## Compound Pressure Transducers

August 9, 2018

In a compound pressure transducer, the sensor is zero-referenced to atmospheric pressure. In other words, the transducer has been calibrated to provide the zero-pressure output value at full vacuum range of -14.7 PSI (or -1 bar). Typically this is offered for low and medium pressure range, gauge referenced transducers (vented or sealed). It is possible to use a compound pressure range on higher pressures but the tolerances of the transducer, such as full scale error, may exceed the vacuum range of -14.7 PSI. It is not recommended to use compound pressure ranges higher than 500 PSI in order to maintain accuracy in the vacuum range, unless the vacuum portion of the output is used only as an indicator of vacuum condition rather than as an accurate measurement.

Compound pressure transducers are commonly found in building climate controls and refrigeration (HVAC/R) equipment. During normal operation, the transducer will measure the positive pressure in the system. When the system needs to be serviced, the transducer will measure the negative pressure caused by the suction during evacuating the system.

Other applications where compound pressure transducers can be beneficial include:

- Compression Equipment
- Water Pumps (Suction & Discharge)
- Vapor Recovery

#### Output Signal Calculation

When using a compound pressure transducer, it is important to understand how to calculate the output signal at a given pressure in the calibrated range. Here are two examples of how to properly calculate the output signal points on a compound pressure transducer:

*Example 1: -14.7 to 20 PSIg pressure range with 4-20mA output signal:*

The first step is to calculate the mA (milliamps) per PSI value.

- Calculate the full span of the pressure range: 14.7 + 20 = 34.7 PSI
- Calculate the full span of the output signal: 20 – 4 = 16mA
- Calculate the transfer function: 16mA / 34.7 PSI = 0.4611 mA/PSI

Next, we calculate the output signal at zero psi gauge. We do this by taking 14.7 PSI and multiplying by 0.4611 mA/PSI. We use 14.7 because the sensor’s zero psig output is 14.7 PSI above 4mA:

- 14.7 PSI X 0.4611 = 6.778 mA

Finally, we add in 4mA for the offset of the output signal. This is required since the output signal of the sensor starts at 4mA and not 0mA:

6.778mA + 4mA = 10.778mA

The results of the above calculations are as follows:

4mA = -14.7 PSI

10.778mA = 0 PSI

20mA = 20 PSI

*Example 2: -14.7 to 50 PSIg pressure range with 1-5V output signal:*

The first step is to calculate the volts per PSI value.

- Calculate the full span of the pressure range: 14.7 + 50 = 64.7 PSI
- Calculate the full span of the output signal: 5V – 1V = 4V
- Calculate the transfer function: 4V / 64.7 PSI = 0.0618 V/PSI

Next, we calculate the output signal at zero pressure. We do this by taking 14.7 PSI and multiplying by 0.0618 V/PSI. We use 14.7 because the sensors zero psig output is 14.7 PSI above 1 Volt:

- 14.7 PSI X 0.0618 V/PSI = 0.908 V

Finally, we add in 1 volt for the offset of the output signal. This is required since the output signal of the sensor starts at 1 volt and not 0 volts:

0.908V + 1V = 1.908V

The results of the above calculations are as follows:

1V = -14.7 PSIg

1.908V = 0 PSIg

5V = 50 PSIg

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