Troubleshooting Pressure Transducers
Techs must understand how to service refrigeration systems with electronic controls
PRESSURE TRANSDUCERS: Refrigeration systems often incorporate electronic controls, so it is very important for technicians to be able to troubleshoot electronic devices such as pressure transducers. (Courtesy of Joe Marchese)
As more and more refrigeration systems incorporate electronic controls into their design, it is becoming increasingly important for technicians to be able to troubleshoot these electronic devices. One such device is the pressure transducer.
Pressure transducers allow a controller to interrupt a system’s refrigerant or oil pressure by converting a measured pressure into an analog electrical output to the system’s controller. The actual electrical output will vary based on the type of transducer used and the measured pressure. Common outputs are 0.5 vdc to 4.5 vdc, 0 vdc to 10 vdc, and 4 mA to 20 mA. However, technicians may come across a transducer with a different range of electrical outputs. It is always best to verify its specifications provided by the transducer manufacturer. This information will either be listed on the transducer itself or in the literature.
Many controllers will have a display showing the pressure measured by the transducer. If the display shows a transducer error or an incorrect pressure value, technicians will need to determine if the issue is a faulty transducer or failed controller. If the display shows a transducer error, the best first step is to verify the transducer is receiving the required supply voltage to operate. Determine the required supply voltage by the transducer manufacturer and measure the actual voltage applied to it. If there is no voltage, or if the incorrect voltage is applied to the transducer, that is likely the issue.
However, if the applied voltage is correct, the next step is to measure the electrical output of the transducer and verify it is correct for the applied pressure. Using a reliable and accurate pressure gauge, measure the actual pressure applied to the transducer. If the actual pressure does not closely match the displayed pressure, there is either an issue with the transducer not sending the correct electrical output to the controller, or the controller is defective.
If the difference between the displayed pressure and the actual pressure is relatively small, some controllers will have a programmed feature allowing an offset to be entered to allow the actual pressure to match the displayed pressure. When the pressure difference is too great, or the display shows a transducer error, technicians can determine if the pressure transducer is sending the correct electrical signal to the controller by measuring its electrical output.
Based on the applied pressure, technicians will need to determine the correct electrical output for the transducer. Some manufacturers will have a chart showing the correct electrical output for a given pressure. Technicians may also be able to use a formula supplied by the transducer manufacturer to determine the correct output. For example, for 4 mA to 20 mA transducers, use this formula:
I = 4 + 16( (P-P(min)) / (P(max)-P(min)) )mA
I = Calculated output amperage
P = Measured pressure
P(max) = Maximum pressure value
P(min) = Minimum pressure value
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For 0-vdc to 10-vdc transducers, use this formula:
Vo = 10( (P-P(min)) / (P(max)-P(min)) ) vdc
V0 = Calculated output voltage
P = Measured pressure
P(max) = Maximum pressure value
P(min) = Minimum pressure value
For example, if you have a 0-vdc to 10-vdc pressure transducer with a 0 psig to 100 psig range and a measured pressure of 50 psig, the expected output voltage would be:
Vo = 10( (50-0) / (100-0) )vdc
Vo = 5vdc
So, if the transducer is sending a 5-vdc signal to the controller and the displayed pressure is not 50 psig, the issue is not with the transducer but with the controller. If the transducer has the correct supply voltage but its output is not 5 vdc, then the transducer is faulty.
Troubleshooting transducers is generally not a difficult process as long as you have the correct specifications from its manufacturer.
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