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 voltage signal used for the back-end acquisition system. Generally, the sampling resistors will be integrated into the acquisition device.
It is a simulation diagram of 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
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;
Check the wiring according to the label on the transmitter housing or manual. For our products, generally black wire is power supply and the red wire is output positive.
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.
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).
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.
(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.
(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;