umidity Sensors Theory and Behavior Humidity Sensors
HIH Series
142
Honeywell Sensing and Control 1-800-537-6945 USA
1-815-235-6847 International 1-800-737-3360 Canada
REFERENCE AND APPLICATION DATA
HUMIDITY SENSOR THEORY AND BEHAVIOR
SENSOR CONSTRUCTION: Relative humidity sensors use an
industrially proven thermoset polymer, three layer capacitance
construction, platinum electrodes and except for high temper-
ature versions (shown bottom), on-chip silicon integrated volt-
age output signal conditioning. (RHIC Sensor).
In operation, water vapor in the active capacitors dielectric layer
equilibrates with the surrounding gas. The porous platinum
layer shields the dielectric response from external influences
while the protective polymer over layer provides mechanical
protection for the platinum layer from contaminants such as dirt,
dust and oils. A heavy contaminant layer of dirt will slow down
the sensors response time because it will take longer for water
vapor to equilibrate in the sensor.
TEMPERATURE & HUMIDITY EFFECTS: The output of all
absorption based humidity sensors (capacitive, bulk resistive,
conductive film, etc.) are affected by both temperature and
%RH. Because of this, temperature compensation is used in
applications which call for either higher accuracy or wider oper-
ating temperature ranges.
When temperature compensating a humidity sensor, it is best to
make the temperature measurement as close as possible to the
humidity sensors active area, i.e. within the same moisture
micro-environment. This is especially true when combining RH
and temperature as a method for measuring dew point.
Industrial grade Humidity and Dew Point instruments incorpo-
rate a 1000 ohm Platinum RTD on the back of the ceramic sensor
substrate for unmatched temperature compensation measure-
ment integrity. No on-chip signal conditioning is provided in
these high temperature sensors.
VOLTAGE OUTPUT: The RHIC sensor linear voltage output is a
function of V
supply,
%RH and temperature. The output is ratio-
metric, i.e. as the supply voltage rises, the output voltage rises
in the same proportion. A surface plot of the sensor behavior for
temperatures between 0
°
C and 85
°
C is shown in the graph
below. This surface plot is well approximated by a combination
of two equations:
1. A Best Fit Line at 25
°
C, or a similar, sensor specific
equation at 25
°
C. The sensor independent typical Best
Fit Line at 25
°
C (bold line in graph) is:
V
out
V
supply
(0.0062 (%RH) + 0.16)
A sensor specific equation can be obtained from an RH
sensor printout. The printout equation assumes V
supply
5VDC and is included or available as an option on every
sensor.
2. A sensor independent equation which corrects the %RH
reading (from the Best Fit Line Equation) for temperature, T:
True RH (%RH)/(1.0546.00216 T); T
°
C
Or True RH (%RH)/(1.093.0012 T); T
°
F
The equations above match the typical surface plot (Best Fit Line
at 25
°
C) or the actual surface plot (sensor specific equation at
25
°
C) to within the following tolerances:
±1% for T>20
°
C
±2% for 10
°
C<T<20
°
C
±5% for T<10
°
C
Our dewpoint instruments account for the sensor specific ver-
sion of the surface plot directly via a look up table.
NOTE: Convert the observed output voltage to %RH values via
the first equation before applying the second equation. Humidity Sensors
HIH Series
Honeywell Sensing and Control 1-800-537-6945 USA
1-815-235-6847 International 1-800-737-3360 Canada
143
REFERENCE AND APPLICATION DATA
CONDENSATION AND WETTING
CONDENSATION occurs whenever the surface temperature of
the sensors active area drops below the ambient dew point of
the surrounding gas. Condensation forms on the sensor (or any
surface) even if the surface temperature only momentarily drops
below the ambient dew point. Small temperature fluctuations
near the sensor can unknowingly cause condensation to form
when operating at humidity levels above 95%.
While quick to condense, water is slow to evaporate in high
humidity conditions (i.e. when the surface temperature of the
sensor is only slightly above the ambient dew point.) Because of
this, a sensors recovery period from either condensation or
wetting is much longer than its normal time response. During
recovery, the sensor outputs a constant 100% RH signal regard-
less of the ambient RH.
When an application calls for continuous monitoring of RH at
humidity levels of 90% and above, take steps to avoid intermit-
tent condensation. Some strategies are:
1. Maintain a good air mixing to minimize local temperature
fluctuations.
2. The HIH-3602-A and -C use a sintered stainless steel filter to
protect the sensor from splashing. A hydrophobic coating
further suppresses condensation and wetting in rapidly
saturating and de-saturating or splash prone environ-
ments.
3. Heat the RH sensor so that the active area is hotter than the
local dew point. This can be done through an external
heater or by self heating of the CMOS RH chip by operating
it at a higher supply voltage.
NOTE: Heating an RH sensor above ambient temperature
changes its calibration and makes it sensitive to thermal disturb-
ances such as air flow. When contemplating such an approach,
MICRO SWITCH recommends selecting an HIH-3602 type sen-
sor and getting application technical support.
INTEGRATED SIGNAL CONDITIONING
All RH sensors quickly recover from condensation or wetting
with no shift in calibration. However, after 24 hour or longer
exposures to either high >95% RH or continuous condensation,
an upward shift of 2% to 3% RH may occur. This shift is repeat-
able and can be reversed by placing the sensor in a low 10% RH
environment for a 10 hour period.
Silicon integrated humidity sensors (RHIC relative humidity
integrated circuit) incorporate signal conditioning circuitry on-
chip with the sensing capacitor. These RHIC humidity sensors
are laser trimmed so that at V
supply
5 V, the output voltage
typically spans 0.8V to 3.9V for the 0% RH to 100% RH range at
25
°
C. (Sensor specific calibration data printouts and best fit
lines at 25
°
C are either included or available as an option on
every sensor.)
The HIH-3602-C incorporates a RHIC humidity sensor, a 1000
platinum RTD and anti-static protection in a single TO-5 can.
RHIC based sensors are factory calibrated, micro-power de-
vices with either individual calibration and/or good unit-to-unit
interchangeability. These features help OEM manufacturers
avoid in-house humidity calibration costs and extend battery life
in portable instruments. Improved accuracy can be obtained by
tuning system electronics to account an individual sensors Best
Fit Line at 25
°
C.
Reference/Index