20 ANALYTICAL & LABORATORY EQUIPMENT
Achieving accurate humidity measurement in cleanrooms
Choosing the right type of measurement instrumentation is important in order to reach the best humidity measurement results. Calibration should also be carried out regularly, and to traceable standards. Daisuke Fujisawa explains.
Il est important de choisir le bon type d’appareillage de mesure afin d’obtenir les meilleures résultats de relevé d’humidité. L’étalonnage doit également être effectué régulièrement et selon des normes de traçabilité. Explications de Daisuke Fujisawa.
Die Auswahl der richtigen Messinstrumente ist für optimale Feuchtigkeitsmessungen besonders wichtig. Auch Kalibrierung sollte regelmäßig und im Rahmen nachvollziehbarer Standards durchgeführt werden. Ein Bericht von Daisuke Fujisawa.
pressure are often controlled, as these parameters can have serious effects on product quality and production efficiency. Relative humidity (per cent RH) describes
P
the amount of water vapour that exists in a gaseous mixture of air and water. It is a ratio of the amount of water vapour present compared to how much could be present at a given temperature. Issues at production sites, such as expansion and contraction, and hardening and softening of material, change in viscosity of liquid, growth of microbes, increase in static electricity, and corrosion and rust, are largely affected by humidity. Dewpoint (Td) is a temperature at which
dew, or condensation, forms on cooling a gas. Dewpoint is a parameter suitable for expressing very small water content in a gas like air. In the micromachining of semiconductors the conditions are very dry as water molecules are regarded as contaminants. In this condition relative humidity is practically stagnated at 0 per cent RH but dewpoint scale is still sensitive for water content changes in the measured gas.
Applications and needs
A pharmaceutical manufacturing plant often has a large number of cleanrooms. The control and recording of temperature and humidity is strictly designated by Good Manufacturing Practice (GMP). The most important feature required from humidity sensors is small deviation. It is important to be able to perform precise calibration to check that the sensor does not drift over the long-term. In food processing plants, it is
necessary to keep the manufacturing site at or below certain humidity. For example, 40 per cent or below seems to be a commonly used value. This helps in restricting the growth of germs and bacteria that can cause food poisoning. In semiconductor and electronics
product plants, the generation of products changes more and more rapidly. As a
roducts manufactured in cleanrooms cover a wide range, including pharmaceuticals and semiconductors. Humidity, temperature, particles and
result, the control of humidity and dewpoint in the manufacturing process has become stricter. In the manufacturing mini-environments, very high level control with an accuracy of ±per cent RH is often required. Humidity control is also important at liquid
crystal display plants and paint plants. In this case, the durability and accuracy of the humidity sensor is very important. These plants use various gases, which can affect sensor elements.
Humidity and dewpoint sensors
Humidity sensors, which measure water content in the air, are broadly divided into two types. One measures humidity and the other dewpoint. In an atmosphere where the humidity level is at least 10 per cent RH, humidity measurement is often used, while in low humidity, dewpoint measurement is preferred. In some cases it is convenient to use dewpoint measurement even in high humidity conditions.
Fig. 1. Vaisala’s own cleanroom produces sensors for radiosondes as well as different humidity, barometric pressure and carbon dioxide measurement products.
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