Differential Pressure Transmitter Calibration Procedure


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In the Differential Pressure Transmitter, as flow increases, the differential pressure increases, and when flow decreases the differential pressure decreases.

Differential Pressure Transmitter Calibration

For example, an orifice plate is placed in a pipe to restrict the fluid flow. This restriction creates a pressure drop that can be converted to flow rate.

The differential pressure transmitter measures the pressure drop created, by measuring pressure at two different points, upstream and downstream.

Differential pressure, then is the difference between the higher pressure or upstream reading and the lower pressure or downstream reading.

Differential Pressure = High Pressure - Low Pressure.

Input and Output Measurement Standards

Differential pressure is usually measured in inches of water.

Use a low pressure calibrator to both furnish and measure the input pressure.

A milliammeter is an appropriate output standard to measure the transmitter’s output.

Differential Pressure Transmitter Connections

Connect the transmitter to the pressure calibrator as shown in the manufacturer’s instructions for the calibrator. The air supply requirements for the calibrator are also found in the manufacturer’s instructions.

Connect the output from the pressure calibrator to the high pressure port on the transmitter to provide signal pressure. Vent the transmitter’s low pressure port to atmosphere to provide a reference point for the differential pressure measurement.

To measure the transmitter output, connect a milliammeter to the transmitter. Then connect a 24-volt power supply in series with the transmitter and milliammeter.

Differential Pressure Transmitter Five-Point Check

Typically, inputs at 10%, 30%, 50%, 70% and 90% of span are used as test points.

Check for Hysteresis. Hysteresis is the tendency of an instrument to give a different output for a given input, depending on whether the input resulted from an increase or decrease from the previous value.

Often the data from an instrument test is recorded on a calibration data sheet to help identify instrument errors.

Adjusting for Error Correction

Adjust the zero first, since span error is corrected only after an accurate zero is established. Zero is properly set when a 10% input produces a 10% output.

Adjust the span at 90%. Since zero and span frequently interact, after one of these errors has been corrected, the other may require readjustment.

Square Root Extractor (Required in OLD Models, For SMART Transmitters NOT REQUIRED)

Flow rate which may be represented by Q, is the square root of the calculated pressure drop across a restriction. Q = square root of the Differential Pressure.

Differential pressure transmitters may include an integral square root extractor, which provides a linear output signal.

However, if a square root extractor is not part of the transmitter circuitry in the process, a separate square root extractor may be installed in the output signal loop.

Input and Output Measurement Standards

In a loop, a 4-20 mA output from a differential pressure transmitter provides an input to the square root extractor.

So, in the calibration, a milliamp source would provide an appropriate input standard. The output measurement standard is also a milliammeter.

To complete the setup, connect a power supply in series with a square root extractor and milliammeter. Manufacturer’s instructions specify the input values and expected outputs.

The square root of the input determines the output.