How a Dead Weight Tester Works ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

How a Dead Weight Tester Works

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A dead weight tester is a very handy piece of instrument for calibrating a pressure gauge or other pressure transducers in an industrial plant. But how does this device work? Lets find out:
A deadweight tester consists of a pumping piston with a screw that presses it into the reservoir containing a fluid like oil, a primary piston that carries the dead weight, W, and the pressure gauge or transducer to be calibrated as shown in the schematic above. It works by loading the primary piston (of cross sectional area A),
with the amount of weight (W) that corresponds to the desired calibration pressure (P = W/A). The pumping piston then pressurizes the whole system by pressing more fluid into the reservoir cylinder.                                            

When the screw is turned the increase in fluid pressure is applied to both the gauge and the weights.  When the weights start to lift the gauge pressure should be the same as the pressure indicated by the weights. You can calibrate pressure gauges and pressure transducers very accurately if the weights are correct and there is minimum friction between the weight piston and the cylinder.

A good quality deadweight tester has a motor which keeps the weights spinning all the time.  This reduces the friction.  If the tester has no spinning motor, you should spin the weights by hand. There are many kinds of deadweight testers.  The operating fluid can be oil, water or air, depending on the manufacturer.  You might not be able to use an oil type dead weight tester on the job site in certain applications especially sanitary industrial environment. The oil left in the gauge can contaminate process fluids, particularly some gases.

Today’s deadweight testers are more accurate and more complex than the schematic shown above but the essential operating principles are the same. Sophisticated features include temperature compensation and the means to rotate the primary piston in its cylinder to reduce or eliminate the effects of friction. Modern dead weight testers do not use weights these days.  The weights are replaced with a digital read out.  These modern units  ,now used for calibrating industrial pressure transducers, contain a precision power supply, an accurate digital readout, and a high-accuracy resonant (quartz) pressure sensor or Piezo-electric sensor as the unit for sensing pressure. These new generation dead weight testers are precise enough to be used to calibrate most industrial pressure transducers, but must be NIST (National Institute of Standards & Technology)-traceable to be used as an official calibration standard in the U.S