Using a Differential Pressure Transmitter as a Gauge Pressure Transmitter
Gauge pressure transmitter compares a process pressure to atmospheric pressure measured on the low-pressure side of the transmitter. Unfortunately, gauge pressure transmitters are designed to measure relatively high pressures. So, how do you measure spans that are less than the minimum required for gauge pressure transmitters? Just use a DP transmitter and leave the low-pressure side open to the atmosphere. Leaving the port open effectively turns the DP transmitter into a GP transmitter. Since DP transmitters minimum span goes lower than gauge pressure transmitters, you can measure much lower gauge pressures.
Manifolds
Differential Pressure (DP) transmitters usually use either 3-Valve or 5-Valve manifold, controlling the process to both the low-pressure or high-pressure ports of the transmitter.
3-Valve and 5-Valve manifolds have an equalizing valve that links the high-pressure side and the low-pressure side. If a DP transmitter with a 3 or 5-Valve manifold is utilized as a GP transmitter and the equalization valve is open, a leak path is opened, releasing process medium into the atmosphere.
Therefore, when using the DP transmitter in this way, it is advisable to use a 2-Valve manifold. Since the 2-Valve manifold does not have an equalizing valve, the potential leak path is eliminated. When using a DP in this manner, make sure that the low-pressure side port is open to the atmosphere and not covered by the 2-Valve manifold.
