A vacuum indication is useful only when the instrument responds fast enough and measures at a representative point. A gauge near the receiver may show strong vacuum while the mold pressure remains high because of a restriction or delayed valve. A blocked sensing port can hold an apparently stable reading after the actual pressure has changed.
Validate the instrument
The gauge or transducer should be compared with a calibrated reference over the operating range. Zero, span, response time, sample rate, filtering, units, and absolute-versus-gauge pressure configuration should be confirmed. Tubing to the sensor must be short, clear, and protected from condensate or debris.
For troubleshooting, pressure should be measured close to the tool as well as at the receiver. Control alarms and recipe limits should not be based on an unverified signal. Replacing pumps, valves, or vents in response to a false reading can move the process farther from control.