Recently BJ PPS Leeds dried a client system with nitrogen. Dewpoint was measured using a Michell ADM dewpoint meter which was connected to the system at different locations via a one metre long rubber hose. The instrument was allowed to stabilise for approximately five minutes before each reading was taken. Average result was in the region of -27.5 °C. A check of our nitrogen supply and the client's nitrogen supply showed a dewpoint of -30 °C. Clearly the nitrogen supply readings indicated that it was highly likely that the measurement of dryness was not correct as nitrogen is usually much drier than -30 °C and in fact we had dried the client system to a lower level than we could demonstrate !
The above project details were recorded just before BJ PPS Leeds were due to carry out dewpoint measurement trials at Moisture Control and Measurement and highlighted the need to raise our awareness of what actually happens when we measure dryness and how accurate our readings actually are!
For the purpose of testing MCM calibration house facilities utilising dewpoint generators that are calibrated to National Physical Laboratory and International Standards were employed. Reference standards used were a 1.8 ppm by volume (- 70 °C) air supply and generated set dewpoints of - 60 °C, -40 °C and - 20 °C.
Please note that all measurements were made under laboratory conditions. We actually take measurements in the field where conditions are far from ideal and uncertainties are far greater.
Areas of investigation were limited to the following :-
- The effect on dewpoint measurement created by sample line composition.
- The performance of six different dewpoint meters through a dry to wet to dry cycle.