Calibration of relative humidity
instruments in low pressure, low
temperature, CO2 environment
M. Genzer, J. Polkko, T. Nikkanen, M. Hieta, A.-M. Harri
Humicap®
Humicap® humidity sensor chip is a capacitive
sensor. It contains an active polymer film that
changes capacitance as function of relative
humidity and temperature. The capacitance is also
affected by CO2 gas, and the magnitude of the
effect is dependent on both temperature and
pressure.
Accurate housekeeping temperature measurements
as close to Humicap® as possible are needed to
interpret the humidity results.
Humicap® can be regenerated or defrosted with an
integrated heating resistor.
Humicap® used in REMS-H and DREAMS-H
2
Humicap® capacitance varies as
function of temperature and relative
humidity. Dry point (~0%RH)
capacitance changes only a little with
temperature, while saturation point
(~100% RHice) capacitance falls as
temperature gets colder, thus
narrowing the dynamic range. Also
time response of the Humicap® gets
longer in cold temperatures.
In each temperature point,
capacitance between dry point and
saturation point is nearly linear.
Capacitance
Humicap®
behavior in
cold
Saturation points
Temperature / °C
Dry points
3
CO2 effect on Humicap
DREAMS-H ‘d’ Difference in capacitance
between vacuum
and ~8hPa CO2
0
0,00
-40
-20
-0,01
-80
0
-60
-40
-0,01
0
Humi 1 diff
Humi 2 diff
-0,02
-0,02
Humi 3 diff
-0,03
Humi 1 pF difference
-0,03
average diff
-0,04
Humi 2 pF difference
-0,04
-0,05
-0,05
Humi 3 pF difference
-0,06
-0,06
-0,07
-0,07
Poly. (pF difference
average)
-0,08
-0,08
T °C
-20
pF difference
average
pF
-60
pF
-80
REMS-H REF difference in capacitance
between vacuum and ~8 hPa CO2
-0,09
T °C
-0,09
Dry conditions (dry CO2)
The CO2 effect is larger in higher pressures and cold temperatures. Taking into
account also the dynamic range of the Humicap®‚ in Martian pressure range the
effect is negligible in temperatures > ~ -10°C.
4
Humidity in CO2 testing system set-up
J. Polkko, FMI, 23.3.-17
Temperature controlled cabinet
Pressure
transmitter
Vacuum gauge
Test
chamber
I/F system
Gas
(CO2 and water vapour)
Water level
Transparent lid
CO2
Water
Saturated conditions
Humicap capacitance reaction to wet gas
-60°C, 9.5 mbar, CO2+H2O
-60°C -> -70°C, CO2+H2O
6,30
6,30
6,20
6,20
6,10
6,10
6,00
pF
pF
6,00
5,90
5,90
Series1
5,80
5,80
5,70
5,70
5,60
5,60
5,50
5,50
0
3600
7200
10800
Time (s)
14400
Switch from dry to wet gas
31.3.2017
18000
0 14400
28800
43200
57600
72000
86400
100800
115200
129600
144000
158400
172800
187200
201600
216000
230400
244800
Time (s)
Temp switch -60C…-70C
7
-70C -> -60°C, 9 mbar, CO2+H2O
6,30
6,20
6,10
pF
6,00
5,90
Humicap pF
5,80
5,70
5,60
5,50
0
7200 14400 21600 28800 36000 43200 50400 57600 64800 72000 79200 86400 93600
Time (s)
Temp switch -70C…-60C
31.3.2017
8
Dry CO2 flow through the chamber
-60°C, 9.5 mbar, switch to dry CO2
6,1
6
pF
5,9
5,8
Humicap
5,7
5,6
5,5
0
7200
14400
21600
28800
36000
43200 Time
50400
(s) 57600
64800
72000
79200
86400
93600 100800
Switch to dry CO2
31.3.2017
10
-60°C, 9.5 mbar CO2 -> vacuum
6,30
10
9
6,20
8
6,10
7
6
5,90
5
4
5,80
mbar
pF
6,00
Humicap pF
Pressure
3
5,70
2
5,60
5,50
93000
1
96600
100200
0
103800
Time (s)
31.3.2017
11
-60°C, CO2+H2O, 7 mbar -> 9.5 mbar
6,050
10
6,045
9,5
6,040
9
6,035
pF
6,025
8
6,020
7,5
mbar
8,5
6,030
Humicap pF
P ref
6,015
7
6,010
6,5
6,005
6,000
0
31.3.2017
3600
7200
10800
14400
Time (s)
18000
21600
25200
6
28800
12