UNCERTAINTY IN RELATIVE HUMIDITY – CALIBRATION OF PSYCHROMETERS

1. Tempmeko 2004 - Workshop on Uncertainty in Humidity Measurements UNCERTAINTY IN RELATIVE HUMIDITY – CALIBRATION OF PSYCHROMETERS Anders Kentved NATIONAL REF. LAB. FOR HUMIDITY DELTA, DK Cavtat – Dubrovnik, 21 June 2004

2. Uncertainty in Relative Humidity – Calibration of Psychrometers:Summary • Introduction • Calibration of psychrometers at DELTA • Psychrometer basics – a brief review • Psychrometer equations and sensitivity coefficients • Sources of uncertainty related to calibration of psychrometers • Worked example – Calibration of an Electropsychrometer • Sources of uncertainty related to use of psychrometers • Typical calibration results • Conclusion • Discussion

3. Calibration of psychrometers at DELTA:Background – Capability • Need for reliable instruments for measuring relative humidity during climatic testing of electronics • Typical test conditions:10 %RH – 97 %RH / 15°C – 85°CTolerance level < ± 2-3 %RH • Calibration method:Comparison in climatic chamber using dew-point hygrometer and thermometers • Capability:10 %RH – 97 %RH / 15°C – 85°CWorst case uncertainty < ± 1 %RH (k=2) for entire range(3 psychrometers calibrated in parallel)

4. Psychrometer basics – a brief review:Measuring principle • Two thermometers:- A “dry”, for measuring the air temperature t [°C]- A “wet”, sheathed in wet wicking, for measuring the “wet bulb temperature”tw [°C] • A fan draws air across the thermometers • Evaporation from the wick cools the wet thermometer • Relative Humidity is calculated from t and tw

5. Psychrometer basics – a brief review:Psychrometric chart

6. t = Air temp. [°C] e’s(tw,P) = Sat. vap. pres. at tw and P [Pa] tw = Wet bulb temp. [°C] e’s(t,P) = Sat. vap. pres. at t and P [Pa] P = Total pressure [Pa] A and B = Constants Psychrometer equations and sensitivity coefficients:Typical equations

7. Psychrometer equations and sensitivity coefficients:Sensitivity coefficients – Effect on RH of 1 °C change of t and tw

8. Psychrometer equations and sensitivity coefficients:Sensitivity coefficients – Effect on RH of 1 hPa change of P

9. Sources of uncertainty related to calibration of psychrometers:Most significant contributions – Typical values • Uncertainty of reference temperature measurement in calibration chamber – typical increase of uncertainty due to psychrometers:± 20 mK (stability in time) • Uncertainty of reference dew-point measurements in calibration chamber – typical increase of uncertainty due to psychrometers:± 20 mK (stability in time) • Resolution of psychrometer temperature measurement, typically:± 10 mK to ± 100 mK • Stability (short term) of psychrometer temperature measurement, typically: ± 10 mK • Readings of psychrometer, typically: ± 0.1 %RH • Reproducibility of psychrometer, typically: ± 0.2 to ± 0.5 %RH(if caused by contamination of wick, typically:+ 0.2 % to + 0.5 %RH)

10. Sources of uncertainty related to calibration of psychrometers:Often overlooked contributions – Typical values • Uncertainty of reference temperature measurement in calibration chamber – typical increase of uncertainty due to psychrometers:± 100 mK (stability in calibration space) • Changes in atmospheric pressure (P) during calibration can cause uncertainty of psychrometer reading up to ± 1 %RH – measure atmospheric pressure during calibration! • Psychrometers are very non-linear, uncertainty calculation is valid only for the specific calibration point (t, %RH) • Readings of mercury thermometers, typically: ± 50 mK • Improper fitting of psychrometer wick, typically:± 0.5 %RH to ± 1.0 %RH

11. Worked example – Calibration of an Electropsychrometer:Uncertainty budget for calibration at 30° / 70 %RH Calculated reference value = 70.3 %RH Average of psychrometer readings = 68.9 %RH Correction to be applied = 70.3 %RH – 68.9 %RH = 1.4 %RH Expanded uncertainty (k=2) of correction = 0.7 %RH

12. Sources of uncertainty related to use of psychrometers: Most significant contributions – Typical values • Long term stability – drift of thermometers and measuring electronics, typically: ± 0,5 %RH to ± 1.0 %RH • Thermal drift of measuring electronics during use at high or low temperatures – correlated and cancel out to some extent, typically ± 0.2 %RH to ± 0.5 %RH • Uncertainty of readings from psychrometric charts • Changes in atmospheric pressure (P) during use causes uncertainty of psychrometer reading up to ± 1 %RH – correction possible if pressure is known • Improper fitting of psychrometer wick, typically:± 0.5 %RH to ± 1.0 %RH • Contamination of psychrometer wick – don’t reuse wick, change it!

13. Typical calibration results:DELTA Electropsychrometer EP02 - Corrections

14. Uncertainty in Relative Humidity – Calibration of Psychrometers:Conclusion • Psychrometers increase the uncertainty of reference temperature measurement in calibration chambers. Reasons: Power dissipated in fan and evaporation of cooled air/water vapour from wet bulb • Psychrometers increase the uncertainty of reference dew-point measurement in calibration chambers: Reason: Water evaporation from wet bulb • The influence of a psychrometer on calibration uncertainty depends significantly on the size of the calibration chamber, stirring and where the reference temperature is measured • The calibration measurement capability could be based on a small psychrometer capable of stable and high resolution temperature measurement

15. Uncertainty in Relative Humidity – Calibration of Psychrometers:Discussion • Which uncertainty contributions do we need to include in CMC’s? • How many different calibration methods exists for calibration of psychrometers? • Is it common practice to calibrate the psychrometer coefficient A instead of direct comparison of %RH reading? • Is it common practice to include readings of atmospheric pressure (P) in calibration certificates? • ----