level Measurement.

In liquid storage tanks or deep wells, pressure transmitters can measure liquid levels to ensure they remain within safe limits.

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  1. Leak Detection:

On tanks or pipelines, pressure transmitters detect real-time pressure changes and monitor for leaks.

  1. Environmental and Weather Monitoring:

Pressure transmitters are used in weather stations and environmental protection equipment to monitor atmospheric pressure changes, influencing weather forecasts and environmental monitoring.

Pressure transmitter 4-20mA working principle

The working principle of a 4-20mA pressure transmitter is to convert pressure into a 4-20mA standard current signal output.

First, the medium pressure acts on the pressure sensor diaphragm, causing the diaphragm to deform. This causes a change in resistance or capacitance. The signal from the measuring element is filtered, amplified, and temperature compensated before being converted into an analog signal (4-20mA current) for transmission to the control system.

Read more about: Analog output signal of pressure transmitter: DC voltage 0~10 V, DC voltage 1~5 V, DC voltage 1~10 V

Pressure transmitter 4-20mA wiring

Wiring Principle

  • Power supply positive (24V+) → transmitter positive (+).
  • Transmitter negative (-) → DRO current input positive (AI+).
  • DRO current input negative (AI-) → power supply negative (24V-), forming a closed circuit.

Key Point: The signal and power supply share a common circuit; ensure that the series connection has no branches.

Pressure transmitter wiring

The power supply and current signal circuit of the two-wire transmitter is: 24V power supply positive terminal → transmitter positive terminal → transmitter negative terminal → digital display positive terminal → digital display negative terminal → 24V power supply negative terminal. The digital display receives the current signal. If a resistor R is connected in parallel between the DRO positive and negative terminals, the DRO receives a voltage signal.

Example Wiring Steps:

Using a 24V DC power supply, connect the red wire to the transmitter positive terminal, the blue wire (signal line) to the DRO AI+, and short the black wire (power supply negative) to the DRO AI-.

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

How to Test the 4-20mA of a Pressure Transmitter?

The 4-20mA signal is the converted value of the output generated by a pressure transmitter. At zero-scale output, the transmitter displays a 4mA signal output, while at maximum or full-scale output, the readout scale displays a 20mA signal.

Verifying the 4-20mA current loop signal is a critical step in pressure transmitter calibration and troubleshooting. Below are two recommended methods for measuring the 4-20mA current loop signal.

Use a process clamp meter to measure the 4-20mA signal.

The 4-20mA current loop signal can be measured without disconnecting the two-wire circuit.

Open the transmitter’s wiring terminals and connect the process clamp meter to the signal wires.

Identify the mA output signal at the zero value of the process clamp meter. The mA output value must be within the range of 4mA to 20mA.

If a reading between 4mA and 20mA is not displayed, troubleshooting and calibration are necessary. If the value is within this range, the transmitter’s performance is guaranteed.

Use a loop calibrator or multimeter to measure the 4-20mA signal.

Set the multimeter to current measurement mode. Connect the red test lead to the positive output terminal of the transmitter and the black test lead to the negative terminal. The value displayed on the meter will indicate the measured current.

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8 Types 4-20mA Pressure Transmitter Troubleshooting

Okay, now let’s look at some common 4-20mA pressure transmitter faults and their solutions.

Troubleshooting and Testing:Troubleshooting Steps:
First, check if the power supply is functioning correctly and if the power cable is properly connected.Ensure the power supply polarity is correct.
Measure the voltage at the transducer’s power input; it should be 24V DC.The power supply voltage to the transmitter must be ≥12V (i.e., the voltage at the transmitter’s power input terminal must be ≥12V). If there is no power, check for any open circuits or incorrect selection of the measuring instrument (input impedance should be ≤250Ω).
If the transducer has a local display, check if the display is damaged. Short-circuit the two wires to the display; if it then functions normally, the display itself is faulty.If the display is faulty, replace the display unit. If the display is working correctly, the transmitter is functioning properly; check other instruments in the circuit.
Connect an ammeter in series with the 24V power supply to check the current; it should be within the normal range.If the current reading is abnormal, the pressure sensor may be faulty and needs replacement.
Ensure the power supply is connected to the correct power input terminals of the transducer.Connect the power cable to the power terminals.

Check and Test:Troubleshooting Steps:
Is the power supply to the transducer functioning correctly?If the power supply voltage to the transmitter is less than 12VDC, check for any heavy loads in the circuit. The input impedance of the transmitter load should meet the requirement: RL ≤ (Transmitter Supply Voltage – 12V) / 0.02A Ω.
Is the actual pressure exceeding the selected range of the pressure transducer?Select a pressure transmitter with an appropriate measurement range.
Is the pressure sensor damaged? Severe overloads can sometimes damage the diaphragm.If the pressure sensor is damaged, send it back to the manufacturer for repair.
Are the connections loose?Ensure all connections are secure and tightened.
Are the power supply wires connected correctly?Connect the power supply wires to the corresponding terminals.

Check and test:Troubleshooting Steps:
Is the transmitter power supply functioning correctly?If the power supply voltage to the transmitter is less than 12VDC, check for any heavy loads in the circuit. The input impedance of the transmitter load should meet the requirement: RL ≤ (Transmitter Supply Voltage – 12V) / 0.02A Ω.
Is the actual pressure exceeding the selected range of the pressure transmitter?Select a pressure transmitter with an appropriate measurement range.
Is the pressure sensor damaged? Severe overloads can sometimes damage the diaphragm.If the pressure sensor is damaged, it must be sent back to the manufacturer for repair.

Check and test:Troubleshooting Solutions
Is the power supply to the transmitter normal?If the transmitter’s power supply voltage is less than 12VDC, check for any heavy loads in the circuit. The input impedance of the transmitter load should meet the requirement: RL ≤ (Transmitter Supply Voltage – 12V) / (0.02A) Ω.
Is the reference pressure value accurate?If the accuracy of the reference pressure gauge is low, replace it with a higher-precision gauge or pressure transmitter.
Does the pressure indicator’s range match that of the pressure transmitter?The pressure indicator’s range must match the pressure transmitter’s range.
Are the inputs and wiring of the pressure indicator correct?If the pressure indicator’s input is 4-20mA, the transmitter output signal can be connected directly. If the pressure indicator’s input is 1-5V, a 250Ω resistor with an accuracy of 0.1% or better must be connected in parallel to the indicator’s input, and then the transmitter’s output can be connected.
The input impedance of the transmitter load should meet the requirement: RL ≤ (Transmitter supply voltage – 12V) / (0.02A) ΩThe input impedance of the transmitter load should meet the requirement of RL ≤. If it does not, take appropriate measures depending on the situation: such as increasing the supply voltage (but must be below 36VDC) or reducing the load.
When the multi-point chart recorder is not recording, is the input open circuit?If a multi-point chart recorder is not recording and the input is open, then: 1. Do not connect any other load; 2. Use a different recorder with an input impedance ≤ 250Ω when not recording.
Is the equipment enclosure grounded?Ground the enclosure of the equipment.
Are the power cables and other cables separated?Keep the wiring separate from AC power and other power sources.
Is the pressure sensor damaged? Severe overload can sometimes damage the diaphragm.If the pressure sensor is damaged, send it back to the manufacturer for repair.
Are there any sand, debris, or other contaminants blocking the pipeline? Contaminants can affect measurement accuracy.If the pipeline is clogged with sand or other debris, clean the debris and install a filter screen before the pressure port.
Is the pipeline temperature too high? The operating temperature range for a standard pressure sensor is -25°C to 85°C, but it is best to keep it within -20°C to 70°C in actual use.If the pipeline temperature is too high, add a cooling tube to dissipate heat. Before use, it is best to add some cold water to the cooling tube to prevent hot steam from directly impacting the sensor, which could damage it or reduce its lifespan.

The choice of pressure transmitter installation location affects its measurement performance. Based on our experience, the following points should be considered during installation:

When measuring gas pressure, the pressure tapping point should be located in the upper half of the process pipeline.

When measuring liquid pressure, the pressure tapping point should be located in the lower half of the process pipeline, within an angle of 0 to 45° with the horizontal centerline of the process pipeline.

When measuring steam pressure, the pressure tapping point should be located in both the upper and lower half of the process pipeline, within an angle of 0 to 45° with the horizontal centerline of the process pipeline.

The pressure sampling component should be installed in a pipe section where the process medium flow is stable.

When the pressure sampling component and the temperature sampling component are installed on the same pipeline, the pressure sampling component should be installed upstream of the temperature sampling component.

The end of the pressure sampling component should not protrude beyond the inner wall of the process equipment or the process pipeline. When measuring the pressure of turbid media such as dust, solid particles, or sediment in vertical process pipelines, the source component should be installed at an angle upward, with an angle greater than 30 degrees to the horizontal. On horizontal process pipelines, it should be installed at an acute angle along the flow beam.

The pressure transmitter should be installed in a well-lit location that is easily accessible for operation and maintenance. It should not be installed in areas subject to vibration, humidity, high temperatures, corrosive conditions, or strong magnetic fields.

For low-pressure pressure transmitters, the installation height should be consistent with the pressure tapping point, especially when measuring liquids and condensable gases.

When measuring gas pressure, the transmitter should be installed above the pressure tapping point. When measuring liquid or steam pressure, the transmitter should be installed below the pressure tapping point to minimize the need for additional venting and draining equipment.

During pressure measurement, the indicated pressure sometimes does not change with operating conditions. After opening the drain valve, only a small amount of wastewater flows out, but then no water flows out. This is because the water or compressed air may carry a small amount of dust, which is carried along with the water flow into the pressure conduit pipe and settles. Over time, the pressure conduit pipe wall corrodes and accumulates, leading to blockage.

  • After several years of operation, the accuracy, sensitivity, and stability of a pressure transmitter will degrade. Therefore, we recommend annual calibration.
  • If the pressure is high, the diaphragm or integrated circuit of the pressure transmitter may also be damaged and malfunction.
  • The pressure transmitter may experience zero offset. This means that the output value of the pressure transmitter is not zero when there is no pressure. This may be caused by improper adjustment of the measurement circuit or accumulated errors in the pressure sensing element. For this type of failure, the zero and span of the measurement circuit should be readjusted and the pressure sensing element should be calibrated.
  • If the output value of the pressure transmitter changes over time, this may be due to range drift caused by aging of the pressure sensing element and temperature drift. Regular calibration and maintenance of the transmitter is necessary, and attention should be paid to temperature changes in the operating environment.

If the pressure transmitter itself is not faulty but still experiences significant signal fluctuations, environmental interference may be the cause.

  1. First, consider environmental factors such as temperature, corrosion, dust, and humidity. Extreme environmental conditions can cause problems such as melting of sensor coating materials, cracking of solder joints, and structural changes in stress within elastomers.
  2. In industrial production, a large number of electrical equipment, such as motors, transformers, and distribution cabinets, generate strong power-frequency electromagnetic fields. When these devices operate, an alternating magnetic field with a primary frequency of 50Hz or 60Hz forms in the surrounding area. The electronic components and signal transmission lines within the pressure transmitter are highly susceptible to power-frequency electromagnetic fields.
  3. With the continuous advancement of modern communication technology and industrial automation, various communication devices, high-frequency heating equipment, and radio frequency identification (RFID) systems generate high-frequency electromagnetic radiation. This high-frequency electromagnetic radiation can cause additional noise in the electronic components within the pressure transmitter, affecting the proper functioning of the signal processing circuitry and even damaging the electronic components.
  4. Static electricity is easily generated during industrial production. Static interference can cause dust or impurities to adsorb onto the measuring diaphragm, affecting its normal pressure sensing performance. Furthermore, electrostatic discharge can generate transient high voltages and large currents, damaging the electronic components within the pressure transmitter.

How do you convert 4-20 mA to pressure?

We know that the 4-20mA signal of a pressure transmitter and the pressure measurement range are linearly proportional. They can be converted to each other.

For example, if the pressure transmitter has a range of 0-40MPa and the output current is 12mA, what is the pressure value?

First, map the transmitter’s output current (4-20mA) to the corresponding pressure range (0-40MPa). The calculation formula is as follows:
Pressure = (Output Current – 4) * (40 – 0) / (20 – 4)

Based on the above formula, the result is:
Pressure = (12 – 4) * (40 – 0) / (20 – 4) = 8 * 40 / 16 = 20MPa

Therefore, when the output current is 12mA, the corresponding pressure is 20MPa!

Of course, there are many existing tools that can perform this conversion directly for you. For example, there is an online 4-20mA scaling calculator from DIVIZE b.v.

How do you convert 4-20 mA to pressure

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In the industrial automation field, the 4-20mA current signal, the standard output for pressure transmitters, has become a core sensing method for process control due to its strong anti-interference capabilities and long transmission distance.

Sino-Inst’s 4-20mA pressure transmitters can be directly connected to computers, control instruments, and display instruments. They are easy to install and offer high vibration and shock resistance, ensuring long-term operation in harsh environments. Customizable measurement parameters are also supported.

If you require pressure measurement or have any technical questions, please feel free to contact our sales engineers!

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