1. 1 GE Infrastructure Sensing NCSL Conference Ken Soleyn The Effect of Air Laden Soluble Salts on Dew Point Measurement Using Condensation Hygrometers Ken Soleyn Product Manager Metrology GE Infrastructure Sensing 500 Research Drive Wilmington, MA 01887 USA T: 978.203.9000 F: 978.203.1920 E: ken.soleyn@ge.com W: www.gesensing.com

2. 2 GE Infrastructure Sensing NCSL Conference Ken Soleyn Summary  Theory & operation of condensation hygrometers  Definitions of humidity terms  Psychrometric relationships derived from the measurement of the partial pressure of water  Ambiguity of humidity measurements when condensate species (water or ice) is not known  Raoult’s Law and how it applies to condensation hygrometers  Airborne contaminants & the mitigation of their effects  Investigation of the effects of salts deposited on condensation surface of condensation hygrometers which occurs when used in salt-laden environments  Review of experimental set up and data  Conclusions

3. 3 GE Infrastructure Sensing NCSL Conference Ken Soleyn Theory & Operation of Condensation Hygrometers Thermoelectric Heat Pump Control OP AMP -40.0 Dew Point Temperature Indicator Emitter Regulation Precision 4-Wire PRTD Gas Flow Power Supply Rhodium or Platinum Mirror Thermoelectric Heat Pump Control OP Amp -40.0 Dew Point Temperature Indicator Emitter Regulation Optical Balance Precision 4-Wire PRTD Gas Flow GaAs IR Emitters Photo Detectors TEC Heat Pump Power Supply Rhodium or Platinum Mirror Thermoelectric Heat Pump Control OP AMP -40.0 Dew Point Temperature Indicator Emitter Regulation Precision 4-Wire PRTD Gas Flow Power Supply Rhodium or Platinum Mirror Thermoelectric Heat Pump Control OP Amp -40.0 Dew Point Temperature Indicator Emitter Regulation Optical Balance Precision 4-Wire PRTD Gas Flow GaAs IR Emitters Photo Detectors TEC Heat Pump Power Supply Rhodium or Platinum Mirror Schematic of Optical Chilled Mirror Condensation Hygrometer Thermoelectric Cooling Module4-Wire Pt RTD  Thermoelectric cooling sometimes supplemented with other cooling/heating sources is used to control the temperature of a condensation target to an equilibrium point where the mass of condensate is constant  Various methods of detecting the the condensate may be used including: optical (light scattering), acoustic wave propagation, microbalance, resonant frequency etc.  A precision RTD is used to measure the temperature of the target  Dew or Frost Point temperature (Td) is measured directly  Typical uncertainty of 0.2°C Td with 95% confidence is achieved  Simultaneous measurement of Temperature (Ta) and Pressure enable other humidity parameters (%RH, MR, AH etc) to be calculated with high precision  These measurement systems are characterized by minimal drift

4. 4 GE Infrastructure Sensing NCSL Conference Ken Soleyn Various Condensation Hygrometers

5. 5 GE Infrastructure Sensing NCSL Conference Ken Soleyn Saturation Water Vapor Pressure: When water vapor in a mixture of other gases is at the saturation point, the space has reached the maximum capacity to hold water vapor in the gaseous phase. Any water vapor in excess of the saturation point will be converted to the liquid phase. Dew Point Temperature (Td): The temperature at which the water vapor pressure in gas mixture equals the saturation water vapor pressure. Usually expressed in  F or  C. Relative Humidity (%RH): The ratio of the partial pressure of water vapor to the saturation water vapor pressure at a given temperature multiplied by 100. Absolute Humidity: The ratio of the mass of water vapor to volume of air or gas. Most commonly expressed as grains (1 grain = 1/7000 of a pound) per cubic foot, grams per cubic meter or pounds per million cubic feet (used for natural gas). Volumetric Mixing Ratio: The ratio of the partial pressure of water vapor to the partial pressure of the carrier gas. Usually expressed as PPMv. Mass Mixing Ratio: The ratio of the mass of water vapor to the mass of carrier gas. Usually expressed as grains per pound, milligrams per kilogram or PPMw Definitions of Humidity Terms

6. 6 GE Infrastructure Sensing NCSL Conference Ken Soleyn Equations for Saturation Water Vapor Pressure Reference: ITS-90 Formulations for Vapor Pressure, Frost Point Temperature, Dew Point Temperature and Enhancement Factors in the Range of –100 to +100°C Bob Hardy Proceedings of the Third International Symposium on Humidity and Moisture, was held in April 1998 at the National Physical Laboratory in the United Kingdom. Condensation Hygrometers measure dew/frost point temperature which may be used to calculate e (partial pressure of water vapor)

7. 7 GE Infrastructure Sensing NCSL Conference Ken Soleyn Psychrometric Relationships e s = Partial pressure of water vapor (mbar) over water e i =Partial pressure of water vapor (mbar) over ice P = Total pressure (mbar) t d = Dew Point temperature (  C) Water Vapor Pressure Over Water Water Vapor Pressure Over Ice Relative Humidity Absolute Humidity Volumetric Mixing Ratio Mass Mixing Ratio A.L Buck Equations General Eastern’s Humidity Handbook. Document No. A401103384, Revision B.00, Date: May, 1993. General Eastern Instruments, Woburn, MA.General Eastern Humidity Handbook 28.96 = molecular weight of dry air. For other carrier gases substitute molecular weight of the gas.

8. 8 GE Infrastructure Sensing NCSL Conference Ken Soleyn Saturation Water Vapor Pressure

9. 9 GE Infrastructure Sensing NCSL Conference Ken Soleyn Frost Point vs Dew Point Since the saturation vapor pressure over water is different that the vapor pressure over ice, condensation hygrometers in the 0 to -20°C range can give ambiguous readings Super cooled water can exist typically to -20°C particularly on plane clean surfaces Optical verification is one of the best methods of confirming condensation species

10. 10 GE Infrastructure Sensing NCSL Conference Ken Soleyn Raoult’s Law Raoult’s law, named for the French chemist Francois Marie Raoult (1830-1901), who first expressed it, states: “the vapor pressure of a solution is the sum of the vapor pressure of each component multiplied by the ratio of molecules (mole fraction) of each component”. In a binary solution consisting of water and a salt, the vapor pressure (e) will be equal to the product of the mole fraction of the water/salt and vapor pressure of pure water. e = Xe o e = Vapor pressure X = Mole fraction of the solvent = moles of solvent / total number of moles of solution e o = Vapor pressure of pure water Per Raoult’s law it is expected that condensed mixed with salt deposits will have a lower vapor pressure than that of pure water. A condensation hygrometer seeks to establish an equilibrium with the surrounding water vapor by controlling the temperature of the condensation surface such that the mass of condensate remains constant. Raoult’s law predicts that the dew point of a target with soluble salt deposits will read higher than that of a clean condensation surface.

11. 11 GE Infrastructure Sensing NCSL Conference Ken Soleyn The Effect of Salts on the Freezing & Boiling Points of Water Colligative Property: A property that depends only on the amount of solute in a solution and not the identity of the solute. Due to the colligative properties salt dissolved in water, the freezing point decreases and the boiling point increases directly proportional to the mole fraction of salt.  T f = K f c m  T = Freezing Point Depression K f = The freezing-point-depression constant. C m = Molal concentration of the solution K f for water = 1.86 (Freezing-Point Constant) K b for water = 0.52 (Boiling Point Constant) Molarity = Moles of solute/Liters of solution Molality = Moles of solute/Kilograms of solvent Depressing the freezing point due to salt is significant when using condensation hygrometers – Is the condensate species ice or water?

12. 12 GE Infrastructure Sensing NCSL Conference Ken Soleyn Airborne Contaminants Solid airborne contaminates can be classified into two basic categories: Insoluble Contaminants Dust Fibers Pollen Oil Mist Sand Soot Soluble Contaminants Salts Water Miscible Aerosols In costal or marine environments the concentration of airborne salts can vary from 0.1 to 5ppm by weight. One cubic meter of air at one atmosphere and room temperature weighs about 1.28kg (1.28g/l). Salts can exist suspended in air as solids or as aerosols dissolved in water

13. 13 GE Infrastructure Sensing NCSL Conference Ken Soleyn Mitigation of Contaminants Filter: Non-Hygroscopic media. Ex; Spun Borosilicate glass, PTFE etc. 5  M porosity nominal. Stainless steel housing recommended. Sampling Pump: Pull atmospheric samples through the hygrometer rather than “push” through. Tubing: Use stainless steel tubing. Clean when required. EUT Contamination: The EUT should be downstream of the reference hygrometer (particularly sensors installed in industrial applications) Flow Rate: Use low flow rates. 0.25 to 1 L/min nominal Clean the Condensation Surface: Use prepared solutions (water/methanol) or distilled water. Use only cotton swabs…“Q-tips” have lanolin. Automatic Cleaning & Balancing: A number of hygrometers are equipped with automated cleaning and balancing of the optics

14. 14 GE Infrastructure Sensing NCSL Conference Ken Soleyn Clean Mirror Contaminated Mirror Mirror after PACER cycle 85% clean surface The PACER Cycle for Cleaning & Optical Balance 1.The mirror is chilled to develop a thick dew layer. Soluble salts dissolve in the condensate. 2.The mirror is rapidly heated. Some contaminants are “flash” evaporated. The residue accumulates in discreet “islands” due to the surface tension of water. 3.The light reflecting off the mirror is optically balanced against a reference emitter/detector pair. 4.If there is not enough “gain” to balance the optics an alarm is annunciated. 5.The system may be programmed to transmit the last prevailing dew point measured prior to initiating the PACER. It may also be run on a timed basis.

15. 15 GE Infrastructure Sensing NCSL Conference Ken Soleyn Chilled Mirror Hygrometer (Analyzer) Two-Stage Chilled Mirror Sensor Temperature Sensor Dew Point Generator Test Apparatus DPG-300: Dew Point Generator. Volumetrically mixes dry compressed air with moist air produced built in saturator. Needle valve/rotameters are used. Dry Compressed Air: Delivered from zero air generator at 6.2 Bar (90 PSI) at -80°C Td. Regulated to 3.1 Bar. Optica Two Stage Hygrometer: Analyzer provides control of chilled mirror sensor. Measures temperature of mirror. Sensor is equipped with a “view port”. Analyzer provides data logging. Salt Solution: NaCl & Distilled Water. Laboratory balance used to weigh 0.28 g then diluted in volumetric flask. Graduated cylinder used to calibrate droplet volume. Chilled Mirror (Condensation Target)

16. 16 GE Infrastructure Sensing NCSL Conference Ken Soleyn Test Procedure 1.The Dew Point Generator was set to target values of –20, -10, 0, +5 & +10  C respectively. The chilled mirror sensor was connected via stainless steel tubing to the sensor and the flow rate was regulated to 0.5 l/min. 2.The chilled mirror sensor was cleaned by heating the mirror, wiping the mirror with a cotton swab saturated with a prepared cleaning solution, followed by wiping the mirror with a cotton swab saturated with distilled water. The water was allowed to dry. The optical bridge was balanced per the unit’s instructions and the PACER cycle initiated. Once the PACER cycle was completed, 90 minutes elapsed prior to recording the readings. A base-line reading for each set point was established. 3.The mirror was then heated to the maximum level (106ºC) by using the Optica’s “heat switch”. One drop of the prepared salt solution was placed on the mirror and allowed to dry. The unit was taken out of the heat mode and allowed to establish a reading. 90 minutes elapsed and the reading was recorded. 4.The Pacer cycle was engaged and a reading recorded 90 minutes later. 5.The mirror was then heated, clean by the identical procedure as described in section (2) and a reading was taken after 90 minutes. 6.At each reading, the mirror was observed via the view port to determine if the condensate species was either dew or frost.

17. 17 GE Infrastructure Sensing NCSL Conference Ken Soleyn Test Results

18. 18 GE Infrastructure Sensing NCSL Conference Ken Soleyn Normalized Test Results

19. 19 GE Infrastructure Sensing NCSL Conference Ken Soleyn The effect of depositing approximately 0.68mg of NaCl on an optical chilled mirror hygrometer caused the dew point measurement to increase. The increase was approximately 1  C at –20 & - 10  C Td. 0.5  C at 0  C and 3.5  C at 5 &10  C. The condensate species below 0ºC was observed on all readings to be frost (the crystalline structure of ice observed). This was observed through the chilled mirror’s viewport. Sufficient dell time at the readings elapsed for the condensate to convert completely to ice. The PACER cycle was effective for automatically cleaning the condensation surface (mirror) and rebalancing the optics. At all data points, the reading returned to the base line readings to within approximately ±0.5  C. Manual cleaning proved to be very effective. After manual cleaning all of the readings returned to within ±0.2ºC Td of the baseline readings. Since manual cleaning is effective, perhaps an automated mechanical means of cleaning might be employed. (A miniature windshield washer/wiper system comes to mind). Conclusions

20. 20 GE Infrastructure Sensing NCSL Conference Ken Soleyn The remote connectivity of the analyzer via the internet and data logging proved to be very useful. One could check the operation of the unit from a remote office or even from the comfort of one’s home. It was not possible to test high concentrations of salt residue on the mirror. NaCl residue appears to the optics as a thick dew or frost layer. (No cooling occurs). Additional studies with a more advanced optical detector systems should be conducted. GE Infrastructure Sensing has patented a condensation hygrometer using a high resolution CCD camera. The camera based system might be able the discriminate between dew, frost and contaminants such as salt residue. Additional studies should be conducted to develop a mathematical function relating the mass of salt deposited on the mirror to the dew point error. The mass of salt deposited might also be correlated to the mass/volume concentration of salt air and “run-time” between cleanings. Conclusions Con’t