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Products Repairs and Service Difference between Range & C alibration

The difference between re-ranging / dry calibration / wet calibration- for differential pressure transmitters Let's start with the basics:

What is calibration?

Ca libration is a procedure to maintain accuracy of an instrument. The measured value of an instrument is comparedto a known value or pre ferably to standards. The deviation of the measured quantity from the standard value isminimized through the calibration process. Accurate calibration will teach an instrument to produce accurate resultswith unknown samples. I fa ct, 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 andchemical quantities cannot be measured directly, and these are to be measured indirectly. Like flow can never bemeasured dire ctly, it is always measured indirectly. Differential pressure transmitters measures physical quantitiesindirectly 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 onesingle instrument will be used to measure so many different physical quantities? In all these cases, the D PTransmitter will measure the differential pressure in some form or other and then the result is interpolated to getthe desired physical quantity.

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

There ar e different methods of calibration like re-ranging, dry calibration and wet calibration. The choice of calibration methods will entirely de pend on the application of the DP Transmitter. Lets try to understand thedifference between these three calibration methods. To do this, we will fir st find out how to perform DP Tr ansmittercalibration by these calibration methods.

Did you know? Coulton offers calibration services eve n to very high static pressure.

1. Re-ranging a Differential Pressure Transmitter:

Differential Pressure Transmitters are 2-wire transmitters. These are loop powered de vices with 4-20mAcurrent loop protocol as the standard. The lower range is 4mA, and the upper range is 20mA. Thedifference between lower range and upper range is called span of the transmitter. The span requirementof the application pressure range is to be matched to the transmitter span. The transmitter re ading ismost accurate when it uses most of its span during measureme nt. Lets explain this phenomenon with an

example.

Example:

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Fuji DP Transmitter

Calibration Services

Say, the 4 and 20mA points of a DP Transmitter are set to 0 and 100H2O as original calibration (Pleasesee Fig-1). In this case, the span will be 100 10 =90H2O. Now there is some change in the process andthe 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 C alibration or Dry Leg Ca libration method of a Differential Pressure Transmitter is performed in aclosed, pressurized tank. This is a common method and one of the easiest to do. The method is almostsimilar to open tank level calibration. In a closed tank, the bottom most point is the HP or High Pressurepoint, 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 isconnected to the bottom most position of the vessel through an Isolation valve. The LP inlet of thetransmitter is connected through a pipe to the Top most point. This is shown in Fig-2.

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The calibration process is ea sy. Make bo th the inlet pressures a t HP & LP equal to the atmosphericpressure by ope ning both the valves V1 & V2. The 0mA (Minimum span) calibration is done. Now, closeV1 & V2, open the isolation valve. The HP inlet is subjected to maximum span and calibrates thetransmitter to 20mA. Now, open the dry reference leg valve, the LP side will be subjected to theminimum span. Re-range the transmitter. The dry me thod of calibration is complete.

3. Wet Calibration of Differential Pressure Transmitter:

Dry ca libration holds true only when the liquid in the tank is at ambient temperature. When the liquid ishot 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 calibrationmethod is to be adopted.

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

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We may conclude that re-ranging calibration method is required for all types of Differential Pressure Transmitterapplications. Dry leg C alibration is used only for level measurement in a closed tank with liquids at ambienttemperature. Wet leg calibration method is required for level mea surement 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 espe cially suitable for tough

application typically found in the oil, gas or power genera tion industries.

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