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Easy and Reliable Calibration with the HMK15 Humidity Calibrator

Easy and Reliable Calibration with the HMK15 Humidity Calibrator
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Easy and Reliable Calibration with the HMK15 Humidity Calibrator - 1

he use of sa t u r a t e dsalt solutions is anaccepted and reli-able method for cali-brating humidity instruments.M a ny leading laboratories usethis method, which has beengenerally known since the early19 0 0 s. The operating principleis basedon the properties ofsalt and water solutions, whichgenerate a known relativehumidity in the air abovet h e m.Based on these reference v a l u e s, the reading of the humid-ity instrument can then beadjusted and calibrated. Satu-rated salt solutions provide areliable measurement standardas the humidity levels generat-ed by these solutions and theirbehavior at different tempera-tures are well documented andhave been verified by many re-s e a r ch e r s. The Greenspan table( Table 1) is one of the mostw e l l -known references.Several different salts aresuitable for reference measure-m e n t s, providing a number ofoptions for the user. With highquality instruments, however,no more than two referencepoints are needed to adjust themeasurement over the entirerelative humidity range. Jan GrҶnblad, M. S c. (E n g. )Product ManagerSensor Systems DivisionVa i sala HelsinkiF i n l a n d > Other new additions include sa l tp a ckages with calibration cer-tificates from an accredited la-b o r a t o ry. Examples of the sa l tsolutions suitable for the HMK 15calibrator include lithium ch l o-ride LiCl (11% RH), magne-sium chloride MgCl > (EN45001)(ISO/IEC Guide 25) 2 ( 3 3 %RH), sodium chloride NaCl( 75% RH) and potassium sul-phate K > Vaisalas Measurement Standards Labo ratory at the Vaisala headquarters inHe l s i n k i, Finland, is a FINAS accredited calibration fa c i l i t y. For morei n f o r m a t i o n, please visit w w w. e u ro pe a n-a c c re d i t a t i o n. o r g 2 SO > 4 ( 97% RH). Allthese salts are delivered fromthe factory in dry format toavoid any problems that mayoccur with ready-made salt so-l u t i o n s. Dry salts also offer am u ch longer storage life thanr e a d y -mixed salt solutions. The ready-dosed salt pack- > During the development of theHMK 15, Va i salaҒs latest humid- i t ycalibrator based on sa t u r a t e dsalt solutions, the main goalswere to improve the ease of useand reliability of the system. Th es h o rtened thermal and humiditystabilization time of the HMK 15 ,w h i ch is one of the main im-provements in the new design,speeds up calibration andimproves the reliability of ther e s u l t s. By combining an alu-minum chamber and properm e chanical design, the sa l tchamber is optimized for fasttemperature stabilization. Th einner space of the chamber isshaped to promote fast humid-ity stabilization and the genera-tion of a homogenous atmos-phere above the salt solution. > 18 148/1998 size="-3">

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Easy and Reliable Calibration with the HMK15 Humidity Calibrator - 2

* The use of an LiCl solution at temperatures below +18 C is not recommended as its equilibrium humidity can change permanently. 1 G r e e n s p a n, L.: Journal of Research of the National Bureau of Standards Ж A Physics and Chemistry Vo l. 81 A, No. 1 January F e b r u a ry 1977, pp. 89֖95 Table 1. Gr e e n s pans calibration table > 1 for generated humidity levels for four commonly used salts over at e m p e rature range of 0 to 50 ҰC. In each column, the numbers on the right indicate Gr e e n s pans uncertainty forthe reference humidity of the salt. > 148/1998 19 size="-1">

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Easy and Reliable Calibration with the HMK15 Humidity Calibrator - 3

* The uncertainty given in Greenspans calibration table at the calibration temperature, e.g. the accuracy of LiCl sa l tat +20 ҰC is (1.0 + 0.3) % RH = ѱ1.3% RH Temperature difference between the humidity probe and calibrationc h a m b e r As relative humidity is a very temperature dependent physical parameter, it is impor-tant to have both the calibrator and the humidity probe at the same temperature. At e m p e r a t u re diff e rence of ұ 1 C at + 20 аC between the air in the chamber andthe sensor causes an error of 3% RH at 50% RH and ѱ6% RH at 97% RH. To avoidt e m p e r a t u re diff...

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