# 10 RH sensor calibration and measurements by 4jV64J

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```									           RH Calibration and Measurements
Humidity Definitions and Sensors
   Relative and Absolute RH Definitions
   RH Measurements and Sensitivity to Temperature
   Response Time, Stabilization Time and Ambient Airflow
   Sensor Types and Principles of Measurement
Humidity Providers and Reference Standard
   Humidity Generation Principle and Calibration
   Saturated Salt Solutions According to OIML R121:1996
Uncertainty in RH Measurement and Calibration
   Sensor Long Term Stability, Hysteresis, Contamination and Regeneration
   Traceability Diagram
   Uncertainty Budgets on RH Reference and Sensor
Calibration of RH Sensors
   Calibration Procedure
   Certificates of Calibration

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Humidity Definitions
Evaporation of water generates humidity in the air
   Humidity is the presence of water vapor in the air.
   Water vapor in the air comes from evaporated water.
   When water evaporates the steam contributes to a raise in the
barometric pressure.

When water evaporates from lakes and
seas and mixes with the air molecules
O2 and N2, the ambient pressure
will raise. This happens because
the total no of molecules in the air raises

Evaporation will proceed until the no of molecules escaped from the surface is
the same as the no of molecules returned to the liquid form, and the air becomes
saturated with evaporated water at the specific temperature

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Humidity Definitions
Summary of the presence of humidity
   We are talking about two pressures when water evaporates.
   Actual water vapor pressure exists as soon as water
evaporates and the molecules escape into the air. We
abbreviate this as pw.
   Saturation vapor pressure exists when the air holds the full
capacity of water vapor at a given temperature. We
abbreviate this as pws.
   Daltons Law : The total pressure in a ”closed system” is the
sum of all the partial pressures.

P  p  p  pw
Total    N2      O2        H 2O
p   Other

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Humidity Definitions
Absolute Humidity
   Absolute humidity is the water vapor concentration in the air. It is described
as the mass of evaporated water present in a volume of moist air in SI-unit
as g/m3.
   The RH sensor is internally measuring the absolute amount of humidity in
the air. Use this figure together with the ambient temperature for the
calculation of the relative humidity in accordance with the definition given
below.
Relative Humidity
   The relative humidity determines how saturated the air is with evaporated
water at a specific temperature.
   The definition
pw
RH       100 %
   Where:                      pws
   pw : Actual vapor pressure (found indirectly by the sensor)
   pws : Saturation vapor pressure (calculated)

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Humidity Definitions
RH level of 100 % !
   An RH level of 100 % does not mean water is in a liquid form?

   This is due to the fact that the RH level is determined as the amount of
evaporated water in a steam form in the air at a specific temperature.
   When the ambient temperature in the cloud meets the dew point
temperature, then condensation begins and rain occurs.

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Humidity Definitions
Experiment divided into two different states
   Effects of pressure and temperature changes on relative humidity

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Humidity Definitions
pw and pws relation to the barometric pressure
   pw : Water vapor pressure and pws sat. water vapor pressure
   An increase of the barometric pressure does increase the water vapor pressure
(Daltons law). The air’s capability to hold water vapor remains constant.

The graph shows that relative
humidity is not very sensitive
to changes in the barometric
pressure

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Humidity Definitions
pw and pws relation to the ambient temperature
   pw : Water vapor pressure and pws Sat. water vapor pressure
   An increase of the air temperature does increase the air’s capability to hold
water vapor, while the actual water vapor pressure remains constant.

The graph shows that relative
humidity is very sensitive
to changes in the ambient
temperature

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Humidity Definitions
Relation between Isobaric heating and the %RH
   An increase of the temperature does decrease the relative humidity
because the saturation vapor pressure (pws) increases. The actual vapor
pressure (pw) remains constant.

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Humidity Definitions
Relation between Isothermal compression and the %RH
   An increase of the barometric pressure does increase the relative humidity
due to a raise in the actual vapour pressure pw, while the saturation vapour
pressure pws remains constant.

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Relative Humidity Definitions
RH Sensitivity To Temperature
   Green line is the temperature and blue line is the RH level.

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Relative Humidity Definitions
The Dew Point
   The dew point is the temperature where condensation begins, which is
the reason why the dew point is a saturation point when water vapor
starts condensation.
   Dew appears when the ambient temperature is equal to the dew point
temperature, and this is the point where the air becomes saturated with
water vapor.
   The dew point increases when the amount of moisture in the air
increases, which is equal to an increase in actual water vapor pressure
(pw).
   The bigger the difference between air temperature and dew point, the
lower the relative humidity.

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Relative Humidity Definitions
The Dew Point Temperature
   The dew point is associated with relative humidity.
   On the graph: When increasing the dew point, the actual water vapor pressure
increases while the saturation vapor pressure remains constant.

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Relative Humidity Definitions
Dew Point
When the dew point remains constant and when isobaric heating occurs, the relative
humidity will decrease due to an increase of the saturated water vapor pressure (pws). The
actual vapor pressure (pw) remains constant.

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Relative Humidity Definitions
Dew Point
A low dew point temperature means a low water vapor pressure
(pw) is present in the air.

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RH Sensor

RH Measurement Sensibility to Temperature
Problem : The RH sensor is not thermally stabalized with the environment temperature
where the RH is being measured due to the fact that the sensor has not been in the
environment long enough.
Environmental temperature is 23°C and the relative humidity is 50%. There is a
temperature difference between sensor and environment of 1°C due to a sensor
temperature of 24°C.
50%
With pws 23C ) 2811 Pa which is giving pw 
(                                          2811 Pa  1406 Pa
100 %
pw 23C )
(                 1406
t  1C pws 24C ) 2986 Pa giving RH % 
(                                         100 %        100 %  47.1 %RH
pws 24C )
(                 2986

The temperature difference of 1°C between the RH sensor and the environment
causes a systematic error in the RH measurement of 3% RH.
Conclusion: It is vital that the humidity sensor is thermally stabalized with the
environment being measured.

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RH Sensor

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RH Sensor

Response Time and Stabilization Time
   Response time is the time taken to register 63% of the step
change, which is shown below, in the applied RH reference level
in the RH calibration chamber.

100 %

63 %

Time

t       tStab
tStart Res

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RH Sensor

RH Digital Sensor Response Time

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RH Sensor

Response Time at Different Flow Rates

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RH Sensor

Sensor Type
The Classic Plate Capacitor Type
   The capacitive thin-film polymer sensor uses porous conductive alumina
films on a substate. Any change in moisture is detected as a change in
capacitance. The capacitance is measured between two electrodes, shown
in the figure to the left.
   The sensor’s polymer layer is hygroscopic and acts like a sponge. The intake
of water vapor alters the dielectric constant between the plates, which finally
result in a changes the capacitance.
   The sensor characteristic is shown on the graph.

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RH Sensor
Other RH Sensor Types
   Dew Point Hygrometer
   Optical Dew-Point Types
   Measures the absolute humidity when water condenses on a cold
surface (mirror)
   The measurement capability for dew point temperature is
performed with an uncertainty is ±0,1°C for the best hygrometers
   Impedance Dew-Point Types
   Measuring principle is the plate capacitor type
   The dew point temperature can be found with an uncertainty of
app. ±2°C
   Wet and Dry-Bulb Psycrometer (Least Accurate)
   Wet and Dry Consist of Two Matched Temperatures
   Approximate method to convert temperatures to RH
   Uncertainty app. 2-5% RH

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Humidity Providers and References

RH providers used in calibration of RH sensors
The two-pressure/two temperature generator

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Humidity Providers and References
Calculating the RH level
pwsSat (TSat )  f (TSat , PSat )  P
RH %                                        Chm  Sat
pwsChm (TChm )  f (TChm , PChm ) PSat
 Sat is efficiencyof the saturator which is 100%

   The generated level of RH is primarily dependent on the level and
uncertainty of the two temperatures and the two pressures.
   The enhancement factors (f) are used to correct from ideal to real air
properties in both chambers.
   The two different values of pws are the saturation vapor pressures in
the saturation chamber and in the test chamber.

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Humidity Providers and References
The RH Calibration Chamber
   The chamber is based on the 2P/2T principle for humidity generation.
   A liquid flow in all the chamber walls gives the best possible
temperature uniformity during calibration of the RH sensors.
   Mixed flow generation principle is applicable when RH is below 10%.
This requires a dew point hygrometer to be used as reference.

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Humidity Providers and References
Summary of an RH Calibration
   The table below shows a summary of reference calibration results used in the
calculation of the reference RH level during calibration of the RH sensors. The
RH level is found using the equation given in the previous slide.

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Humidity Providers and References

Saturated salt solutions used in verfication of RH sensor reading

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Humidity Providers and References

Saturated Salt Solutions
   Consist of a closed chamber containing a saturated
solution of a specific chemical salt
   The purpose of using the salt solutions is to verify RH
sensor readings before and after a validation study
   Results of the verification shall determine whether the
sensors need a new calibration
   The verfication using the saturated salt solution should
not replace a regular RH calibration due to lack of
traceability

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Humidity Providers and References

Saturated Salt Solutions (Humidity Fix Points)
   Saturated salt solutions generate characteristic values of relative
humidity depending on a chemical salt in a saturated solution in
demineralized water.
   HFP should be used in accordance with the standard OIML R121.
   In the OIML standard the RH values do not vary significantly with
temperature in the range from 20 to 30°C.
   The salt solutions must never be dried out or completely dissolved.
Remaining salt must always be visible in the liquid.
   The uncertainty of using HFP is app. ±1% RH.
   Only use the HFP’s for indicative measurements on RH sensors and
use the results to assess whether a calibration is necessary.

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Humidity Providers and References

Procedure For Using HFP’s
   Start the RH data logging in ValSuite.
   Install the data loggers/sensors in the HFP chambers using
   Put the chambers and sensors in a temperature stable
environment to stabalize for 3 hours.
   Remove the data loggers/sensors and read the loggers
using ValSuite.
   Perform a statistical analysis of the aquired data.

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RH Sensor Measurement Uncertainty
Measurement Uncertainty In General

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RH Sensor Measurement Uncertainty

Contamination and Regeneration
   RH sensors used in customer applications can be contaminated by
dust, chemical substances and water soluables which can alter the
sensors element dielectric constant and subsequently the sensor
   RH sensors are typically de-contaminated by peforming a
regeneration.
   The regeneration process is consisting of:
   24 hours of baking at 100°C
   48 hours of rehydration at 100% RH
   The sensor is typically returned to the initial reading state by the
regeneration process.
   Regeneration should always be performed before any adjustment
of the sensor in order to avoid concealing any possible
contaminants.

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RH Sensor Measurement Uncertainty

Regenerating the ETO Sensor

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RH Sensor Measurement Uncertainty

Regenerating the Digital RH Sensor

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RH Sensor Measurement Uncertainty

Preventing Contamination of the RH Sensor
   Protecting the sensor element by sealing it in an envelope
of thin film with a semi-permeable membrane such as
Goretex.
   The disadvantages are that the response times are
dramatically extended beyond what we see today.

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RH Sensor Measurement Uncertainty

Uncertainty Budget
   The uncertainty budget for an RH sensor is based off of the
following main uncertainty contributors.
   Uncertainty of the RH Reference
   Temperatures Tsat,TChm
   Temperature Accuracy
   Temperature Uniformity
   Temperature Resolution
   Pressures Psat,PChm
   Uncertainty of the Calibration Process
   Repeatability
   Uncertainty From the Sensor
   Hysteresis
   Non-Linearity (as left data)
   Long Term Stability (as found data)
   Repeatability

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RH Sensor Measurement Uncertainty

Hysteresis in RH Measurements
   Hysteresis of the sensor reading has a ” directional effect” as shown in the
diagram below.

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RH Sensor Measurement Uncertainty

Hysteresis in RH Measurements

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RH Sensor Measurement Uncertainty

Summary of an Uncertainty Budget for the RH Digital
Sensor

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RH Sensor Measurement Uncertainty

RH Calibration Traceability Diagram

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Calibration of RH Sensors
Process of Calibrating RH Sensors
   RH as found calibration
   Temperature sensor element adjustment and
calibration
   Regeneration the RH sensor element
   RH as left adjustment and calibration
   If no reconditioning takes place, then the sensor
accuracy and measurement uncertainty is
dramatically changed

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Calibration of RH Sensors

Calibration Process Time for RH Sensors
   RH as found calibration   18 hours
   Temperature calibration    5 hours
   Regeneration              62 hours
   RH as left calibration    18 hours
   Total lead time           1 week

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Calibration of RH Sensors
Calibration Certificates

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