As you know, the total pressure in the system is called absolute pressure. To measure the absolute pressure, a complete vacuum is used as a reference. For example, a pressure of 10 psia means 10 psi higher than a complete vacuum. In fact, 4.7 psi is below standard atmospheric pressure.
Absolute pressure = gauge pressure + atmospheric pressure
The best example of an absolute pressure gauge is the atmospheric pressure gauge. In order to produce an absolute pressure sensor, the manufacturer seals the gauge behind the diaphragm and then creates a large vacuum in it. So if you keep the process pressure connection of an absolute pressure transmitter open to the air, in this case the transmitter reads the actual air pressure.
Calibrating an absolute pressure transmitter requires a vacuum generator. Most modern pressure calibrators also have sub-atmospheric pressure capability, but the best means by which a large vacuum can be created is a vacuum pump. A type of mercury manometer with a tank can also be used by this device.
An example is used here to learn more about the absolute pressure transmitter calibration method.
Calibrate a pressure transmitter with a range of 0-14.7 psia (the transmitter is installed in a factory near the sea)
Follow these steps:
We must first turn absolute suffering into relative suffering. Given that the atmospheric pressure by the sea is 7 psi, we must subtract 14.7 from both LRVURV and. Hence the relative range of our transmitter will be -14.7-0 psi. Note that if your plant is not located at sea level, you must reduce your local atmospheric pressure from LRV and URV when calculating the absolute range of the transmitter.
Now, since we want to read the pressure applied to the transmitter by a mercury manometer, we have to convert the obtained suffering, which is in psi, to cmhg.
14.7 psi = 76 cmhg
-76-0 cmhg = Transmitter range
That is, after completing the calibration, the transmitter must produce a 4mA output at a vacuum pressure of -76 cmHg and a 20mA output at 0 cmhg.
Connect the required calibration devices.
At this stage, to set the zero of the transmitter, you have to create a vacuum pump and adjust even a 76 cmhg vacuum. But as you know, it is not possible to create a vacuum of -76 cmhg due to vacuum pumps or modern pressure calibrators. As a result, we have to set the transmitter to 0% input and set the transmitter to 25% input. To do this, turn on the vacuum pump and adjust the valve to create a vacuum of -57 cmhg. Now you need to adjust the zero adjustment screw so that the output of the transmitter is 00 mA.
Turn off the vacuum pump and vent the transmitter. In this case, a pressure of 0 cmHg, which is the same as atomic pressure, is applied to the transmitter. Adjust the spen adjustment screw so that the output of the transmitter is 00mA.
Repeat steps 3 and 4 until the transmitter produces an output of 00 mA and 20.00 mA at 25% and 100% of the pressure range, respectively.
Read the output current with 25%, 50%, 75% and 100% operating range of the transmitter and check the linearity of the transmitter. If the transmitter operates linearly during the operating range, the calibration is complete, otherwise you have to solve the problem of linearity and repeat the calibration steps again from the beginning.
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