Wednesday, May 6, 2009

Reference Hygrometer

Even in normal concentrations and pressures, water vapor can present a challenge for seemingly straightforward HVAC applications such as cleanrooms and energy management systems and for handling gases in the industrial domain.

While no single device or technology is suitable for all applications, understanding the role of the reference hygrometer is often central to understanding the performance of other instruments. Because of the cost and complexity of the reference hygrometer, they are not usually deployed directly into the field. Instead, they are used in the lab for critical measurements, process development, and as a reference for checking process instruments.

Their value is not only accuracy, but also the high degree of confidence they provide that its measurement are in fact correct; properly deployed and maintained, they do not report incorrect results. Part of this is due to the fact that reference instruments live in the lab and see only clean, inert gases under carefully controlled conditions. It is the fundamental nature of the reference hygrometer, however, that sets it apart from the secondary devices normally used to make process measurements.

One happy fact about water vapor is the direct correlation between the partial pressure of water vapor and the dew point/ frost point temperature. Dew point is simply the temperature at which condensation occurs when a gas is cooled; frost point refers to condensation in the solid phase. (In this article, I’ll use “dew point” to mean condensation in both forms.).

If one knows the dew point, one can look up or calculate the partial pressure of water vapor (with additional knowledge of pressure and temperature, one can determine virtually any humidity parameter). This principle has been exploited by the condensation hygrometer, an instrument that accurately measures the temperature at which dew forms. This is a powerful tool for metrologists, as temperature measurement is a well developed science and the relationship between SI units of temperature and the partial pressure of water vapor is direct. Secondary sensors, those that correlate the behavior of a material to the parameter of interest, usually lack the long term repeatability that is the hallmark of a reference instrument.

The condensation hygrometer is the reference standard of choice for dew point temperatures ranging from -70°C to +60°C. Some of these instruments will measure as low as –95°C (about 40 ppb). This class of device typically samples a gas stream at flow rates on the order of 1 liter per minute. The gas is passed over a surface that is temperature controlled. The temperature of the surface is monitored with an accurate temperature detector, often a four-wire platinum resistance temperature detector (RTD). When condensation is detected on the surface, a control system varies the cooling of the surface to maintain condensation in an equilibrium condition. Upon achieving equilibrium, the surface is by definition at the dew point temperature. Devices of this type can be as accurate as ±0.2°C and offer long term repeatability of ±0.05°C.

Cooling the sensing surface in the condensation hygrometer can be accomplished in several ways, but most reference instruments use one or more Peltier coolers. These thermoelectric heat pumps are controlled by varying the current supplied to the heat pump. However, Peltier coolers become inefficient for achieving low temperatures, so most devices capable of measuring -60°C dew point or lower require some sort of auxiliary cooling. This may be integrated into the device itself as a mechanical refrigeration system, or it may be externally provided, usually in the form of a recirculating chiller.

Detection of condensation on the sensor surface has traditionally been accomplished optically. The cooled surface is highly reflective and is illuminated with a light source. The reflected light is measured by a photodetector. Formation of dew or frost causes some scattering of the light, which manifests as a change in output of the photodetector. These devices are known as “chilled mirrors.”

A more recent innovation is the surface acoustic wave (SAW) detector . The cooled surface is a piezoelectric quartz chip (Photo 1). The quartz is excited with an RF signal, setting up a mechanical wave across the surface of the sensor. The wave is re-converted to an electrical signal on the “far side” of the sensor and is monitored for changes in amplitude and frequency. The presence of dew or frost on the sensor alters the mechanical wave in a repeatable fashion.

All condensation hygrometers suffer from at least two shortcomings. First, it is possible to have water in the liquid phase at temperatures below 0°C. This is a problem for a reference hygrometer because the vapor pressure over water is different from the vapor pressure over ice at the same temperature. Ambiguity between dew and frost can easily result in measurement errors of 1°C or more;–about five times the stated accuracy of the instrument! Second, water soluble contaminants, particularly salts, can go into solution with the condensate on the sensor surface, causing a reduction in vapor pressure. This is known as Raoult Effect, and it too can cause substantial measurement errors (see Table 1).

A SAW-based reference hygrometer that addresses these issues has been developed (Photo 2). Dew/frost ambiguity is eliminated via a combination of SAW behavior and signal processing. In short, dew and frost behave differently with respect to transmission of a mechanical wave across the sensor surface. The SAW device is able to determine the phase of the condensate and is therefore always able to report the correct dew point or frost point. Similarly, ý salt detection scheme can be manually initiated or set to run automatically at a user–defined interval. The SAW device measures the impedance of the sensor surface, comparing the result to the impedance of a clean sensor. Significant impedance changes indicate the presence of salts on the sensor and the likelihood of Raoult Effect errors. The instrument reports that all is well, or prompts the user to clean the sensor.

If you are considering the purchase of a reference hygrometer, you should consider a number of factors before making a decision, including the measuring range you require, desired accuracy and repeatability, and the intended use of the instrument. Some instruments offer excellent performance but “bare bones” user interfaces. Others offer a host of features for ease of use, such as built in datalogging, graphical interfaces, and integrated sample pumps (see Photo 2).

Still others offer the highest possible accuracy. Prices range from several thousand dollars to well over $50K. Talk to the instrument manufacturers and ask lots of questions. They will be happy to help guide you to the instrument that best meets your needs.

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