October 2001
Surface acoustic waves to the mission control
by James Tennerman
Humidity measurement technology keeps pace with ever more complicated processes.
Humidity measurement is important in a wide variety of industrial processes but paradoxically, water vapor content remains one of the most poorly understood physical parameters.
Many of us have wrestled with difficult humidity problems, not the least of which is sorting out the different technologies to determine which is most appropriate for a given measurement requirement. Buried somewhere in this milieu is the concept of the reference instrument, the one that can be used with confidence to end all arguments. In the world of humidity, the reference instrument of choice is the condensation hygrometer.
This device is usually in the calibration laboratory but can also find its way into the field for audit purposes, and with careful consideration it can even make full-time process measurements.
Recent advances in condensation hygrometry offer very relevant benefits to industrial hygrometer users, as well as to calibration and metrology personnel.
Condensation hygrometry is a technique used to measure the dew point of a gas. By definition, dew point is the temperature at which water begins to condense from the gaseous to the liquid phase.
This seemingly trivial phenomenon is important because dew point has a direct correlation to the partial pressure of water vapor. Once the partial pressure of water vapor is known, we might supplement our knowledge by measuring the total pressure of the gas, telling us how much of the total gas pressure is due to the presence of water vapor.
Because the capacity of gas to hold water vapor is a function of temperature, a temperature measurement of the gas can tell us whether the gas is saturated with water vapor and therefore how likely to condense to water. In fact, once we know dew point, total pressure, and temperature, we can calculate almost any humidity parameter!
The beauty of the condensation hygrometer—and one of the reasons it is the reference standard of choice—is that it determines the water vapor content of a gas by measuring the temperature at which vapor condenses to liquid. We understand the parameter temperature quite well.
There are three things every condensation hygrometer must do:
- Cool a surface in a controlled fashion.
- Measure the temperature of that surface.
- Detect the presence or formation of liquid water on that surface.
Over the years, cooling has occurred in many ways, including the use of dry ice, the expansion of compressed gas, mechanical refrigeration, and thermoelectric coolers. Most modern automatic condensation hygrometers use thermoelectric coolers, which easily control temperature using varying electrical current flow through the cooler. Some devices use a combination of mechanical refrigeration and thermoelectric coolers to attain very low temperatures.
Temperature measurements in the condensation hygrometer use liquid-in-glass thermometers, thermistors, thermocouples, and resistance temperature detectors (RTDs). Modern condensation hygrometers usually use thermocouples or RTDs. The most precise reference instruments use platinum RTDs.
The human eye originally detected condensate (as you might guess, these were the days when folks used dry ice and liquid-in-glass thermometers, too). At least one device measures change in electrical conductance, while another measures change in capacitance as these parameters vary, due to the presence or absence of liquid condensate on a surface.
The most commonly used detection method in the modern condensation hygrometer is optical reflectance, the chilled mirror technique. This technique involves illuminating a cooled surface with a light source and measuring the reflected light. Condensation on the surface scatters the light, which manifests as a change in reflectance.
Finally, the new generation of condensation hygrometers utilizes surface acoustic wave (SAW) detection.
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Single quartz chip surfaces
A radio frequency (RF) signal fed to the transmitting antenna converts—and this is why the sensor material has to be piezoelectric—to a mechanical wave. The mechanical wave propagates along the sensor surface to the receiving antenna, where the mechanical wave converts back to a RF signal. The beauty of the technology lies in the fact that the wave can propagate along the surface of the thin quartz chip only. This serves to concentrate the energy of the wave on the very surface of the chip. Consequently, the wave is very sensitive to any material on the surface. Compared with traditional optical parameters, such as reflectance and dispersion, measurable from a chilled mirror, these electrical parameters measurable from a SAW sensor, such as attenuation and frequency shift, have a more repeatable relationship to the condensate mass. A clean, dry sensing element shows a specific signature at the receiver. The presence of liquid condensation on the element alters the signature in a repeatable way. Freezing the condensate on the element causes a distinctly different signal change, enabling one to determine whether dew or frost is on the element. |
Analysis of the received wave in terms of frequency and amplitude provides the information required to control the elements temperature in such a way that a thin layer of dew or frost remains in equilibrium on the element. Additionally, analysis determines the presence or absence of contaminants on the element and, to some extent, the composition of any contaminants.
Experiments have shown that the SAW element can be covered by solid particulate contaminants on the quartz surface but still detect frost or dew in the presence of such contamination. The element also detects the presence of salts on the quartz surface.
Lie between dew and frost
Combining SAW technology and the excellent characteristics of existing chilled mirror technology results in a superb condensation hygrometer. Thermoelectric coolers reduce the temperature of the SAW crystal. A platinum RTD monitors the quartz surface.
The complete instrument offers accuracy, repeatability, and measurement range comparable to the most advanced chilled mirror devices. Above and beyond the existing technology, the SAW hygrometer offers the following:
SAW detects very small amounts of condensate—as little as one-tenth of that required by an optical detection system. This means substantially faster response times, particularly at low dew/frost points.
SAW automatically differentiates between dew and frost on the sensing element. This is primarily of interest to laboratory users, who are routinely required to note dew vs. frost when performing measurements between 0 and –30°C.
Optical detection methods require visual verification of the condensate phase status or a manipulation of the system to ensure frost at dew point temperatures below 0°C, a significantly more unwieldy and less accurate process.
Proprietary signal processing enables certain SAW hygrometers to detect contaminants, including salts, on the sensor surface. While all condensation hygrometers are subject to contamination, SAWs’ ability to differentiate salt is a step forward for industrial users. Because salt alters the vapor pressure of water in its vicinity, it is a significant and insidious source of measurement error in condensation hygrometers.
The SAW hygrometer operates reliably even with substantial particulate contamination on the sensing element. This means longer maintenance intervals in industrial applications.
The use of quartz wafers in a SAW sensor results in excellent resistance to aggressive chemicals. Wetted parts can be limited to stainless steel, PTFE, tantalum, and quartz. Material of the sensing element itself can be crucial, as gases may go into solution with the condensate forming corrosive liquids on the sensing element.
No single technology is appropriate for all humidity measurement applications. However, SAW condensation hygrometry does offer substantial improvements over the current state of the art for a broader range of more demanding industrial applications. IT
Figures and Graphics
- Piezoelectric quartz substrate
- Surface acoustic wave
- Surface acoustic waves to the mission control (pdf version)
- Tiny chemistry lab
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Behind the Byline
James Tennermann is a sensor technology wonk and has written about temperature for InTech magazine before. He’s a member of ISA and a delegate to the National Conference of Standards Laboratories. Tennermann works at Vaisala, Inc., where his main responsibility is to introduce new technology to market.
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