Text version of the page

Table 2. The specified accuracy of an HM K 15 c a l i b rator delivered with a calibration certificate.

be calibrated by a national calibra-tion laboratory (e.g. CET I AT,NIS T, NPL, PT B) to obtain then e c e s sa ry traceability to thenational standard.

Lithium chloride LiCl± 1.0% RH + Greenspan’s uncertainty * Magnesium chloride MgCl

2

± 1.0% RH + Greenspan’s uncertainty *Sodium chloride NaCl± 1.4% RH + Greenspan’s uncertainty *Potassium sulfate K

2

S O

4

± 1.5% RH + Greenspan’s uncertainty *

Easy and Reliable Calibration with the HMK15 Humidity Calibrator - 7108 Thermometer as ana c c e s s o r y

* The uncertainty given in Greenspan’s calibration table at the calibration temperature, e.g. the accuracy of LiCl sa l tat +20 °C is ±(1.0 + 0.3) % RH = ±1.3% RH

A thermometer with a calibra-tion certificate is included withthe HMK 15 calibrator. Th epurpose of the thermometer isto measure the temperature ofthe air above the salt solution.The temperature reading isused primarily to select the cor-rect reference humidity valuefrom Greenspan’s table, whichis printed on the calibrator.In addition, the accuracy ofthe thermometer is sufficientfor ch e cking the temperaturechannel of the humidity instru-m e n t. The thermometer hasbeen calibrated at six pointsand is traceable to the FinnishNational Measurement Stan-dards La b o r a t o ry* for tempera-ture. For more accurate results,the corrections given in the cali-bration certificate or their inter-polated values can be used.

Typical sources of errors in humidity calibration

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 ± 1 °C at + 20 °C between the air in the chamber andthe sensor causes an error of ±3% RH at 50% RH and ±6% RH at 97% RH. To avoidt e m p e r a t u re diff e rences, it is recommended to allow the humidity instrument and cali-brator to stabilize for at least 30 minutes in the same ambient temperature. The probe should be handled as little as possible to avoid the warming effect ofthe hands. To prevent the salt chamber and other parts of the calibrator from warm-ing up and causing errors in the readings, they should not be held during calibra-t i o n . Insufficient stabilization time The response time of thin film polymer sensors is very fast, usually less than 10 sec-onds for the sensor element itself. After the humidity probe is inserted in the calibra-t o r, however, it takes some time before the air around the humidity sensor elementreaches equilibrium with the generated humidity level. A temperature diff e re n c ebetween the humidity probe and calibration chamber increases the stabilization time.The recommended stabilization time for reliable calibration is from 10 to 20 minutes.

On-site checks possible

In addition to laboratory use,the calibrator is suitable for on-site ch e cks. During transport a-t i o n, the chambers can betightly closed with specially de-signed transit covers that pro-vide a complete seal and pre-vent any leaks of the salt solu-t i o n. The optional transit bagm a kes it easy to transport thec a l i b r a t o r. Th a n ks to the fasttemperature stabilization of thec a l i b r a t o r, the waiting time foro n -site calibration is compara-tively short.

O verly narrow range between the calibration points Multiple calibration points ensure the accuracy of the calibration results over a widehumidity range. If the instrument reading must be adjusted (i.e. if the output re a d i n gis changed to correspond to the re f e rence value), two points are sufficient for a high-quality humidity instrument that is used over a wide humidity range. The first point isused for zero point (offset) adjustment and the second point for span (gain) adjust-ment. A diff e rence of 50% RH or more is recommended between these two points. Dirty or wet eq u i p m e n t To avoid contamination, the salts and equipment must be handled carefully and keptabsolutely clean. Even a tiny drop of water on the probe near the sensors can dis-t o rt the re a d i n g s .

*

The Finnish National Measurement Standards La b o r i t o ry is part of the Centre for Metrology and Accreditation.

20

148/1998