Humidity sensors. Behavior in different environments. Supervisor: Vaclav Vacek Collaborators: Martin Doubek, Martin Janda and Michal Vitek 28.9.

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1 Humidity sensors Behavior in different environments Supervisor: Vaclav Vacek Collaborators: Martin Doubek, Martin Janda and Michal Vitek CZECH TECHNICAL UNIVERSITY IN PRAGUE CERN

2 Content Content Introduction Hygrometrix HMX22 and Honeywell HIH 4 sensors Sensors description Behavior of the humidity sensors in air Testing facility Calibration in the climate chamber Behavior of the humidity sensors in nitrogen and carbon dioxide ambience Testing facility Sensors calibration Time response Thermal stability Hygrometrix Xeritron sensor Introduction Sensor description Calibration in nitrogen and carbon dioxide atmosphere Nitrogen Carbon dioxide Comparison of the sensors behavior in carbon dioxide and nitrogen Time response Summary Hygrometrix and Honeywell sensors Hygrometrix Xeritron Reference Behavior of the humidity sensors being used within the ID ATLAS volumes Page 2

3 1 Introduction The motivation of the performed tests was to examine the behavior of various humidity sensors under different ambient conditions. Three types of sensors that are currently used for the monitoring of the Atlas Pit volume were used for the tests: Honeywell HIH-4 Series (further as Honeywell), Hygrometrix HMX 22 and Hygrometrix Xeritron. Sensors were due to the unavailability of the Hygrometrix HMX 22 sensor tested in two batches. The first batch composed of Honeywell HIH and Hygrometrix HXM sensors was tested in air. Later the second batch composed from all sensors was tested in nitrogen and carbon dioxide ambience. Details of the performed tests of the Honeywell HIH and Hygrometrix HMX sensors are described in the chapter 2, for the Hygrometrix Xeritron see chapter 3. Behavior of the humidity sensors being used within the ID ATLAS volumes Page 3

4 2 Hygrometrix HMX22 and Honeywell HIH 4 sensors 2.1 Sensors description Honeywell HIH and Hygrometrix HXM sensors operate on the basis of different physical principles.the Honeywell sensor is based on the thermo set polymer capacitive sensing element. It is radiation soft and it came with the factory fit (see Table 2). The Hygrometrix sensor works on the basis of piezo-resistive strain gauge with integral temperature element and it is radiation hard. The company which developed Hygrometrix sensors is no longer in operation. No factory fit for those sensors was available. The output signal of the Hygrometrix sensor was read through patch panel which schematic is on the Figure 1 By this patch panel the signals from sensor s resistance bridge are buffered by IC2 and IC3, then they go to the IC4 which is a differential amplifier (gain = 1).Output of this amplifier is the output of the patch panel. In this paper the term The Hygrometrix sensor means a set composed form the Hygrometrix HMX22 and the patch panel. The Honeywell and Hygrometrix sensor s performance specifications delivered by manufacturer can be found in Table 1,Table 2 and Table 3. Parameter Min Typical Max Unit Accuracy ±3.5 %RH Repeatability ±.5 %RH Response time 15 s Operating o C temperature Operating humidity 1 %RH Table 1 Honeywell sensors performance specifications Model HIH4 Channel 336 Wafer t1 MPR t1 Calculated values at 5 V V %RH V 75.3 %RH.785 V 3.4 V Linear output for 2 %RH 25 o C Zero offset Slope RH Ratio metric response for % To 1 %RH V out Table 2 Honeywell factory fit.785 V mv / %RH (V out -zero offset) / slope (V out -.785) /.299 V supply * (.157 to.7559) Parameter Min Typical Max Unit Response time 1 s (1 to 9%) Operating o C temperature Full scale detection range.1 1 %RH Table 3 Hygrometrix sensor performance specifications Figure 1 Hygrometrix patch panel schematic Behavior of the humidity sensors being used within the ID ATLAS volumes Page 4

5 ` 2.2 Behavior of the humidity sensors in air Testing facility For the tests where atmospheric air was used as a medium (see Figure 2), the air testing facility and data acquisition system (DAQ) were prepared. Essential element of the setup was climate chamber Vötsch VC 22. The climate chamber is a digitally controlled device used for maintaining constant conditions of relative humidity (RH) and temperature inside its insulated volume. Measurements in the chamber were limited by the range of the allowed values of temperature and relative humidity, See Chart 1. Redundant NTC temperature sensors and calibrated relative humidity sensors were placed inside this climate chamber. Uniformity of the conditions inside the chamber is ensured by the stirrer fan, so the placement of the sensors inside the chamber was not critical. The dew point meter DewMaster was used as a reference humidity sensor. It is a high precision chilled mirror hygrometer with integrated digital control and automatic calibrating cycle. During the measurements this hygrometer was located next to the climate chamber and the air from the chamber was flowing into the hygrometer s sensor by the short tube and the fan. The temperature of passing air through the hygrometers sensor is not important, because the absolute content of water as well as dew point temperature is not influenced by the temperature of the air. The only important information is the temperature of the air near the relative humidity sensors that are being calibrated and dew point temperature. A relative humidity can be calculated from these values A calibrated embedded local monitor board (ELMB 128) with standard motherboard unit and specially prepared PVSS II project were used for the readout of the sensor s outputs. Input range of the ELMB (ELMB 128 plugged into the motherboard) channels was set to 1 mv and the precision of the ELMB s readings was ±.5% from the full input scale ; Relative humidity % Chart 1 - Allowed values of climate chamber Hygrometrix 9V Honey well 5V Table 4 Sensors supply Clim ate chamber 2 C ha m b er s fa n 3 Cham ber s reference hum idity sensor 4 Calibrated hum idity sensor 5 Area of calibrated hum idity sensors 6 Reference dew point m eter 7 Dew point sensor of reference dew point m eter 8 Fan m aintaining air flow throught the Dew m eter 9 Power supply 1 ELM B unit 11 Notebook with CAN Bus card Figure 2 Air testing facility schematic Behavior of the humidity sensors being used within the ID ATLAS volumes Page 5

6 2.2.2 Calibration in the climate chamber Measurements in the climate chamber were performed during multiple runs within period of almost three weeks ( and ). Such a long time was necessary because of the assembly and verification of the experimental setups. The individual measurements have also proved themselves to be very time consuming and often error prone. Initial discrepancies were caused by problems with the condensation in the tube supplying the air to the reference sensor and by the long transition time of the chamber between different environmental set points. The experiment was divided into two steps. The first step consisted from two tests: one with the increasing humidity at constant temperature 4 C and one with the decreasing humidity at temperature 2 C. Second step of the experiment was composed from two measurements in the air with rising humidity but at different temperatures. The humidity set-points used during the runs were: 2%, 3%, 4% and 5% RH and the temperatures were 3 C and 4 C. Voltage [ V ] Calibration in climat chamber 2 o C % 28 7% % % :12 14:24 15:36 16:48 18: 19:12 2:24 21:36 time [hh:mm ] Honeywell Hygrometrix Temperature Chart 2 Climate chamber calibration in the air at 2 C Temperature [ oc ] Voltage [ V ] Calibration in climat chamber 4 o C 3 7% % 6 4% 5% % 3% % :24 9:36 1:48 12: 13:12 14:24 15:36 16:48 18: 19:12 time [hh:mm ] Honeywell Hygrometrix Temperature Chart 3 Climate chamber calibration in the air at 4 C Temperature [ oc ] Behavior of the humidity sensors being used within the ID ATLAS volumes Page 6

7 Summary of the calibration runs in climate chamber Honeywell sensor: Temperature: 38.6 C Factory fit: RH=33.398U Relative humidity [%] Calibration of Honeywell relative humidity sensor (~38.6 C) FIT 1 Honeywell in air at 38.6 C Honeywell RH = x Factory fit RH = x Voltage [V] Honneywell Factory fit Honeywell Reference (DewMaster) Factory fit Error Voltage [V] RH [%] RH [%] RH [%] Average temp [ C] Temperature: 19.4 C Factory fit: RH=33.398U Relative Humidity [% Calibration of Honeywell relative humidity sensor (~19.4 C) RH Honeywell RH = x Factory fit RH = x Voltage [V] Honeywell Reference (DewMaster) Factory fit Error Voltage [V] RH [%] RH [%] RH [%] Average temp [ C] Honneywell Factory fit FIT 2 Honeywell in air at 19.4 C Behavior of the humidity sensors being used within the ID ATLAS volumes Page 7

8 Hygrometrix sensor: Temperature 38.6 C Relative humidity [%] Calibration of Hygrometrix relative humidity sensor (~38.6 C) FIT 3 Hygrometrix in air at 38.6 C RH = 71,941x - 4, Voltage [V] Hygrometrix Reference (DewMaster) Voltage [V] RH [%] Average temp [ C] Temperature 19.4 C Relative Humidity [%] Calibration of Hygrometrix relative humidity sensor (~19.4 C) RH = 15.47x Voltage [V] Hygrometrix Reference (DewMaster) Voltage [V] RH [%] Average temp [ C] FIT 4 Hygrometrix in air at 19.4 C Behavior of the humidity sensors being used within the ID ATLAS volumes Page 8

9 Comparison of the sensors outputs for the air of the same humidity and different temperatures The aim of the test was to compare sensor outputs for the same humidity levels but different temperatures. Four different humidity levels in the range from 2% RH to 5% RH for two temperature set-points (29 C and 38.5 C) were used for the comparison. The results are summarized in Table 5. Honeywell Hygrometrix Average temperature Reference(DewMaster) Voltage [V] Voltage [V] [ C] RH [%] Table 5 Summary of the tests with the same moisture set-points and 1 C temperature difference Relative humidity [%] Hygrometrix Air 29 C: RH= x C: RH = x Relative humidity [%] Honeywell Air 29 C: RH = x C: RH = 35.89x Voltage [V] Voltage [V] 29 C 38.5 C 29 C 38.5 C Chart 4 Comparison of the sensors fits for the same moisture set-points and 1 C temperature difference Behavior of the humidity sensors being used within the ID ATLAS volumes Page 9

10 2.3 Behavior of the humidity sensors in nitrogen and carbon dioxide ambience Testing facility An installation capable of increasing humidity of dry gases was prepared for the second set of the tests (see Figure 3). Bottles of 99.9% CO 2 and % N 2 were used as a source of a dry gas. The gas was brought from the bottle through pipes to the bubbler filled with the distilled water that provided the required moisturizing level. The humidity level was controlled by the needle valve located on the bypass around the bubbler. The gas then went through flow-meter and heated pipe to the sealed insulated plastic container with installed humidity and NTC temperature sensors. The dew meter DewMaster placed behind the box was used as a reference. The heating was turned on only during the thermal stability testing otherwise the gas was at a room temperature. Same ELMB based DAQ system was used for the sensors readout as in Figure 3 Nitrogen and carbon dioxide testing facility schematic Behavior of the humidity sensors being used within the ID ATLAS volumes Page 1

11 2.3.2 Sensors calibration The main goal of the experiments performed between and was to find out the calibration equations for the Honeywell and Hygrometrix sensors in the nitrogen and carbon dioxide atmosphere. Data were collected during multiple runs on various humidity levels to ensure minimal error of the measurement. Nitrogen atmosphere: Flow: 15 Nl /s Temperature: C and 3.64 C RH % RH % Hygrometrix N2 ~21,79 o C y = x Sensor output V Honeywell N 2 ~21,79 o C y = x Sensor output V Honeywell Hygrometrix Reference (DewMaster) Voltage [V] Voltage [V] RH [%] Average temp [ o C] Honeywell Hygrometrix Reference (DewMaster) Voltage [V] Voltage [V] RH [%] Average temp [ o C] 3.64 FIT 5 Hygrometrix ad Honeywell sensor in nitrogen Behavior of the humidity sensors being used within the ID ATLAS volumes Page 11

12 Carbon dioxide atmosphere: Flow: 15 Nl /s Temperature: 21.8 C and 3.31 C RH [%] RH [%] Hygrometrix CO 2 ~21.8 o C y = 14.32x Sensor output [V] Honeywell CO 2 ~21.8 o C y = 5.142x Sensor output [V] Honeywell Hygrometrix Relative humidity DewMaster Voltage [V] Voltage [V] RH [%] Average temp [ o C] 21.8 Honeywell Hygrometrix Relative humidity DewMaster Voltage [V] Voltage [V] RH [%] Average temp [ o C] 3.31 FIT 6 Hygrometrix and Honeywell in carbon dioxide Behavior of the humidity sensors being used within the ID ATLAS volumes Page 12

13 Flow: 2 Nl /s Temperature: 2.64 C Sensors behavior in different gases Honeywell sensor output [V] Pure N 2 Pure CO Hygrometrix sensor output [V] Reconection of pipes.3 9:57 1:4 1:12 1:19 1:26 1:33 1:4 1:48 time [hh:mm] Honeywell Hygrometrix Chart 5 Comparison of the sensors outputs in nitrogen and carbon dioxide. Behavior of the humidity sensors being used within the ID ATLAS volumes Page 13

14 2.3.3 Time response The aim of the test was to measure sensors response due to the step change in humidity and to compare the reaction time of the tested sensors. Steps in humidity were done in both directions (from lower to higher humidity and vice versa). The test was repeated several times with the step size between 1%, and 2% RH. The testing facility didn t allow us to set exactly the same moisture content in all tests. Flow: 25 Nl /s Temperature: C Output singnal of Honeywell sensor [ V ] Response to step change in humidity in Nitrogen ~23 C VOLTAGE OUTPUT RH 8.48% RH 21% : 2:53 5:46 8:38 11:31 time [ mm:ss ] Output singnal of Hygrometrix sensor [ V ] Chart 6 Response to humidity change Flow: 25 Nl /s Temperature: C Honeywell Hygrometrix Output singnal of Honeywell sensor [ V ] Response to step change in humidity in Nitrogen ~23 C VOLTAGE OUTPUT This peak is caused by "bubbler" disconection RH 21% RH 4.97% : :7 :14 :21 time [ mm:ss ] Output singnal of Hygrometrix sensor [ V ] Chart 7 Response to humidity change Honeywell Hygrometrix Behavior of the humidity sensors being used within the ID ATLAS volumes Page 14

15 2.3.4 Thermal stability Thermal stability test was aimed to detect any effect of temperature on the output signal of the sensors located in a very dry gas ambience. Verification of the effect of temperature variations were made for both sensors in pure nitrogen and carbon dioxide. In these cases of the very dry gas was possible to exclude the influence of temperature on the actual value of the relative humidity. The fits from chapter were used (used fits are referred in the chart titles). Response of Hygrometrix sensor to the temperature change [~21.8 o C CO 2 FIT ] RH [%] : 12:28 12:57 13:26 13:55 14:24 14:52 15:21 time [hh:mm] Temperature [ o C] Hygrometrix Average temperature Chart 8 Temperature influence on Hygrometrix sensor in carbon dioxide RH [%] Response of Honeywell sensor to the temperature change [~21.8 o C CO2 FIT ] : 12:28 12:57 13:26 13:55 14:24 14:52 15:21 time [hh:mm] Temperature [ o C] Honeywell Average temperature Chart 9 Temperature influence on Honeywell sensor in carbon dioxide Behavior of the humidity sensors being used within the ID ATLAS volumes Page 15

16 Response of Honeywell sensor to the temperature change [~21.8 C N 2 FIT ] RH [%] Temperature [ C] -2 15:36 16:48 18: 19:12 2:24 21:36 time [hh:mm] Honeywell Average temperature Chart 1 - Temperature influence on Honeywell sensor in nitrogen Response of Hygrometrix sensor to the temperature change [~21.8 C N 2 FIT ] RH [%] Temperature [ C] 15:36 16:48 18: 19:12 2:24 21:36 time [hh:mm] Hygrometrix Average temperature Chart 11- Temperature influence on Hygrometrix sensor in nitrogen Behavior of the humidity sensors being used within the ID ATLAS volumes Page 16

17 3 Hygrometrix Xeritron sensor 3.1 Introduction The behavior of Hygrometrix Xeritron sensors has been already examined before and after the irradiation in the nitrogen environment, for the test results see [3]. The motivation of our test was to examine behavior of the sensor in two different gases ambience (carbon dioxide and nitrogen). Two samples of the sensor type were used during the tests. The tests were performed using the same facility as in chapter Sensor description Hygrometrix Xeritron is based on organic and inorganic crystals. These crystals react on moisture by hygromechanical stress. Pair of silicon strain gauges reacts on this by changing their electrical resistance. Therefore the sensor can be considered as an electrical circuit composed from two resistors, see Figure 4. Parameter Min Typical Max Unit Accuracy ±1-3 %RH Repeatability ±1 %RH Response time 3 min Operating o C temperature Operating humidity 1 %RH Table 6 Hygrometrix Xeritron performance specifications Figure 4 Electrical equivalent circuit of Hygrometrix Xeritron sensors. The resistance of the resistor R1 is decreasing with rising humidity and the resistance of the resistor R2 is increasing with rising humidity. The relative humidity in the surrounding environment can be calculated from the formula: R1 RH = a + b. (1) R1 + R2 Behavior of the humidity sensors being used within the ID ATLAS volumes Page 17

18 3.3 Calibration in nitrogen and carbon dioxide atmosphere The goal of the calibration was to determine coefficients a and b of the equation (1) for both used pieces of the Hygrometrix Xeritron sensor Nitrogen Flow: 15 Nl /s Temperature: 22. C Relative humidity [%] Hygrometrix Xeritron sn: 2833 N 2 22 C y = x R 2 =.989 Serial number: 2833 Hygrometrix Xeritron Reference (DewMaster) R1/(R1+R2) RH [%] Average temperature: 22 C R1/(R1+R2) FIT 7 Hygrometrix Xeritron nitrogen at 22 C, serial number: 2833 Relative humidity [%] Hygrometrix Xeritorn sn: N 2 22 C RH = x R 2 =.9774 Serial number: Hygrometrix Reference Xeritron (DewMaster) R1/(R1+R2) RH [%] Average temperature: 22 C R1/(R1+R2) FIT 8 Hygrometrix Xeritron in nitrogen at 22 C, serial number: Behavior of the humidity sensors being used within the ID ATLAS volumes Page 18

19 3.3.2 Carbon dioxide Flow: 15 Nl /s Temperature: 22. C Relative humidity [%] Hygrometrix Xeritron sn: 2833 CO 2 23 C RH = x R 2 = R1/(R1+R2) Serial number: 2833 Hygrometrix Reference Xeritron (DewMaster) R1/(R1+R2) RH [%] Average temperature: 23 C FIT 9 Hygrometrix Xeritron carbon dioxide at 23 C, serial number: 2833 Relative humidity [%] Hygrometrix Xeritron sn: CO 2 23 C RH = x R 2 = R1/(R1+R2) Serial number: Hygrometrix Xeritron Reference (DewMaster) R1/(R1+R2) RH [%] Average temperature: 23 C FIT 1 Hygrometrix Xeritron carbon dioxide at 23 C, serial number: Behavior of the humidity sensors being used within the ID ATLAS volumes Page 19

20 3.4 Comparison of the sensors behavior in carbon dioxide and nitrogen In this test the sensors were firstly placed in pure CO 2 with stable flow. Then after their outputs had stabilized the circumfluent gas was changed to the pure nitrogen. Flow: 2 Nl /s Temperature: C Hygroemtrix Xeritron sensor behavior in different gases R1 resistance [ohm] Pure CO 2 Pure N :26 13:55 14:24 14:52 15:21 15:5 16:19 16:48 time [hh:mm] sn: sn: 2833 Chart 12 R1resistance in nitrogen and carbon dioxide response Hygroemtrix Xeritron sensor behavior in different gases R1 resistance [ohm] Pure CO 2 Pure N :26 13:55 14:24 14:52 15:21 15:5 16:19 16:48 time [hh:mm] sn: sn: 2833 Chart 13 R2 resistance in nitrogen and carbon dioxide response Behavior of the humidity sensors being used within the ID ATLAS volumes Page 2

21 Hygroemtrix Xeritron sensor behavior in different gases.382 R1/(R1+R2) resistance ratio Pure CO 2 Pure N :26 13:55 14:24 14:52 15:21 15:5 16:19 16:48 time [hh:mm] sn: 2833 Chart 14 R1/(R1+R2) ratio in different gases for Hygrometrix Xeritron sn: 2833 Hygroemtrix Xeritron sensor behavior in different gases R1/(R1+R2) resistance ratio Pure CO 2 13:26 13:55 14:24 14:52 15:21 15:5 16:19 16:48 time [hh:mm] sn: Pure N 2 Chart 15 R1/(R1+R2) ratio in different gases for Hygrometrix Xeritron sn: Behavior of the humidity sensors being used within the ID ATLAS volumes Page 21

22 3.5 Time response The aim of the test was to measure speed of the sensors response on the step change in humidity and to compare the reactions of the tested sensors. Flow: 25 Nl /s Temperature: 22 C.45 Response to step change in humidity in nitrogen 22 C R1/(R1+R2) resistance ratio RH 16% RH 31%.4 : :28 :57 1:26 1:55 2:24 2:52 3:21 3:5 4:19 time [hh:mm] R1/(R1+R2) sn: R1/(R1+R2) sn: Chart 16 Time response of Hygrometrix Xeritron sensors Behavior of the humidity sensors being used within the ID ATLAS volumes Page 22

23 4 Summary 4.1 Hygrometrix and Honeywell sensors The initial calibration of the Honeywell and Xeritron sensors was performed in the air environment. The calibration was performed in the climate chamber during four runs with a constant temperature setpoint. The temperature was monitored by set of NTC resistors placed near to the humidity sensors. The average temperatures during the runs were 19.4 C, 29 C, 28.5 and 38.6 C. The highly precise sensor Dew Master was used as a reference for the humidity measurements. The comparison between factory fit and relative humidity calculated from the DewMaster readings was made for the Honeywell sensor (See FIT 1 and FIT 2). The maximal error of the factory fit against our reference Dew Master hygrometer is below 3.5% RH for the values of RH below 45% and below 5% for the values of the RH over 45% toward our calibration (in terms of sensors error). The factory fit of the Hygrometrix sensor was not provided therefore the sensor was only calibrated. From the performed calibrations is clear that the Hygrometrix RH sensor is in air very temperature sensitive. The slope of the calibration curve for the temperature 19.4 o C (See FIT 4) is almost 1.5 times higher than for the temperature 38.6 o C (See FIT 3). Moreover the acquired data for 19.4 o C proofs that the Hygrometrix output is not strictly linear (dispersion of the linear fit is R 2 =.9315). Chart 4 and Chart 5 show how the Honeywell and Hygrometrix sensor outputs differ for the air with the same humidity but different temperature ( t 1 C). The Hygrometrix and Honeywell sensors were calibrated also in the nitrogen and carbon dioxide ambiences after the tests performed in the air. The behavior of both sensors differs significantly depending on the surrounding gas as can be seen from the collected data. Sensors outputs are different in pure nitrogen, in pure carbon dioxide (gases without humidity) and even in comparison with the calibration equations obtained for the air. The test aiming to compare the behavior of the sensors in the pure gases was performed aside the standard calibration. The sensors were exposed to the pure nitrogen and immediately after their outputs stabilize the gas was switched to carbon dioxide. The flow and the temperature of both gases were the same. The Chart 5 shows the difference in the sensors outputs - the step change in the output is evident especially for the Hygrometrix sensor. The results also confirm the temperature dependence of the Hygrometrix sensor seen in the air (see chapters 2.2.2). The difference in the sensor s output was 9% of RH for the temperature change of 8.4 C (from 21.1 C to 29.5 C), constant humidity and flow. The RH value given by the Hygrometrix sensor is higher the higher is the temperature which is the opposite of the behavior of relative humidity which is inversely proportional to the temperature. Therefore it is possible that in the environment with some stable relative humidity level and rising temperature the Hygrometrix sensor may give stable output because the behavior of the sensor and behavior of the relative humidity will cancel each other out. The difference of the Honeywell sensor output was for the same temperature step less than 2%. The reaction of both sensors reflecting the step change in humidity is, with regards to different voltage output, the same (see Chart 6 and Chart 7). A problematic feature was found for the Hygrometrix output in nitrogen with the humidity lower than 1% RH. Those values were at the edge of the resolution for the patch panel circuits. The patch panel s output was almost V and the changes of the voltage were so small that they were undetectable by the used electronics. Behavior of the humidity sensors being used within the ID ATLAS volumes Page 23

24 4.2 Hygrometrix Xeritron Two pieces of the Hygrometrix Xeritron sensor were calibrated during tests in the nitrogen and carbon dioxide atmosphere. Since this type of sensor has long response time and our test facilities were not able to keep highly stable humidity level for a long time periods (hours) the accuracy of the results is limited. The fits, the resistances R1, R2 and resistance ratios for every each sensor are different (see FIT 7, FIT 9,FIT 8 and FIT 1), but with respect to a stability of the humidity level both fits seem to be linear. Those parameters also differ depending on the used gas. The difference between the sensor ratios for nitrogen and carbon dioxide is approximately.1 which corresponds to ~6% step of the RH when the sensor is moved from the CO 2 to the N 2 respectively ~9% when is moved vice versa (with the corresponding fits from chapter 3.3). The Hygrometrix Xeritron sensors are giving reasonable response in the whole range of the humidity (from % RH), but their response time is very long. The sensor outputs stabilize after more than three hours for the humidity step of 15% RH as can be seen on the chart Reference [1]: Honeywell HIH-4 Humidity sensors, [2]: HMX22 Humidity/Temperature Sensor with Built in I 2 C E PROM [3]: Evaluation of a humidity sensor for use in an environment exposed to radiation. Richard Brenner, Nils Bingefors, Bjarte Mohn, Journal of Testing and Evaluation, Sept 22 Vol XX, No. X Behavior of the humidity sensors being used within the ID ATLAS volumes Page 24