Two moisture readings can appear to disagree when one sensor measures compressed air at line pressure and another measures a sample expanded toward atmosphere. Before diagnosing the dryer, establish whether both readings describe the same pressure condition.
Pressure is part of the result
Vaisala explains that changing gas pressure changes dew point. A sample expanded before measurement will not show the same dew-point temperature as the pressurized process, even when the sample is otherwise representative.
Pressure dew point refers to the condition at the stated pressure. A result at atmospheric pressure needs an appropriate conversion if the acceptance requirement is expressed at process pressure. Comparing the temperatures alone can create a false impression of improvement or deterioration.
Draw the sampling arrangement
Vaisala’s sampling guidance emphasizes representative sampling and identifies pressure at the sensor as a potential source of error. The measurement location belongs in the setup record.
Mark the process connection, any pressure reduction, the sample cell and the outlet. Record the pressure where the sensor actually operates. If the sensor and process are at different pressures, retain the basis and method of any conversion.
Keep a complete measurement record
| Item | Why it matters |
|---|---|
| Process location | Connects the result with an air user or treatment stage |
| Sensor pressure | Establishes the dew-point pressure basis |
| Sampling arrangement | Makes the test reproducible |
| Instrument and settling record | Helps distinguish measurement behavior from a process change |
| Required air-quality condition | Provides the acceptance target |
Ask the instrument supplier to review the sampling components, flow and stabilization requirements for the intended range. Avoid declaring a failed dryer from a reading taken before the measurement has settled or without a known pressure basis.
When investigating a change, reproduce the earlier arrangement where practicable or convert both results consistently. Then compare process conditions and dryer behavior. A traceable result at a defined condition makes the next action much easier to justify.
