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I am trying to figure out how to go about calibrating this transmitter we have. What we have is a Rosemount 1151 Absolute pressure transmitter that is measuring the condenser pressure in relation to barometric pressure. The Low side of this transmitter is sealed with no process connections. Our process connection is on the high side tied to a pressure tap on our condenser. The range of this transmitter is 0-32inHg = 4-20mA respectively. Currently the barometric pressure according to the local airport is 30.05inHg which is close to what we are reading in the control room. The problem I am having is that I have no means of drawing a vacuum of greater than -30.05 on the high side of this transmitter. It would be an easy fix if there were a process connection on the low side. Any suggestions? I have never come across a transmitter used in this application before and nobody around here seems to know much about it. They say that it has caused problems in the past with how to calibrate it.