10 RH sensor calibration and measurements by 4jV64J

VIEWS: 26 PAGES: 43

									           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


www.ellab.com
                           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



 www.ellab.com
           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




www.ellab.com
           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)




www.ellab.com
           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.


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




www.ellab.com
                         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




www.ellab.com
                        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




www.ellab.com
           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.




www.ellab.com
           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.




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




www.ellab.com
           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.




www.ellab.com
           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.




www.ellab.com
           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.




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




www.ellab.com
           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.




www.ellab.com
           RH Sensor




www.ellab.com
           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



www.ellab.com
           RH Sensor

                RH Digital Sensor Response Time




www.ellab.com
           RH Sensor

                Response Time at Different Flow Rates




www.ellab.com
           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.




www.ellab.com
           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




www.ellab.com
           Humidity Providers and References

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




www.ellab.com
           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.




www.ellab.com
           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.




www.ellab.com
           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.




www.ellab.com
           Humidity Providers and References

                Saturated salt solutions used in verfication of RH sensor reading




www.ellab.com
           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




www.ellab.com
           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.




www.ellab.com
           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
                     the black adaptors from Ellab.
                    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.




www.ellab.com
           RH Sensor Measurement Uncertainty
                Measurement Uncertainty In General




www.ellab.com
           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
                     reading.
                    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.




www.ellab.com
           RH Sensor Measurement Uncertainty

                Regenerating the ETO Sensor




www.ellab.com
           RH Sensor Measurement Uncertainty

                Regenerating the Digital RH Sensor




www.ellab.com
           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.




www.ellab.com
           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
                                Inadequate Temperature Compensation



www.ellab.com
           RH Sensor Measurement Uncertainty

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




www.ellab.com
           RH Sensor Measurement Uncertainty

                Hysteresis in RH Measurements




www.ellab.com
           RH Sensor Measurement Uncertainty

                Summary of an Uncertainty Budget for the RH Digital
                Sensor




www.ellab.com
           RH Sensor Measurement Uncertainty

                 RH Calibration Traceability Diagram




www.ellab.com
           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




www.ellab.com
           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




www.ellab.com
           Calibration of RH Sensors
                Calibration Certificates




www.ellab.com

								
To top