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CZECH TECHNICAL UNIVERSITY IN PRAGUE CERN

Humidity sensors

Behavior in different environments

Supervisor: Vaclav Vacek Collaborators: Martin Doubek, Martin Janda and Michal Vitek

28.9.2009

Behavior of the humidity sensors being used within the ID ATLAS volumes

Content Content ...................................................................................................................................................... 2 1 Introduction....................................................................................................................................... 3 2 Hygrometrix HMX2200 and Honeywell HIH 4000 sensors ............................................................ 4

2.1 Sensors description..................................................................................................................... 4 2.2 Behavior of the humidity sensors in air...................................................................................... 5

2.2.1 Testing facility .................................................................................................................... 5 2.2.2 Calibration in the climate chamber ..................................................................................... 6

2.3 Behavior of the humidity sensors in nitrogen and carbon dioxide ambience .......................... 10 2.3.1 Testing facility .................................................................................................................. 10 2.3.2 Sensors calibration ............................................................................................................ 11 2.3.3 Time response ................................................................................................................... 14 2.3.4 Thermal stability ............................................................................................................... 15

3 Hygrometrix Xeritron sensor .......................................................................................................... 17 3.1 Introduction .............................................................................................................................. 17 3.2 Sensor description .................................................................................................................... 17 3.3 Calibration in nitrogen and carbon dioxide atmosphere .......................................................... 18

3.3.1 Nitrogen ............................................................................................................................ 18 3.3.2 Carbon dioxide.................................................................................................................. 19

3.4 Comparison of the sensors behavior in carbon dioxide and nitrogen ...................................... 20 3.5 Time response........................................................................................................................... 22

4 Summary ......................................................................................................................................... 23 4.1 Hygrometrix and Honeywell sensors ....................................................................................... 23 4.2 Hygrometrix Xeritron............................................................................................................... 24

5 Reference ........................................................................................................................................ 24


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-4000 Series (further as Honeywell), Hygrometrix HMX 2200 and Hygrometrix Xeritron. Sensors were due to the unavailability of the Hygrometrix HMX 2200 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.


2 Hygrometrix HMX2200 and Honeywell HIH 4000 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 = 100).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 HMX2200 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 ±0.5 %RH Response time 15 s Operating temperature

-40 85 oC

Operating humidity

0 100 %RH

Table 1 – Honeywell sensors performance specifications

Model HIH4000 Channel 336 Wafer t1 MPR t1 Calculated values at 5 V Vout @ 0 %RH Vout @ 75.3 %RH

0.785 V 3.040 V

Linear output for 2 %RH Accuracy @ 25 oC Zero offset Slope RH

0.785 V 29.942 mV / %RH (Vout-zero offset) / slope (Vout-0.785) / 0.0299

Ratio metric response for 0% To 100 %RH Vout

Vsupply * (0.1570 to 0.7559)

Table 2 – Honeywell factory fit

Table 3 – Hygrometrix sensor performance specifications

Parameter Min Typical Max Unit Response time (10 to 90%)

10 s

Operating temperature

-40 85 oC

Full scale detection range

.001 100 %RH

Figure 1 – Hygrometrix patch panel schematic


2.2 Behavior of the humidity sensors in air

2.2.1 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 2020. 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 100 mV and the precision of the ELMB’s readings was ±0.05% from the full input scale.

0 10 20 30 40 50 60 70 80 90 100

100

90

80

70

60

50

40

30

20

10

0

Relative humidity %

Chart 1 - Allowed values of climate chamber

Hygrometrix 9V Honey well 5V

Table 4 – Sensors supply

Figure 2 – Air testing facility schematic

;

`

1

2

3 2

4

4

5

6 678

1 0

9

9

1 13 1 0

1 C lim a te c h a m b e r2 C h a m b e r ’s f a n3 C h a m b e r ’s r e fe r e n c e h u m id i t y s e n s o r4 C a l ib r a te d h u m id i t y s e n s o r5 A r e a o f c a l ib r a te d h u m id i t y s e n s o r s6 R e fe r e n c e d e w p o in t m e te r7 D e w p o in t s e n s o r o f r e f e r e n c e d e w p o in t m e te r8 F a n m a in t a in in g a ir f lo w t h r o u g h t t h e D e w m e te r9 P o w e r s u p p ly1 0 E L M B u n i t1 1 N o te b o o k w it h C A N B u s c a r d

1 1


2.2.2 Calibration in the climate chamber

Measurements in the climate chamber were performed during multiple runs within period of almost three weeks (27.7.2009 - 12.8.2009 and 7.9.2009 – 9.9.2009). 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 40°C and one with the decreasing humidity at temperature 20°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: 20%, 30%, 40% and 50% RH and the temperatures were 30°C and 40 °C.

Calibration in climat chamber 20oC

00.5

11.5

22.5

33.5

4

13:12 14:24 15:36 16:48 18:00 19:12 20:24 21:36

time [hh:mm ]

Vol

tage

[ V

]

1012141618202224262830

Tem

pera

ture

[ oC

]Honeywell Hygrometrix Temperature

90%70%

50%

30%

Chart 2 – Climate chamber calibration in the air at 20°C

Calibration in climat chamber40oC

0

0.5

1

1.5

2

2.5

3

8:24 9:36 10:48 12:00 13:12 14:24 15:36 16:48 18:00 19:12

time [hh:mm ]

Volta

ge [

V ]

25

30

35

40

45

50

55

60

65

Tem

pera

ture

[ oC

]

Honeywell Hygrometrix Temperature

10%20%

30%40% 50%

60%70%

Chart 3 – Climate chamber calibration in the air at 40°C


2.2.2.1 Summary of the calibration runs in climate chamber Honeywell sensor:

Temperature: 38.6°C Factory fit: RH=33.398U-26.217 Reference sensor: DewMaster

FIT 1 – Honeywell in air at 38.6°C

Temperature: 19.4°C Factory fit: RH=33.398U-26.217 Reference sensor: DewMaster

FIT 2 – Honeywell in air at 19.4°C

Calibration of Honeywell relative humidity sensor (~38.6°C)

Factory fitRH = 33.398x - 26.217

HoneywellRH = 36.996x - 30.405

0

10

20

30

40

50

60

70

1 1.5 2 2.5 3

Voltage [V]

Rela

tive

hum

idity

[%]

Honneywell Factory fit

Honeywell Reference (DewMaster)

Factory fit

Error

Voltage [V] RH [%] RH [%] RH [%]

1.189 13.230 13.509 0.2781.191 13.285 13.569 0.2841.413 21.894 20.985 0.9091.413 21.496 20.975 0.5211.624 30.057 28.006 2.0511.625 29.622 28.050 1.5721.842 38.135 35.317 2.8181.843 38.566 35.320 3.2452.063 46.022 42.669 3.3532.063 46.114 42.669 3.4462.298 55.263 50.536 4.7272.301 54.622 50.630 3.9912.565 63.429 59.456 3.973

2.563 64.223 59.368 4.855

Average temp [°C] 38.644

Calibration of Honeywell relative humidity sensor (~19.4°C)

Factory fitRH = 33.398x - 26.217

RH HoneywellRH = 31.232x - 23.965

30

40

50

60

70

80

90

1.7 2.2 2.7 3.2 3.7

Voltage [V]

Rel

ativ

e H

umid

ity [%

Honneywell Factory fit

Honeywell Reference (DewMaster)

Factory fit

Error

Voltage [V] RH [%] RH [%] RH [%]

3.454 84.080 89.135 5.0553.457 84.595 89.233 4.6383.019 70.059 74.624 4.5653.020 70.032 74.651 4.6192.554 55.151 59.075 3.9232.545 55.266 58.788 3.5221.988 38.490 40.170 1.680

1.987 38.463 40.153 1.690

Average temp [°C] 19.450


Hygrometrix sensor:

Temperature 38.6°C Reference sensor: DewMaster

FIT 3 – Hygrometrix in air at 38.6°C

Temperature 19.4°C Reference sensor: DewMaster

FIT 4 – Hygrometrix in air at 19.4°C

Calibration of Hygrometrix relative humidity sensor (~38.6°C)

RH = 71,941x - 4,5342

0

10

20

30

40

50

60

70

0.2 0.4 0.6 0.8 1

Voltage [V]

Rel

ativ

e hu

mid

ity [%

]

Hygrometrix Reference (DewMaster)

Voltage [V] RH [%]

0.246 13.230

0.248 13.285

0.365 21.894

0.366 21.496

0.479 30.057

0.478 29.622

0.593 38.135

0.591 38.566

0.704 46.022

0.709 46.114

0.825 55.263

0.826 54.622

0.951 63.429

0.950 64.223 Average temp [°C] 38.644

Calibration of Hygrometrix relative humidity sensor (~19.4°C)

RH = 105.47x - 7.649630

40

50

60

70

80

90

0.4 0.5 0.6 0.7 0.8 0.9

Voltage [V]

Rel

ativ

e H

umid

ity [%

]

Hygrometrix Reference (DewMaster)

Voltage [V] RH [%]

0.820 84.080 0.824 84.595 0.753 70.059 0.754 70.032 0.659 55.151 0.653 55.266 0.410 38.490

0.410 38.463 Average temp [°C] 19.450


2.2.2.2 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 20% RH to 50% 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 [%]

1.653 0.342 29.184 24.8181.880 0.456 29.229 32.5372.122 0.572 29.184 40.2612.373 0.690 28.755 48.172

1.501 0.438 38.719 21.0571.732 0.549 38.412 29.358

1.970 0.662 38.622 37.2502.218 0.775 38.477 46.355

Table 5 – Summary of the tests with the same moisture set-points and 10°C temperature difference Chart 4 – Comparison of the sensors fits for the same moisture set-points and 10°C temperature difference

Hygrometrix Air

29°C: RH= 67.238x + 1.8132 38.5°C: RH = 74.639x - 11.736

20

25

30

35

40

45

50

0.3 0.4 0.5 0.6 0.7 0.8

Voltage [V] 29°C 38.5°C

Rel

ativ

e hu

mid

ity [%

]

HoneywellAir

29°C: RH = 32.366x - 28.513 38.5°C: RH = 35.089x - 31.602

20

25

30

35

40

45

50

1.5 1.7 1.9 2.1 2.3 2.5

Voltage [V] 29°C 38.5°C

Rel

ativ

e hu

mid

ity [%

]


2.3 Behavior of the humidity sensors in nitrogen and carbon dioxide ambience

2.3.1 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% CO2 and 99.995 % N2 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 2.2.1.

Figure 3 – Nitrogen and carbon dioxide testing facility schematic


2.3.2 Sensors calibration The main goal of the experiments performed between 12.8.2009 and 5.9.2009 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: 150 Nl /s

Temperature: 21.79°C and 30.64°C Reference sensor: DewMaster

FIT 5 – Hygrometrix ad Honeywell sensor in nitrogen

Honeywell Hygrometrix Reference (DewMaster)

Voltage [V] Voltage [V] RH [%]

0.899 0.018 2.5870.899 0.018 3.0340.996 0.017 5.2461.041 0.017 5.8791.036 0.017 6.662

1.089 0.017 7.7091.110 0.017 8.070

1.125 0.019 8.7041.161 0.019 9.9961.220 0.051 11.9441.189 0.052 11.9901.258 0.089 14.5761.255 0.102 14.8761.357 0.106 17.3411.433 0.163 20.0861.416 0.200 20.8241.560 0.218 24.7611.625 0.253 26.8971.779 0.322 32.3052.793 0.818 66.821

2.939 0.881 72.169 Average

temp [oC] 21.79

Hygrometrix N2 ~21,79 oC

y = 72.158x + 8.3098

0

10

20

30

40

50

60

70

80

0.00 0.20 0.40 0.60 0.80 1.00

Sensor output V

RH

%

Honeywell N2 ~21,79 oC

y = 34.454x - 29.186

0

10

20

30

40

50

60

70

80

0.8 1.3 1.8 2.3 2.8 3.3

Sensor output V

RH

%

Honeywell Hygrometrix Reference (DewMaster)


1.500457 0.346702 25.101681.524261 0.371213 26.536881.574747 0.394135 28.617051.834193 0.509291 40.40579

1.76092 0.470574 36.69787 Average temp

[oC] 30.64


Carbon dioxide atmosphere:

Flow: 150 Nl /s Temperature: 21.8°C and 30.31°C Reference sensor: DewMaster

Honeywell Hygrometrix Relative

humidity DewMaster


0.850 0.309 1.020

0.922 0.340 3.091

0.939 0.348 3.775

0.957 0.362 4.629

0.968 0.359 5.059

0.995 0.362 6.289

1.030 0.379 8.029

1.006 0.382 8.113

1.045 0.385 9.293

1.093 0.405 11.268

1.157 0.434 14.511

1.227 0.483 17.588

1.937 0.828 54.470

Average temp [oC] 21.80

Hygrometrix CO2 ~21.8 oC

y = 104.32x - 31.809

0

10

20

30

40

50

60

0.3 0.4 0.5 0.6 0.7 0.8 0.9

Sensor output [V]

RH [%

]

Honeywell CO2 ~21.8 oC

y = 50.142x - 43.166

0

10

20

30

40

50

60

0.7 1.2 1.7 2.2

Sensor output [V]

RH [%

]

Honeywell Hygrometrix Relative humidity

DewMaster


0.849681 0.390042 1.050324

1.336793 0.588403 25.60678

1.377069 0.602397 27.72586

1.47877 0.64789 33.43123

1.51116 0.661455 35.29255

1.666429 0.728427 43.37218

Average temp [oC] 30.31

FIT 6 –Hygrometrix and Honeywell in carbon dioxide


Flow: 200 Nl /s Temperature: 20.64°C Reference sensor: DewMaster

Sensors behavior in different gases

0.3

0.5

0.7

0.9

1.1

1.3

1.5

9:57 10:04 10:12 10:19 10:26 10:33 10:40 10:48

time [hh:mm]

Hon

eyw

ell s

enso

r out

put

[V]

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Hyg

rom

etrix

sen

sor

outp

ut [V

]

Honeywell Hygrometrix

PureN2

PureCO2

Reconection of pipes

Chart 5 – Comparison of the sensors outputs in nitrogen and carbon dioxide.


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 10%, and 20% RH. The testing facility didn’t allow us to set exactly the same moisture content in all tests.


Response to step change in humidity in Nitrogen ~23°C VOLTAGE OUTPUT

0.5

0.7

0.9

1.1

1.3

1.5

1.7

00:00 02:53 05:46 08:38 11:31time [ mm:ss ]

Out

put s

ingn

al o

f H

oney

wel

l sen

sor [

V ]

00.050.10.150.20.250.30.350.40.450.5

Out

put s

ingn

al o

f H

ygro

met

rix s

enso

r [ V

]Honeywell Hygrometrix

RH 8.48%

RH 21%

Chart 6 – Response to humidity change


Response to step change in humidity in Nitrogen ~23°CVOLTAGE OUTPUT

0.5

0.7

0.9

1.1

1.3

1.5

1.7

0:00 0:07 0:14 0:21time [ mm:ss ]

Out

put s

ingn

al o

f Ho

neyw

ell s

enso

r [ V

]

00.050.10.150.20.250.30.350.40.450.5

Out

put s

ingn

al o

f Hy

grom

etrix

sen

sor [

V ]

Honeywell Hygrometrix

RH 21% RH 4.97%

This peak is caused by "bubbler" disconection

Chart 7 –Response to humidity change


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 2.3.2 were used (used fits are referred in the chart titles).

Response of Hygrometrix sensor to the temperature change[~21.8 oC CO2 FIT ]

0

2

4

6

8

10

12

12:00 12:28 12:57 13:26 13:55 14:24 14:52 15:21

time [hh:mm]

RH

[%]

05101520253035404550

Tem

pera

ture

[o C

]

Hygrometrix Average temperature

Chart 8 – Temperature influence on Hygrometrix sensor in carbon dioxide

Response of Honeywell sensor to the temperature change[~21.8 oC CO2 FIT ]

-2

0

2

4

6

8

10

12

12:00 12:28 12:57 13:26 13:55 14:24 14:52 15:21

time [hh:mm]

RH

[%]

0510152025

3035404550

Tem

pera

ture

[o C

]

Honeywell Average temperature

Chart 9 – Temperature influence on Honeywell sensor in carbon dioxide


Response of Honeywell sensor to the temperature change [~21.8 °C N2 FIT ]

-2

-1

0

1

2

3

4

5

15:36 16:48 18:00 19:12 20:24 21:36

time [hh:mm]

RH [%

]

0

5

10

15

20

25

30

35

Tem

pera

ture

[°C

]

Honeywell Average temperature

Chart 10 - Temperature influence on Honeywell sensor in nitrogen

Response of Hygrometrix sensor to the temperature change [~21.8 °C N2 FIT ]

0

2

4

6

8

10

12

14

16

18

15:36 16:48 18:00 19:12 20:24 21:36

time [hh:mm]

RH

[%]

0

5

10

15

20

25

30

35

Tem

pera

ture

[°C]

Hygrometrix Average temperature

Chart 11- Temperature influence on Hygrometrix sensor in nitrogen


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 2.3.1.

3.2 Sensor description

Hygrometrix Xeritron is based on organic and inorganic crystals. These crystals react on moisture by hygro-mechanical 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 temperature

-40 125 oC

Operating humidity

0 100 %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:

baRR

RRH ++

=21

1.

(1)


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.

3.3.1 Nitrogen Flow: 150 Nl /s Temperature: 22.0°C Reference sensor: DewMaster

Hygrometrix Xeritron sn: 28303N2 22°C

y = 657.84x - 247.09R2 = 0.9809

0

5

10

15

20

25

30

35

0.37 0.38 0.39 0.40 0.41 0.42 0.43

R1/(R1+R2)

Rel

ativ

e hu

mid

ity [%

]

Serial number: 28303

Hygrometrix Xeritron

Reference (DewMaster)

R1/(R1+R2) RH [%] 0.372927 00.387722 6.1180.396085 12.8540.400861 16.1810.418264 26.8960.419519 31.154

Average temperature: 22°C

FIT 7 – Hygrometrix Xeritron nitrogen at 22°C, serial number: 28303

Hygrometrix Xeritorn sn: 28164N2 22°C

RH = 689.32x - 278.84R2 = 0.9774

0

5

10

15

20

25

30

35

0.4 0.41 0.42 0.43 0.44 0.45

R1/(R1+R2)

Rel

ativ

e hu

mid

ity [%

]

Serial number: 28164

Hygrometrix Xeritron

Reference (DewMaster)

R1/(R1+R2) RH [%]

0.401505 00.416395 6.1180.424425 12.8540.428898 16.1810.444809 26.8960.446284 31.154


FIT 8 – Hygrometrix Xeritron in nitrogen at 22°C, serial number: 28164


3.3.2 Carbon dioxide Flow: 150 Nl /s Temperature: 22.0°C Reference sensor: DewMaster

Hygrometrix Xeritron sn: 28303CO2 23°C

RH = 922.26x - 359.92R2 = 0.9667

0

5

10

15

20

25

30

0.38 0.39 0.4 0.41 0.42

R1/(R1+R2)

Rel

ativ

e hu

mid

ity [%

]

Serial number: 28303 Hygrometrix

Xeritron Reference

(DewMaster)R1/(R1+R2) RH [%]

0.388400 00.399912 7.9790.399058 6.3310.400569 9.6180.407416 13.4880.405571 15.7070.412884 21.1670.417433 26.352


FIT 9 – Hygrometrix Xeritron carbon dioxide at 23°C, serial number: 28303

Hygrometrix Xeritron sn: 28164CO2 23°C

RH = 927.36x - 387.21R2 = 0.9699

05

1015202530

0.41 0.42 0.43 0.44 0.45

R1/(R1+R2)

Rel

ativ

e hu

mid

ity [%

]

Serial number: 28164 Hygrometrix

Xeritron Reference

(DewMaster)R1/(R1+R2) RH [%]

0.415745 00.427300 7.9790.426059 6.3300.427724 9.6180.434445 13.4890.432832 15.7080.440082 21.1670.444645 26.352


FIT 10 – Hygrometrix Xeritron carbon dioxide at 23°C, serial number: 28164


3.4 Comparison of the sensors behavior in carbon dioxide and nitrogen In this test the sensors were firstly placed in pure CO2 with stable flow. Then after their outputs had stabilized the circumfluent gas was changed to the pure nitrogen.


Hygroemtrix Xeritron sensor behavior in different gases

390

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13:26 13:55 14:24 14:52 15:21 15:50 16:19 16:48

time [hh:mm]

R1

resi

stan

ce [

ohm

]

sn: 28164 sn: 28303

Pure N2

PureCO2

Chart 12 – R1resistance in nitrogen and carbon dioxide response


600

610

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670

13:26 13:55 14:24 14:52 15:21 15:50 16:19 16:48

time [hh:mm]

R1

resi

stan

ce [

ohm

]

sn: 28164 sn: 28303

PureCO2 Pure N2

Chart 13 – R2 resistance in nitrogen and carbon dioxide response



0.37

0.372

0.374

0.376

0.378

0.38

0.382

13:26 13:55 14:24 14:52 15:21 15:50 16:19 16:48

time [hh:mm]

R1/

(R1+

R2)

resi

stan

ce ra

tio

sn: 28303

PureCO2

Pure N2

Chart 14 – R1/(R1+R2) ratio in different gases for Hygrometrix Xeritron sn: 28303


0.4

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13:26 13:55 14:24 14:52 15:21 15:50 16:19 16:48

time [hh:mm]

R1/

(R1+

R2)

resi

stan

ce ra

tio

sn: 28164

PureCO2

Pure N2

Chart 15 – R1/(R1+R2) ratio in different gases for Hygrometrix Xeritron sn: 28164


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: 250 Nl /s Temperature: 22°C Reference sensor: DewMaster

Response to step change in humidity in nitrogen22°C

0.4

0.405

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0.415

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0:00 0:28 0:57 1:26 1:55 2:24 2:52 3:21 3:50 4:19

time [hh:mm]

R1/

(R1+

R2)

resi

stan

ce ra

tio

R1/(R1+R2) sn: 28164 R1/(R1+R2) sn: 28164

RH 16%

RH 31%

Chart 16 – Time response of Hygrometrix Xeritron sensors


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 set-point. 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 19.4oC (See FIT 4) is almost 1.5 times higher than for the temperature 38.6oC (See FIT 3). Moreover the acquired data for 19.4oC proofs that the Hygrometrix output is not strictly linear (dispersion of the linear fit is R2 = 0.9315). 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 ≈ 10°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 10% RH. Those values were at the edge of the resolution for the patch panel circuits. The patch panel´s output was almost 0V and the changes of the voltage were so small that they were undetectable by the used electronics.


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 10), 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 0.01 which corresponds to ~6% step of the RH when the sensor is moved from the CO2 to the N2 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 0% 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 17.

5 Reference [1]: Honeywell HIH-4000 Humidity sensors, www.honeywell.com/sensing [2]: HMX2200 Humidity/Temperature Sensor with Built in I2C 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 2002 Vol XX, No. X

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