7/24/2019 Sys Lab Magazine 595 433 En

1/1

The direct method

9 high accuracy thermo-hygrometers were located at the measuring positions

in Fig. 2., instead of the dew-point hygrometer and the thermometers [2].

For relative humidity, the stability and uniformity were evaluated in the same

way for temperature according to the G-20 in Ref.1.[1]

A comparison of two calibration methods for evaluating the performance of

climatic chambers was carried out. In this work, a 4.5 cubic foot PGC climatic

chamber was calibrated for its instability and uniformity both in terms of air

temperature and relative humidity.

In the first method, the absolute humidity inside the chamber was assumed to

be uniform, and therefore a dew-point hygrometer was used to measure only at

a corner of the chamber. The second method used 9 probes of high accuracy

thermo-hygrometer installed at different positions to evaluate the chamber

performance with out the uniform assumption as in the first method.

For both methods, the dew-point hygrometer and the thermo-hygrometers wascalibrated traceable to a NIMT 2 pressure primary generator. Due to the higher

accuracy of the dew-point hygrometer, the calibration uncertainty was lower

than that by the method using 9 probes of hygrometer.

For calibration results, the stability obtained from the two methods are in good

agreement within 0.1 %rh cover the measurement range, while the maximum

deviation of uniformity was larger than 0.5 %rh at the relative humidity of 92

%rh

COMPARISON OF CALIBRATION METHODS OF CLIMATIC CHAMBER

0.40

0.10

0.60

1.10

1.60

2.10

20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0

Relative Humidit /%rh

Uniformity/%r

indirect

direct

T. Sinhaneti, T. Keawprasert, U. Norranim

Humidity Laboratory, Thermometry Metrology DepartmentNational Institute of Metrology (Thailand)

ABSTRACT

PRINCIPLE

COMPARISON RESULT

Fig. 1 illustrates variation of measurement temperature at different positions in a

climate chamber and an example for evaluation of stability and uniformity of the

23.05

23.1

23.15

23.2

23.25

0 5 10 15 20 25 30 35

Time (min)

Measurementtemperature(oC)

1

2

3

4

5

6

7

8

ref

1.50

1.00

0.50

0.00

0.50

1.00

1.50

20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0

Relative Humidity/%rh

Stability/%rh

indirect

direct

The stability and the uniformity of climatic chamber when compare between

indirect and direct method deviated about +0.1%rh excepting the uniformity at

92%rh (~0.5%rh). Estimate measurement uncertainty in indirect method is lower

than that in direct method because of high accuracy of dew-point hygrometer and

the uniformity assumption in dew-point temperature. However the direct method

is still more convenient without complicating to calculate the relative humidity

via dew-point temperature. Finally, both method are capable to calibrate the

climatic chamber but the indirect method should be carried out due to lower

measurement uncertainty.

CONCLUSION

Figure 2. Schematic diagram for installing

the thermometers and the dew-point

thermometer

REFERENCES

CALIBRATION METHODS

Stability = tmax-tmin

= 23.17-23.14= 0.03oC = +0.015oC

Uniformity = Max (ti-t Ref)

= 23.2-23.07= 0.13oC = +0.065oC

es c am er in empera ure.

Figure 1. Illustration for determination of stability and uniformity for air temperature

Figure 4. Stability by comparison between indirect method and direct method.

Figure 3. Uniformity by comparison between indirect method and direct method

[1] G-20 Guidelines for Calibration and Checks of Temperature Controlled

Enclosures

[2] DKD-R 5-7 Guideline for calibration of Climatic Chamber.

t1

t2 t6

t5

t7t3

t4 t8

tdp.t ref.

The indirect method

9 thermometers and a dew-point

hygrometer were placed at measuring

positions as shown in Fig. 2.

The stability and uniformity in

temperature were determined

according to G-20 guideline [1].

The uniformity in relative humiditywas calculated via the measurement

temperature distribution and dew-point

temperature [2].