Pressure Calibration
An understanding of the proper tools used for the calibration of transmitters is required for an Instrumentation Technician. There is such a wide variety of calibration tools used in industry. Today I will highlight some of the more common calibration tools used on pressure transmitters.
Pressure Calibrator
A pressure calibrator is a pump you use to generate pressure on the sensor of a pressure transmitter. Hand pumps are common in industry.
Calibration Procedure
First, put a pressure gauge in series with the pump to measure the pressure going to the sensor. Now you will pump until you reach the zero range of your scale. You can now set the zero on the transmitter. Next, pump until you reach the upper range value. Finally, you can set the span.
There are more advanced pressure calibrators where you just input a pressure value into an electric pressure calibrator and the output will equal the corresponding pressure with high accuracy. These will be more accurate but more costly than a simple hand pump.
Deadweight Tester
A deadweight tester is a very accurate tool used for pressure calibration.
A deadweight tester set will include a piston, weights, hand pump, pressure port and a fluid(usually oil).
The weights used on a deadweight tester are very precise. Each weight will correspond to a certain pressure. We will load weights on the piston equal to our desired pressure. Keep in mind that the piston itself has a mass and corresponding pressure too.
Before I explain how a dead weight tester works I’d like to share a formula you may be familiar with, Pressure = Force/Area. Our force in this case will be our weights and the piston. They are acted on by gravity to produce a force. The area in our equation is equal to the cross-sectional area of the piston.
Calibration Procedure
First, we will build enough pressure to lift the piston. We do this using the hand pump. When the piston is lifted, it reaches a state of equilibrium where the pressure applied to lift it will be the same as the pressure acting on it from the weights.
Thanks to Pascal’s Law we know that pressure applied to a confined fluid is transmitted in all directions. This means that once we balance the pressure acting on the piston the fluid going to our transmitter will also be that same pressure.
After you have completed your zero and span adjustments it is important to perform a five-point calibration. Check out our article on how to perform one!
