No matter the size, environmental monitoring is one of the most important responsibilities in a cleanroom, each of which has its own set of priorities dependent on the industry involved. Whether utilized in life sciences, biotechnology, semiconductor, or university settings, the need to maintain constant temperatures, exclude (or minimize) particulates, or control the growth of microorganisms is crucial. Additionally, the need to maintain a specific humidity range is also essential throughout most sectors. Otherwise, the damage to sensitive equipment, product being produced or developed, or research being conducted can be enormous in loss of inventory, time, and labor.
The careful measurement of humidity in an environment, along with the ability to keep it within a specific range, changes given the needs of the facility. Among the many problems faced by cleanrooms experiencing high levels of humidity are risk of increased bacteria growth, corrosion of equipment, and — if gone unchecked — unwanted inflation in power consumption as air-handling compressors and dampers try to decrease the moisture to the correct levels. If humidity levels fall below the standards required, the accompanying rise in static electricity can result in unwanted discharges that will damage electrical components. The speed of many chemical reactions is also dependent on specific humidity levels, and so an environment that has excessively dry or wet air can lead to the collection of false data in experiments and flaws in manufacturing. Of course, whether too high or low, humidity levels also have an effect on the staff working in the cleanroom.
The relationship between temperature and humidity in cleanrooms is also incredibly important to maintain. Changes in cleanroom temperature directly correlate to a change in relative humidity in the environment. For example, if the temperature is set to 70 degrees and 50 percent relative humidity, decreasing the temperature to 68 degrees causes the relative humidity to increase to 55 percent. A seemingly minor change in temperature of just two degrees might be well within the operational range of the cleanroom, but the corresponding 5 percent increase in humidity may be outside those acceptable parameters. This could result in increased time and energy costs to decrease the humidity while maintaining the temperature (at best) or the loss of product or research data (among the worst).
There are a variety of methods used to calculate the amount of water vapor in the air (or other gases) and technology and techniques continue to evolve. Additionally, there are several different types of humidity that can be of importance in cleanrooms: absolute humidity, relative humidity, specific humidity, dew point, frost point, etc. The most commonly regulated are:
• Absolute humidity: the ratio of the mass of water vapor to the volume of air or gas, commonly expressed in grams per cubic meter or grains per cubic foot
• Relative humidity: the ratio of the moisture content of air compared to the saturated moisture level at the same temperature and pressure, expressed as a percentage
• Dew point: the temperature and pressure at which a gas begins to condense into a liquid, expressed in degrees (Celsius or Fahrenheit).
In broad terms, humidity is measured with wet or dry bulb hygrometers, dew point hygrometers, or electric hygrometers (which are most commonly associated with the term “humidity sensor”). Electric hygrometers can be further segmented into two types: sensors that utilize the capacitive sensing principle (capacity humidity sensors) or sensors that use resistive effects (resistive humidity sensors). They have come to dominate the humidity sensing market and are worth further exploration.
Capacitive humidity sensors are often used to measure relative humidity and can be widely found in industrial and commercial facilities. While there are variations with every manufacturer, capacitive humidity sensors are composed of a base on which a thin film of polymer or metal oxide is deposited, sandwiched between two conductive electrodes. The change in the dielectric constant of a capacitive humidity sensor is directly (although not totally) proportional to the relative humidity of the surrounding environment. As previously mentioned, relative humidity is both a function of ambient temperature and water vapor pressure. As such, the relationship between relative humidity, the amount of moisture present in the sensor, and sensor capacitance is what dictates the operation of a capacitive humidity sensor.
Capacitive humidity sensors can also be used to measure the dew point in cleanrooms. To accomplish this, the capacitive sensor is paired with a temperature sensor. The data from both sensors is sent through a processor which calculates the dew point. In some advanced sensors, dew point is confirmed through the use of a chilled mirror hygrometer.
Capacitive humidity sensors are generally cost effective, which has helped them to become a common fixture in most cleanrooms. The technology behind the sensors is also well-established, which has helped to create an ease of use. In a subtle bit of irony, some capacitive humidity sensors are manufactured in cleanrooms in order to help obtain uniformity and consistency. While popular, capacitive humidity sensors are not without drawbacks. The sensor calibration can drift over time, which only experienced staff will notice. There can also be additional costs and downtime in the cleanroom if outside firms need to be employed to recalibrate the sensors. Additionally, capacitive humidity sensors are prone to contamination from dust and foreign chemicals within the air that is being sampled, which can alter their performance.
Resistive humidity sensors measure the change in humidity and translate it into a change in the electrical impedance of a hygroscopic medium. They are usually comprised of noble metal electrodes either deposited on a glass or ceramic substrate or wirewound electrodes on a plastic or glass cylinder. The substrate is coated with a conductive polymer. The sensor absorbs the water vapor and ionic functional groups are dissociated, resulting in an increase in electrical conductivity. Because of this, as relative humidity changes, the resistance measured by the sensor changes with it (an increase in relative humidity correlates to lower resistance within the sensor).
Unlike capacitive humidity sensors, resistive humidity sensors do not need calibration once they are replaced. This has made them an attractive option from a cost standpoint, both in terms of installation cost and field replacements. Additionally, the sensors’ interchangeability and long-term stability have made them a popular choice in cleanrooms. On the downside, resistive humidity sensors can become inaccurate when exposed to condensation if a water-soluble coating is used. They are also not suitable for environments with extreme temperature fluctuations (>10 degrees F). Resistive humidity sensors share capacitive humidity sensors’ vulnerability to contamination from foreign particles and chemicals.
Thermal conductivity humidity sensors measure absolute humidity by quantifying the difference between the thermal conductivity of dry air and that of air containing water vapor. The sensor is comprised of two matched negative temperature coefficient thermistor elements in a bridge circuit. One sensor is encapsulated in dry nitrogen while the other is exposed to the environment. As current passes through the thermistors, resistive heating increases their temperatures. The sealed sensor dissipates more heat than the exposed sensor. This difference in heat dissipation results in the two thermistors having a difference in resistance. This difference is directly proportional to the absolute humidity, which is then calculated.
Thermal conductivity humidity sensors perform well in high temperature or corrosive applications that are often found in a variety of industrial and commercial cleanroom environments. They are also an excellent fit in sensitive environments where moisture levels must be maintained at very specific levels, such as chemical manufacturing, battery production, and fuel-cell operation. The main disadvantage of thermal conductivity humidity sensors is that they may respond to any gas that has thermal properties different from those of dry nitrogen. They also tend to be a more expensive solution.
No matter the industry, selection of the correct humidity monitoring technology for a cleanroom is extremely important. There are a variety of sensors available to help accomplish this, each employing specific techniques to measure different types of moisture. It’s crucial for cleanroom operators to establish the type of humidity they’re trying to measure and control — and only then tailor the monitoring solution to the individual needs of the cleanroom. The price is too high, both in terms of dollars lost and consumer safety, to make the decision lightly.
Don Raymond is the Founder and Chief Technology Officer of RLE Technologies. He has more than 31 years of experience developing environmental monitoring and leak detection technologies to mitigate risks in mission critical environments. Additionally, Don holds eight patents related to this work. firstname.lastname@example.org; www.rletech.com
This article appeared in the May/June 2016 issue of Controlled Environments.