Determination of uncertainty and performance of NIS humidity standards

1. Determination of uncertainty and performance of NIS humidity standards By Nabila I. El-sayed, Faten M. Megahed and Yasser A. Abdelaziz National Institute for Standards Giza, Egypt

2. Abstract • The objective of this paper is to present the methodology for estimation of measurement uncertainties in humidity calibration at the National Institute for Standard (NIS-Egypt). • NIS established a number of humidity calibration facilities. • The methodology is applied for different types of hygrometers within the range from 5% rh to 95% rh using NIS primary, secondary or working standards.

3. Experimental Work • The calibration of a climatic chamber serves to determine the deviation of the climatologically characteristics of air temperature and relative humidity in those parts of the chamber volume which are provided for use or in individual points of the chamber volume from values displayed by the indicators of the chamber. • Besides these deviations additional properties such as in homogeneities, stabilities, gradient and the other properties that are frequently determined to characterize the chamber and potential effects on the test material placed in the chamber.

4. Objectives of the chamber calibration • Calibration of the indication for temperature and relative humidity by comparison technique with NIS standards. • Estimation uncertainties of temperature and relative humidity measurements during calibration processes, and determination of the uncertainty for using chamber under defined conditions, using DKD guideline for calibration of environmental chambers.

5. Instrumentation used for the calibration process • Climatic chamber (Votch, model vc 0018, volume 195 liter). In the temperature range from 10 oC to 90 oC for temperature and from 10 %rh to 98%rh for humidity. The chamber has glass window. • Dew point apparatus (MPW calibration, model DP 30), used in the measuring range from -60 to 60 oC DP with resolution 0.01 oC.

6. Instrumentation used for the calibration process • Digital hygrometer, (Testo, model 650) with two sensors, can be used in the measuring range from -20 oC to 70 oC for temperature and from 10 %rh to 95 %rh for humidity, in two locations, one in the center of the climatic chamber and the other in contact to its indicator sensors. • Semi-Standard PT100 thermometers (Quantity; 9) , connecting to AOL temperature scanner which can measure and record the output of all thermometers with suitable synchronization with accuracy ± 0.1 oC.

7. Arrangement of measuring locations • Set up diagram of equipment to assessment the chamber uniformity- the numbered boxes indicate to the sensors locations.

8. Results and discussion Using the previous recorded measurements, We can determine the following characteristics for the climatic chamber; • Temperature gradient: Temperature gradient expressed as the difference between the highest and the lowest temperature value of the temperature sensors on different locations within the working space after achieving stability. The maximum value of this gradient was found to be (0.5 oC) at temperatures 60, 70 and 80 oC as shown in table (1). • Temperature fluctuation: It is the change of the temperature measured on the centre of the working space, and beside the sensor of the chamber after achieving stability. Its maximum value was found to be 0.4 oC at 60 oC, as shown in table (1). • Humidity gradient: Humidity gradient expressed as the difference between the highest and the lowest value of the relative humidity sensors on different locations within the working space after achieving stability. The maximum value was found to be 0.7 % rh at humidity 30, and 60 % rh as shown in table (2). • Humidity fluctuation: It is the change of the relative humidity measured on the centre of the working space, and beside the sensor of the chamber after achieving stability. The maximum fluctuation value was found to be 0.5 % rh at 75 % rh values as shown in table (2).

9. Results and discussion Using the previous recorded measurements, We can determine the following characteristics for the climatic chamber; • Determination of spatial inhomogeneity of temperature The spatial inhomogeneity is determined as the maximum deviation of the temperature of a corner measuring location from the reference location. It was determined for all calibration temperatures the maximum value was found to be 0.3 oC at temperature 70 oC. • Determination of spatial inhomogeneity of humidity The spatial inhomogeneity is determined as the maximum deviation of the relative humidity of a corner measuring location from the reference location. It was determined for all calibration humilities its maximum value was found to be 0.5 % rh at 75 % rh. • Determination of temporal instability The temporal instability for air temperature and air humidity is determined from the registration of the temporal variation of the temperature or relative humidity, respectively, over a period of time of at least 30 minutes after steady-state conditions have been reached. Steady-state conditions are considered to be reached when systematic variation of temperature or relative humidity is no longer measured. The results were found to be 0.1oC for temperature at 30 oC and 0.6 % rh for humidity at 60 % rh. • Determination of loading effect • Climatic chamber was calibrated in the empty and loaded states. The calibration was carried out for reference measuring location with and without load and the maximum difference was taken as the half-width of a rectangular distributed uncertainty contribution. The maximum value was found to be 0.1 oC at 30 oC and 0.6% rh at 40% rh.

10. Uncertainty contribution • Combination of temperature uncertainties

11. Uncertainty contribution • Combination of humidity uncertainties

12. Conclusion • Calibration of hygrometers at NIS laboratory required detailed investigations of the climatic chamber properties. These results on the one hand are of greater interest to the user of the chamber as they describe its properties during use and on other hand are necessary for determining the measurement uncertainty of the calibration results. • It enables NIS laboratory to achieve the tractability of humidity measurement. The used procedure in this work gives a method for calibration of climate chambers and gives a hand for industrial sectors to be sure of their environmental condition and products. After calculating the uncertainty of the chamber the uncertainty and tractability for any calibrated hygrometer can be determined.

13. Thanks Nabila I. El-sayed, Faten M. Megahed and Yasser A. Abdelaziz National Institute for Standards Giza, Egypt