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In this video, we’re going to introduce you to flow measurement using a Differential Pressure Transmitter.
Before we get started, you might want to review two of our other videos Differential Pressure Transmitter Explained (realpars.com/differential-pre..., and What is Sensor Calibration and Why is it Important? (realpars.com/sensor-calibrati....
Flow transmitter calibration is the focus of our discussion, but let’s review some other important terms first. We’re going to define the terms Range and Span and Zero/Span adjustments.
After that we are going to:
- Explain how a Differential Pressure Transmitter is used to measure the volumetric flow rate
- Explain why a Square Root Extractor is needed in Differential Pressure Transmitter flow measurements.
Instrument calibration is a process where the instrument is adjusted in order to achieve the desired output range for a specific input range.
The input Span of an instrument is simply its Lower Range Value often referred to as LRV, subtracted from its Upper Range Value, often referred to as URV.
Zero and Span are the names of the adjustments made in order to calibrate an instrument. There are many ways to adjust zero and span.
For example, they could be physical adjustments such as potentiometers, or software-based push-button adjustments.
A common method of flow measurement is done by using a Differential Pressure Transmitter.
The Differential Pressure Transmitter often referred to as a Delta P transmitter, is placed across an obstruction such as an orifice plate.
The orifice plate will cause a varying differential pressure drop across it as the flow through the pipe changes.
Unfortunately, the Differential Pressure across the orifice is not proportional to the flow rate but is actually proportional to the square of the flow rate. That’s why in applications like this, we need a Square Root Extractor.
Sometimes this square root function is built into the transmitter and sometimes a Square Root Extractor is a separate signal conditioning instrument connected to the output of the transmitter.
In fact, the volumetric flow rate is directly proportional to the square root of the differential pressure.
There is a way for the flow rate to be directly proportional to the differential pressure, and that’s by removing the square root from the equation.
A Square Root Extractor will perform this function.
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3 май 2020
