Ensuring Accurate Humidity Measurement in Cleanrooms

Every cleanroom has environmental-control specifications that define the upper and lower limits for temperature and relative humidity (RH). Pressure, flow, and contamination must also be controlled. Devices that measure RH (e.g., sensors and transmitters) play a relatively small role in cleanroom management, but their failure can cause significant problems. Operators should bear several factors in mind to ensure that sensors function properly and maintain the appropriate humidity.
 
Installation location
The fact that RH depends on temperature affects the placement and installation of RH sensors. For example, if only 50 mm of an instrument’s 200-mm stainless-steel probe is exposed to the process air, the rest of the probe may serve as a heat sink or source, depending on its environment. Thus, the temperature of even a carefully designed humidity-sensing element can change and cause an RH measurement error that depends on the difference between the process-air temperature and the environment surrounding the exposed portion of the probe. This error is easily measured in a laboratory environment.
 
Because local heating can affect wall-mounted RH instruments’ measurements, operators should install the devices away from equipment that generates heat. Good airflow in the cleanroom usually minimizes convective-heat problems, but warm or hot equipment radiates heat, potentially creating a temperature discrepancy and corresponding measurement error. Loop-powered devices with a 4–20-mA output dissipate some power as heat, and this dissipation could affect wall-mounted RH instruments. Some wall-mounted RH devices housed in enclosures heat themselves by several degrees and experience significant measurement errors. RH devices with short probes that isolate the humidity-sensing element from the instrument’s electronics preclude this possibility.
 
Humidity sensors should be protected from moisture. They should be located far enough from cooling coils so that they are unlikely to be affected by entrained water droplets. The sensors also should be kept away from steam-injection or ultrasonic-humidification elements. Many RH probes incorporate filters that eliminate catastrophic errors by protecting the sensing element from water. Water can accumulate on or in the filter material, however, and create a microclimate around the sensor that results in measurement errors.
 
In all of the situations described above, measurement errors result from subtle factors that change over time or occur only seasonally. These situations are among the most difficult to remedy when the sensor has been installed in an inappropriate location. If the errors are big enough to create out-of-specification conditions, they will trigger service calls, calibration requests, and equipment replacement. Personnel can prevent these potentially expensive problems during the design and specification processes for RH sensors.
 
Accuracy
The instrument-specification process is a good time to think about the accuracy specifications of the various RH sensors available. No standard requirement for accuracy has been established, so manufacturers must consider their specific applications to determine the accuracy specifications they need.
 
Although they are important criteria, accuracy specifications should not necessarily be the decisive factors for RH instruments. Sensor vendors emphasize these specifications, but performance claims can sometimes be misleading because “accuracy” is a qualitative term in measurement science that includes uncertainty.
 
Long-term stability
Long-term stability is easily the most important performance characteristic of an RH instrument. Long-term stability is the instrument’s ability to make accurate measurements consistently over a long period of time. Reputable vendors perform long-term tests to characterize their own devices. The results of these tests help personnel understand the sensors’ baseline performance, but the performance in individual cleanrooms may be different because of the cleanrooms’ unique conditions.
 
The most significant threat to the long-term stability of an RH instrument in a cleanroom comes from vapors from the chemicals used in cleanroom processes. Vapors cause incorrectly low RH readings in high-humidity conditions because the vapor blocks water molecules from reaching the sensor. In low-humidity conditions, vapors cause incorrectly high RH readings. Some RH devices are equipped with features that minimize the effect of chemicals. For example, a chemical-purge function heats the sensor quickly to a high temperature (e.g., 160 °C), and thus cleans it of unwanted vapors.
 
Conclusion
Many humidity-measurement problems in a cleanroom such as sensor drift can be anticipated and prevented with thoughtful planning. Care should be taken throughout the entire RH-instrument specification and installation processes. Correct installation location, protection from moisture and vapors, and the instrument’s long-term stability should be taken into account to ensure optimal sensor performance.
 
Jim Tennermann is a regional market manager for controlled-environment products at Vaisala, 10-D Gill St., Woburn, MA 01801, tel. 888.824.7252, fax 781.933.8029, www.vaisala.com.