Working Principle
Figure 1
It is a typical usage of two-wire 4-20mA pressure transmitters for most customers showed in figure 1. After the pressure transmitter is powered on, the loop current is proportional to the pressure to generate a 4-20 mA signal by collecting the pressure. The current flow through the sampling resistor (typical 100 Ω, 250 Ω) which connects serially with the loop, and then it is converted to a voltage signal used for the back-end acquisition system. Generally, the sampling resistors will be integrated into the acquisition device.
Figure 2
It is a simulation diagram of the actual application in figure 2. The blue arrow indicates the current flow of 4-20 mA. Using a multimeter as the acquisition device, the sampling resistance is 250 ohms, the zero current is 4 mA, and the collected voltage is about 0.997 V (0.004 mA × 250 ohm = 1 V). )
Faults and Solutions
Common fault No. 1: The pressure signal is not collected and the value is obviously abnormal.
There are many reasons for this problem, such as the acquisition equipment problems, wiring errors or product quality issues. To solve the problem, please ensure that the transmitter works normally after power-on at first;
Solutions:
a. Check the wiring:
Check the wiring according to the label on the transmitter housing or manual. For our products, generally black wire is power-supplied and the red wire is output positive.
If you find that the wire is reversed, please don't worry about the damage issue. The conventional products have anti-reverse design and please re-wiring to fix the problem.
b. Measuring voltage:
After checking the wiring, adjust the multimeter to the voltage value to measure the voltage between the positive and negative output of the transmitter. The voltage value should be within the transmitter supply voltage range which can be found in the product label or instruction manual.
If the measured voltage value is incorrect, check if the equipment is normal.
c: Measuring loop current:
Disconnect the transmitter "output positive", adjust the multimeter to the current position series into the current loop to test the loop current value. Generally, the non-pressurized state is about 4mA (the gauge pressure transmitter range lower limit is 0).
When performing the above test, pay attention to the setting of the multimeter's current and voltage position. Refer to the settings of the above two multimeters.
After the three steps above, since the multimeter works normal for testing, if the loop current value is still not about 4 mA (0.5%FS accuracy, 3.92~4.08 mA), it can be judged as a product failure at first, please contact the after-sales department for further analysis.
Common fault No. 2: Transmitter test is inaccurate, out of tolerance, no change in pressurization, etc.
Solutions:
(1) It is recommended to statically place the product in the air to check whether the power supply voltage is normal. Put the product in the vertical direction, test circuit current whether the 4mA is accurate. For the products with 0.5%FS accuracy, the zero current should be between 3.92 and 4.08 mA.
(2) If the zero current of the product is normal, it is recommended to check whether the equipment is normal, whether the shut-off valve is open or not, whether the pressure pipe is blocked or not, etc.
(3) Check the pressure sensing diaphragm of the transmitter, if the diaphragm is damaged, the transmitter should only be returned to the factory for repair;
(4) If the above solutions cannot solve the problems, please contact the after-sales department for further analysis
Besides, we summarised 4 typical troubleshooting about 4~20mADC pressure transmitters to help customers solve the problems quickly on-site as below.
3. No Display, No Output
Check and Test | Solutions |
Check if the power supply is reversed. | Put it in the correct way. |
Check the power supply of the pressure sensor if it is 24VDC. | Make sure the pressure transmitter is power supplied ≥12V(intelligent type)≥15V(general type). If there is no power supply, check if the circuit is open and detect the secondary instrument/PLC sampling resistance (input impedance should be ≤250Ω). |
If it is a displayable pressure transmitter, check if the indicator is damaged (you can short the two wires of the indicator first. If it is normal after a short circuit, it means the indicator is damaged). | Replace the indicator. |
Connect the ammeter to the 24V power circuit and check if the current is normal. | If it is normal, the pressure transmitter is normal. In this case, check if other instruments in the circuit are normal. |
Check if the wiring is loose or not. | Make it tightened and well connected. |
Check if the power supply is correctly connected to the power input terminal. | Connect to the power supply terminal correctly according to the power supply requirements. |
4. The Pressure transmitter output is ≥20mA.
Check and Test | Solutions |
Check if the pressure transmitter power supply is normal. | If it is less than 12VDC, check whether there is a large load in the circuit. The load capacity of the pressure transmitter should be in accordance with RL ≤(transmitter supply voltage -12V)/( 0.02A) Ω. |
Check whether the actual pressure exceeds full range limits. | Change a pressure transmitter with an appropriate range. |
Check whether the pressure sensor is damaged. Severe overload sometimes damages isolated diaphragm. | Return the pressure transmitter to the factory for maintenance. |
Check whether the power supply wiring is correct. | Put it in an incorrect way. |
5. The pressure transmitter output is ≤4mA.
Check and Test | Solutions |
Check if the pressure transmitter power supply is normal. | If it is less than 12VDC, check whether there is a large load in the circuit. The load capacity of the pressure transmitter should be in accordance with RL≤ (transmitter supply voltage -12V) / ( 0.02A) Ω. |
Check whether the wiring is loose or not which causes the contact resistance to increase. It is equivalent to the load increase. | Connect the wire and tighten it, the load is in accordance with RL≤ (transmitter supply voltage-12V) /(0.02A) Ω. |
Check whether the actual pressure exceeds the selected range of the pressure transmitter. | Change a pressure transmitter with an appropriate range. |
Check whether the pressure sensor is damaged. Severe overload sometimes damages the isolated diaphragm | Return the pressure transmitter to the factory for maintenance |
6. Wrong display/output signal with the error.
Check and Test | Solutions |
Check if the pressure transmitter power supply is normal. | If it is less than 12VDC, check whether there is a large load in the circuit. The load capacity of the pressure transmitter should be in accordance with RL ≤ (transmitter supply voltage -12V)/( 0.02A) Ω. |
Check if the reference pressure value is correct. | Replace the same type of pressure transmitter, if the error is consistent, check the reference pressure meter. If the meter accuracy is low, you need to change the instrument with higher precision. |
Check if the range of the secondary meter consistent with the range of the pressure transmitter. | Adjust the range of the secondary instrument to make sure it is consistent with the range of the pressure transmitter. |
The capacity of the pressure transmitter load should be in accordance with RL≤ (transmitter supply voltage -12V)/(0.02A) Ω. | If it is not, take measures according to its different conditions: such as raising the power supply voltage (but must be lower than 36VDC), reducing the load, etc. |
Check if the corresponding device housing is reliably grounded. | If not, make it reliably grounded. |
Check whether the strong and weak electricity is separately arranged onsite. | Make sure the wiring is scientific and reasonable, avoiding interference for pressure transmitter signal instability/error. |
The pressure sensor diaphragm is easily deformed. Overload and hard objects may damage isolated diaphragm. | Return the pressure transmitter to the factory for maintenance. |
Check the pipeline When the pipeline has impurities, the measurement accuracy will be affected. | Clean the impurities and add a filter screen before the pressure port. |
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