The difference between re-ranging / dry calibration / wet calibration - for differential pressure transmitters
 

Let's start with the basics:

What is calibration?

Calibration is a procedure to maintain accuracy of an instrument. The measured value of an instrument is compared to a known value or preferably to standards. The deviation of the measured quantity from the standard value is minimized through the calibration process. Accurate calibration will teach an instrument to produce accurate results with unknown samples. I fact, the purpose of all instruments is to measure unknown samples accurately during normal usage.

There are two kinds of measurements, one is direct, and the other is indirect measurements. Some physical and chemical quantities cannot be measured directly, and these are to be measured indirectly. Like flow can never be measured directly, it is always measured indirectly. Differential pressure transmitters measures physical quantities indirectly and then infer the result to find out the actual physical quantity.

Differential pressure transmitters are used to measure Flow, Density, Viscosity and Level etc. of liquid. How one single instrument will be used to measure so many different physical quantities? In all these cases, the DP Transmitter will measure the differential pressure in some form or other and then the result is interpolated to get the desired physical quantity.

This means, the DP Transmitter is to be calibrated differently to measure the particular physical quantity.

There are different methods of calibration like re-ranging, dry calibration and wet calibration. The choice of calibration methods will entirely depend on the application of the DP Transmitter. Let’s try to understand the difference between these three calibration methods. To do this, we will first find out how to perform DP Transmitter calibration by these calibration methods.

1. Re-ranging a Differential Pressure Transmitter:

Differential Pressure Transmitters are 2-wire transmitters. These are loop powered devices with 4-20mA current loop protocol as the standard. The lower range is 4mA, and the upper range is 20mA. The difference between lower range and upper range is called span of the transmitter. The span requirement of the application pressure range is to be matched to the transmitter span. The transmitter reading is most accurate when it uses most of its span during measurement. Let’s explain this phenomenon with an example.

Example:

Say, the 4 and 20mA points of a DP Transmitter are set to 0 and 100H2O as original calibration (Please see Fig-1). In this case, the span will be 100 ­ 10 =90H2O. Now there is some change in the process and the process pressure requires the minimum to be 50 H2O. In this case, the minimum- 0mA is to be Re-range to 50 H2O. Once this re-ranging is done, the Span will also change to 50 H2O.

2. Dry Calibration of a Differential Pressure Transmitter:

Dry Calibration or Dry Leg Calibration method of a Differential Pressure Transmitter is performed in a closed, pressurized tank. This is a common method and one of the easiest to do. The method is almost similar to open tank level calibration. In a closed tank, the bottom most point is the HP or High Pressure point, and the top most point is the LP or the Low Pressure Point.

The transmitter is mounted near the HP or bottom most position, and the HP inlet of the transmitter is connected to the bottom most position of the vessel through an Isolation valve. The LP inlet of the transmitter is connected through a pipe to the Top most point. This is shown in Fig-2.

The calibration process is easy. Make both the inlet pressures at HP & LP equal to the atmospheric pressure by opening both the valves V1 & V2. The 0mA (Minimum span) calibration is done. Now, close V1 & V2, open the isolation valve. The HP inlet is subjected to maximum span and calibrates the transmitter to 20mA. Now, open the dry reference leg valve, the LP side will be subjected to the minimum span. Re-range the transmitter. The dry method of calibration is complete.

3. Wet Calibration of Differential Pressure Transmitter:

Dry calibration holds true only when the liquid in the tank is at ambient temperature. When the liquid is hot or much colder than the ambient temperature, the liquid vapor or the condensate will fill the dry- leg. Under this condition, the dry calibration does not hold true. In such a situation, wet calibration method is to be adopted.

The LP leg of the DP transmitter set-up is to be filled with some buffer solution. The buffer solution normally used in industries is diesel, glycol, glycerin or the same liquid of the tank. Now, you have to follow the dry leg calibration process, and re-ranging to complete the wet leg calibration process.

We may conclude that re-ranging calibration method is required for all types of Differential Pressure Transmitter applications. Dry leg Calibration is used only for level measurement in a closed tank with liquids at ambient temperature. Wet leg calibration method is required for level measurement in a closed tank with hot and intensely cold liquids.

See also
High Performance Differential Pressure Transmitter (0.04%)  A full range of high accuracy, ultra stable pressure transmitters especially suitable for tough application typically found in the oil, gas or power generation industries.

Fuji DP Transmitter

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