Zero checking a differential pressure transmitter is important as it ensures that the transmitter is not deviating from correct zero differential pressure reading. It should be done regularly or when one suspects an error in the readout of the said transmitter.

Before doing anything with a differential pressure transmitter as with other process instruments, be sure that it is isolated from the system process especially when the industrial plant is in operation. This can be done resetting the set points of the transmitter, bypassing signal transmissions and putting it in bypass mode, or putting the transmitter in maintenance mode. It really depends on the system design of the company. Ensure to inform the control room personnel before and after the job. Usually jobs like this are done during plant shutdown and is in for maintenance. Never attempt to adjust, or manipulate any part of the transmitter either physically or with its parameters without proper communications with process and production personnel. It could lead to a plant emergency shutdown which we don't want to happen.

Now, to zero check a differential pressure transmitter, a technician can close either one of the low pressure side or high pressure side isolation valves. usually the high pressure valve is the one that is closed to prevent transmitter damage and must be closed slowly. Open the equalizing valve also slowly. Both can be closed or open at the same time but do it in a very slow manner. Also, ensure that the you know the working pressure of the process to have an insight of how much pressure there would be in case of leakage or any other undesirable events. The three valves can be found in the valve manifold or in line with the impulse lines if installed otherwise. Refer to the picture to see the valves below the transmitter in manifold. The valve in the center (without the label H for high pressure side and L for low pressure side tapping) is the equalizing valve.

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