MEASURING UP TO HUMIDITY STANDARDS

Humidity and control
Humidity is an increasingly important measurement and control parameter in a wide range of industrial and scientific processes as well as in the validation of product performance. At the very highest level, variations in ambient humidity can even affect the uncertainty of fundamental measurement standards; for example, the primary realisation of the kilogramme at Bureau International des Poids et Measures (BIPM), Paris, France, depends upon the accurate determination of atmospheric humidity using a cooled mirror dewpointmeter in order that the effect of air buoyancy can be corrected for.

Traceability of humidity measurement is currently limited to the range -75 to +82 ºC dew point, or its equivalent in other engineering units. This sounds small, but in concentration terms relates to the range 1 to 1,000,000 parts per million (dry basis). Generally speaking the uncertainty of such standards is of the order of a few hundredths of a degree Celsius, or 0.5 to 1 % of measured concentration.

Uncertainty variances and sensor type selection
The transfer of the measurement capability of a National Standard across to a working industrial hygrometer can add significantly to that instrument's measurement uncertainty. A polymer film relative humidity sensor used for measuring the dew-point temperature in a clean dry air line can, for example, have a measurement uncertainty of +/- 100% of measured value, or approximately 7.5 ºC frost point (see table 1. below). Such polymer film probes are commonly quoted as having 1 or 2% humidity uncertainties.

Table 1: Equivalent measurement uncertainty of an RH sensor used to measure frost-point temperature or volumetric humidity in a compressed air line

NB. This table takes no account of extra uncertainty introduced by temperature measurement errors.

Impedance type dew point sensors give a significantly better uncertainty as they are designed and calibrated in the unit of measurement. A good impedance dew-point sensor will have an uncertainty of between 1 and 2 ºC, which relates to an uncertainty in terms of concentration of about +/-15%

In order to reduce the uncertainty of measurement further, a fundamental cooled mirror measurement technique may be employed. Cooled mirror dewpointmeters are widely used as transfer standard devices to provide the link between a national standard (or calibration laboratory) and a field measurement instrument. A typical cooled mirror instrument will have a measurement uncertainty of 0.2 to 0.3 ºC frost point at this level, which equates in volumetric humidity (concentration) terms to 1.5 to 2% of reading.

From the aforementioned range of uncertainties - from 100% of reading for a polymer film RH probe down to 2% of reading for a cooled mirror sensor at 250 ppmv measured humidity - it can be seen that careful selection of the sensor type is just as important as the need to maintain traceability by regular calibration or validation. The fact that an RH probe is fifty times less accurate than a dew point sensor at 250 ppmv throws into question the decision to select this type of sensor for anything but trend measurement at low humidity levels.

Traceability and accreditation
Traceability through calibration has been possible in the UK humidity industry for nearly 17 years. The UK's national humidity standard, operated by the National Physical Laboratory (NPL), was commissioned in 1985 and continues to provide the fundamental reference, with direct traceability itself back to mass and temperature standards. NPL's two-temperature humidity generator is validated periodically against a primary gravimetric hygrometer and is used to calibrate transfer standard dew-point hygrometers by reference against its own transfer standard hygrometer and reference temperature probes to measure critical loop temperature values.

Since 1986 NAMAS (now UKAS) has operated an accreditation service for humidity measurements and the Calibration Laboratory of Michell Instruments was the first to be accredited in May of that year. Continuous, un-broken accreditation has been maintained ever since and Michell continues to be the only UK manufacturer of cooled mirror and impedance dew-point hygrometers to have UKAS accreditation. Through the European Co-operation on Accreditation (EA), International Accreditation Forum (IAF), and International Laboratory Accreditation Co-operation (ILAC) this accreditation has global recognition.

Table 2. Measurement Uncertainty of Michell Instruments' Laboratory (UKAS Accreditation Number 0179)

With a wide range of impedance hygrometers and cooled mirror dewpointmeters, capable of operating over a range from -110 to +90 ºC dew point, Michell can provide a measurement solution for almost any application, from critical corrections to fundamental mass balances to low cost measurement of compressed air lines.

In addition to UKAS accreditation, Michell's Calibration Laboratory offers commercial calibration of Michell and non-Michell products over the full humidity range, with traceability to NPL and NIST over the range -75 to +82 ºC dew point. Our expertise in humidity calibration also allows us to develop and install commercial humidity generators and calibration systems for major industrial users and standards institutes.

Note: all uncertainties mentioned in this article are assumed to have a coverage factor of 2, giving a confidence interval of approximately 95%.

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