Compound Pressure Transducers

• Compound pressure transducers are useful for both positive and negative pressure measurement for applications such as operating climate control equipment.
• Compound pressure transmitters are also commonly used in Vapor Recovery Units (VRU), water pumps, and in compression equipment.
• It is important to understand how to calculate the output signal at a given pressure in the calibrated range.

In compound pressure transducers, the sensor features a zero-reference to atmospheric pressure. In other words, the calibration of the transducer provides the zero-pressure output value at full vacuum range of -14.7 PSI (or -1 bar). Typically, this is ideal for low and medium pressure ranges, gauge references for transducers (vent or seal).

It is possible to use a compound pressure range on higher pressures. However, the tolerances of the transducer, such as full scale error, may exceed the vacuum range of -14.7 PSI. To maintain accuracy in the vacuum range, we do not recommend using compound pressure ranges higher than 500 PSI. However, an exception for this rule would apply when the vacuum portion of the output is in use only as an indicator of vacuum condition rather than as an accurate measurement.

Compound Pressure Transducers

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 servicing, the transducer will measure the negative pressure from the suction process during evacuation of the system.

Other applications where compound pressure transducers can be beneficial include:

• compression equipment
• water pumps (suction and discharge)
• vapor recovery

Our most commonly used compound pressure transducer is the CS10, which is a standard industrial style pressure transmitter. We also offer a wide selection of pressure sensors with hazardous area certifications for applications that require a higher safety rating.

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 PSI gauge pressure range with 4-20 mA output signal

Calculate the mA (milliamps) per PSI value.

• first, calculate the full span of the pressure range: 14.7 + 20 = 34.7 PSI
• next, calculate the full span of the output signal: 20 – 4 = 16 mA
• last, calculate the mA/PSI value: 16 mA / 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 PSI output is 14.7 PSI above 4 mA:

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:

6.778 mA + 4 mA = 10.778 mA

The results of the above calculations are as follows:

4 mA = -14.7 PSI

10.778 mA = 0 PSI

20 mA = 20 PSI

Example 2: -14.7 to 50 PSI gauge pressure range with 1-5 V output signal

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

• first, calculate the full span of the pressure range: 14.7 + 50 = 64.7 PSI
• then, calculate the full span of the output signal: 5 V – 1 V = 4 V
• last, calculate the V/PSI value: 4 V / 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:

0.908 V + 1 V = 1.908 V

The results of the above calculations are as follows:

• 1 V = -14.7 PSI
• 1.908 V = 0 PSI
• 5 V = 50 PSI