Why is the zero point on a pneumatic transmitter typically set to 3 psi instead of 0 psi?

Setting the zero point on a pneumatic transmitter to 3 psi instead of 0 psi is commonly done to allow for a pressure range that accommodates potential variations and issues within the system. In this case, starting at 3 psi can help indicate the presence of a leak or other system faults. If the zero point were set to 0 psi, any small leaks or inaccuracies could potentially be interpreted as normal operating conditions, thus masking issues that require attention.

By establishing a zero point above 0 psi, any deviation from 3 psi can help signal that something is amiss within the system, making it a crucial practice for maintenance and monitoring. This proactive measure not only enhances safety but also ensures the integrity of the system's readings, which is vital for accurate process control.