THE CASE FOR CHILLED MIRROR HYGROMETRY

Transcription

1 THE CASE FOR CHILLED MIRROR HYGROMETRY by Gerald Schultz, PhD and Sumner Weisman INTRODUCTION TO HUMIDITY MEASUREMENT Often, we wish to know several conditions of the ambient air or other gas. For example, we may need its temperature, pressure, or moisture content. For temperature, using a thermometer or solid state sensor is comparatively straight-forward. For pressure, there are many transducers that are relatively easy to configure. Where many gas measurement errors are made, however, is when humidity measurements are attempted. Quantifying the level of water molecules in a gas can be difficult and challenging. If you want to know the humidity of the ambient air in your lab, dry gas in a test chamber, a tank of bottled gas used in a manufacturing process, or outside air in a meteorological application, careful study and planning are necessary. If you need accurate and reliable results, these measurements simply cannot be taken casually. In this paper, the authors will discuss many of the commonly-used humidity sensors, and will provide information that will aid in selection of the optimum sensor for your unique measurement application. You may select a sensor that provides a primary measurement directly in humidity units. Or, you may select a sensor that provides a secondary measurement in units of resistance, capacitance, or impedance with a change in humidity, which is then calibrated to read out in humidity units. Both are commonly used, depending upon the accuracy and cost requirements. With today s microprocessor-based hygrometers, almost any humidity units may be calculated and displayed from a few basic measured parameters. Depending upon the application, you may need data in Dew Point, Percent Relative Humidity, Parts-per-Million by volume or by weight, or other units. DEFINITIONS Dew (or Frost) Point is defined as the temperature that a gas must be cooled so that saturation occurs and liquid water (or ice, below freezing) begins to condense out. It is usually measured in C or F. Relative Humidity is defined as the ratio between the actual partial vapor pressure of a gas and its saturation vapor pressure, at a given temperature. It is measured in Percent R.H. A Hygrometer is an instrument that measures humidity. 1

2 A Primary Measurement results when the number of water molecules is measured directly. A Primary Sensor can be made traceable to NIST or other standards laboratories. A Secondary Measurement results when the number of water molecules is measured indirectly. A Secondary Sensor can never be made traceable. SENSOR TYPES 1. THE CHILLED MIRROR DEW POINT SENSOR LED Light Source Photodetector Chilled Mirror PRT Thermoelectric Cooler The chilled mirror sensor measures dew point temperature directly. A very small highly reflective metallic mirror is mounted to a solid state heat pump, or thermoelectric cooler. An LED light source is reflected off the mirror onto an opposing photodetector, or light receiver. The mirror is cooled thermoelectrically to the temperature at which condensation (dew or frost) first begins to form. This condensate causes the light source to be refracted, resulting in a reduction of light as seen by the photodetector. This signal is sent to a servo amplifier which controls power to the thermoelectric cooler, automatically controlling the mirror at whatever temperature is required to maintain a very thin film of water droplets (or frost, below freezing) on the surface at all times. Thus, the mirror temperature tracks the dew point up and down as it changes. Since the mirror surface is always at the dew point by definition, measuring the mirror temperature provides actual dew point temperature. Temperature data is received from a PRT (platinum resistance thermometer) embedded directly beneath the chilled mirror surface. The PRT is very tightly thermally coupled to the mirror surface, in order to minimize measurement error. 2

3 The advantages of the Chilled Mirror are: It provides a primary rather than a secondary measurement of dew point. High accuracy High repeatability Traceable to N.I.S.T. or other national standards labs Wide dew point range Wide temperature range No hysteresis No drift The disadvantages of the Chilled Mirror are: More complex, may be more expensive Narrow flow rate range Periodic cleaning may be required Applications for the chilled mirror include: Calibration Labs Meteorology Industrial Process Control and Measurement 2. OTHER SENSOR TECHNOLOGIES In addition to chilled mirror hygrometers, there are a number of sensors available that operate on other principles. These devices measure water vapor concentration by means of an electrical change in an hygroscopic medium, usually configured in a thin or thick film, as it reversibly absorbs water vapor from the ambient environment. This defines a secondary measurement sensor. The higher the water vapor pressure, i.e. dew point, the greater the change in electrical property. These sensor based measurements are temperature dependent, some more, some less depending on the technology, by virtue of the effects of temperature on (1) the amount of water vapor absorbed at equilibrium, and (2) on the electrical property. Electrical properties include, resistance, capacitance, and impedance. Some instruments are designed to compensate for the sensor s temperature dependency. Hygroscopic media can be comprised of organic polymers, or inorganic porous materials. Of the organic polymers, there are two major varieties; those that are electrically conductive whose electrical resistance varies inversely with dew point, and those that are electrical insulators whose capacitance varies directly with dew point. In the former case, examples include polyelectrolytes that form the basis of resistive RH sensors. In the latter case, examples include polyimides and polysulfones, that form the basis of capacitive RH sensors. Inorganic porous materials include aluminum oxide, where impedance changes non-linearly with changes in water vapor concentration. 3

4 A third category of inorganic material is based on phosphorous pentoxide, which is a non-porous intensely hygroscopic material. As the phosphorous pentoxide layer absorbs water vapor, the water is electrolyzed into hydrogen and oxygen using a DC voltage. The current flow through such a device is directly proportional to the vapor pressure of water in the ambient environment, and is measured and displayed directly in parts-per-million by volume, making it a primary measurement sensor. The phosphorous pentoxide acts as an infinite moisture sink, so the measurement signal is independent of the phosphorous pentoxide layer thickness making it a primary device. ADVANTAGES AND DISADVANTAGES Each of the above mentioned technologies offer certain advantages and disadvantages with respect to cost of measurement and performance. Contributing factors to cost are materials, fabrication and assembly of device, and most importantly factory calibration and field maintenance of the sensor and instrument system. Factors that define accuracy are repeatability, speed of response, and drift, i.e. device stability. The effects of humidity cover a wide range of applications and water vapor concentrations from electronic fabs, where water vapor is measured and controlled at less than 1 ppm (- 76 o C frost point), to controlled environments where water vapor concentrations of 20,000 ppm (~15 o C dew point) at 25 o C are typical. Table 1 summarizes each device technology with respect to cost, accuracy and useful operating range. It should be pointed out, that where chilled mirror hygrometers are traceable, they are also primary devices, i.e. the measurement of water vapor concentration is based on a fundamental property of water. Devices based on hygroscopic polymers and inorganic materials cannot be made traceable by virtue of the inherent instability of these materials as well as the complexity of interaction between water and the material. For instance, polymers are vulnerable to oxidation, phase transitions, and swelling followed by de-lamination from sensor substrate. Inorganic materials are susceptible to irreversible morphological and chemical changes that lead to sensor drift. 4

5 TABLE 1 A COMPARISON OF SENSOR TECHNOLOGIES SENSOR TECHNOLOGY COSTS TABLE 1 TYPICAL ACCURACY (+/- C) 25C DEW POINT ( o C) COMMENTS Dielectric Polymers Moderate 1* 24 to -15 Ionically Conducting Polymers Low 1* 24 to 0 Aluminum Oxide Phosphorous Pentoxide Chilled Mirror Hygrometers Moderate- High 3 10 to -90 Moderate- High 1** -20 to -90 Moderate- High to -75 Not traceable, secondary standard Not traceable, secondary standard Not traceable, secondary standard. Potential for large drift Primary standard, though P2O5 layer may have to be replenished Traceable and primary standard * at 50%RH at 25 o C ** 5% of reading 5

6 CONCLUSION Water is an extremely interactive amphoteric material; i.e. it is both an acid and a base depending on its chemical environment. Under normal conditions, it can exist as a vapor, liquid, or solid causing expansions and contractions of real materials which can reap havoc on industrial processes. As such, it becomes a factor in many commercial and R&D processes from glove boxes, chemical synthesis, food processing, fabrication of electronic components, to drug manufacture. There are many others. The control of humidity requires a measurement of humidity that has an associated cost for any particular accuracy requirement. The user must first determine the importance of a particular application and choose from a wide range of humidity sensors accordingly. As Table 1 shows, if one is interested in a primary standard of the highest accuracy, with traceability, over a wide operating range, chilled mirror hygrometry is a good option. mailto:ken.murray@edgetech.com 6