To compare different methods of measuring humidity can be somewhat complicated because of both the high number of measurable hygrometric parameters and their lack of homogeneity. Given that there are measurement uncertainties, this comparison is possible through the law of propagation of measurement uncertainties. In this paper the authors use a method for calculating the uncertainties in indirect measurements of both relative humidity and humidity ratio and therefore for comparing the performances of different sensors. This comparison is developed, starting from the manufacturer's specification for the more common air humidity sensors. The indirect measurement uncertainties at different dry bulbs are presented using diagrams for each sensor family. The results provide useful information both for analysing the humidity measurement uncertainty of a specific sensor, and for choosing the most suitable sensor for given measurement conditions.
Ashrae.1993. American Society of Heating, Refrigeration and Air- Conditioning Engineers, Handbook, Fundamentals, chapter 6. Atlanta.
2.
Bindon, H.H.1965. 'A Critical Review of Tables and Charts Used in Psychrometry' , In: A. Wexler and W. H. Wildhack (Eds), Humidity and Moisture Measurement and Control in Science and Industry, Vol 1, 3—15. Reinhold Publishing Corp, New York.
3.
Buonanno, G., Carotenuto, A., Crovini, L., and Dell'Isola, M.1994. 'To compare ideal and real moist air models for calculating humidity ratios and relative humidity in the 213.15 K to 473.15 K range and up to a pressure of 1 MPa, Int J Thermophysic, 15(3), 483-504.
4.
Carotenuto. A., Cascetta, F., Cerra, D., and Dell'Isola, M.1991. 'Valutazione dell'Incertezze nelle Misure Dirette ed Indirette di Umidità dell'Aria', Proceeding of 46° Congresso Nazionale ATI, Cassino, September 25-27, 11155-11168.
5.
Carotenuto. A., Cerra, D., and Dell'Isola, M.1990. 'Misura e Controllo dell'Umidità Ambientale: limiti, applicazioni e futuri sviluppi', Fisica e Tecnologia, 13(4), 237-268.
6.
Carotenuto, A., Dell'Isola, M., Crovini, L., and Actis, A.1995. 'Accuracy of commercial dew-point hygrometers'. submitted to ASHRAE Transactions.
7.
Carotenuto. A., Dell'Isola, M., Vigo, P., Crovini, L., and Actis, A.1994. 'Measurement of the humidity ratio of moist air using a thermo-hygrometer', ASHRAE Transaction.100(1), 122-131.
8.
Harrison, L.P.1965a. 'Fundamental Concepts and Definitions Relating to Humidity' . In: A. Wexler and W. H. Wildhack (Eds), Humidity and Moisture Measurement and Control in Science and Industry, Vol3, 3-69, Reinhold Publishing Corp, New York.
9.
Harrison, L.P.1965b. *Some Fundamental Considerations Regarding Psychrometry' . In: A. Wexler and W. H. Wildhack (Eds), Humidity and Moisture Measurement and Control in Science and Industry, Vol3, 71-103. Reinhold Publishing Corp, New York.
10.
Hyland, R.W., and Wexler, A.1983a. 'Formulations for thermodynamics properties of saturated phase of H2O from 173.15 to 473.15 K', ASHRAE Transactions , 89(2), 520-531.
11.
Hyland, R.W., and Wexler, A.1983b. 'Formulations for thermodynamic properties of dry air from 173.15 to 473.15 K, and of saturated moist air from 173.15 to 473.15 K, at pressure to 5 MPa', ASHRAE Transactions, 89(2), 500-519.
12.
Iso, 1993. Expression of Uncertainty, TAG 4 WGS, OIML, BIPN, ISO Switzerland.
13.
Ku, H.H.1966. 'Notes on the use of propagation of errors formulas', Journal of Research of NBS, 70C(4), 263-273.
14.
Lafarie, J.P.1985. 'Relative Humidity Measurement: A Review of Two-State of the Art Sensors', Proc 1985 Int Symp on Moisture and Humidity, Washington, DC, 15-18 April, 875-889.
15.
Moore, R.L.1982. 'Moisture and Humidity Measurement', In: Basic Instrumentation Lecture Notes and Study Guide, Vol 2, 23-40. Instrument Society ofAmerica.
16.
Nelson, R.M., and Pate, M.1986. 'A comparison of three moist air property formulations for computer applicaticans', ASHRAE Transactions, 89(2), 435-446.
17.
Wexler, A.1975. 'Electric hygrometers', NBS Circular 586, National Bureau of Standard, Washington, DC.
18.
Wiederhold, P.R.1975. 'Humidity measurements: Part 1; Psychromcters and percent RH sensors', Instrumentation Technology, 22(8), 38-44.
19.
Wiederhold, P.R.1985. Process Instruments and Controls Handbook, Considine, MacGraw Hill, New York, chap 10.
20.
Wiley, R.G.1985. 'The Psychrometer Problems, an Accurate Analysis for Laminar Boundary Layer Flows', Proc 1985 Int Symp on Moisture and Humidity, Washington, DC, 15-18 April, 57-74.
21.
Wiley, R.G., and Lalas, T.1985. 'Accurate Psychrometer Coefficients for Wet and Ice Covered Cylinders in Laminar Transverse Airstreams', Proc 1985 Int Symp on Moisture and Humidity, Washington, DC , 15-18 April, 37-56.
