Humidity Measurement in Cleanrooms

Humidity Measurement in Cleanrooms

Choosing the right type of measurement instrument is important in order to reach the best humidity measurement results. Calibration should also be carried out regularly, and to traceable standards. Products manufactured in cleanrooms cover a wide range, including pharmaceuticals and semiconductors. Humidity, temperature, particles and pressure are often controlled, as these parameters can have serious effects on product quality and production efficiency.

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Choosing the right type of measurement instrument is important in order to reach the best humidity measurement results.  Calibration should also be carried out regularly, and to traceable standards.  Products manufactured in cleanrooms cover a wide range, including pharmaceuticals and semiconductors.  Humidity, temperature, particles and pressure are often controlled, as these parameters can have serious effects on product quality and production efficiency.

Relative Humidity

Relative humidity (%RH) describes the amount of water vapor that exists in a gaseous mixture of air and water. It is a ratio of the amount of water vapor 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

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.

Different applications, different needs

A pharmaceutical manufacturing plant often has a large number of cleanrooms. The control and recording of temperature and humidity is strictly designated by GMP (Good Manufacturing Practice).

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.

Humidity and dewpoint sensor technologies

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%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.

Humidity and dewpoint sensors include:

  1. A psychrometer is a simple form of a hygrometer, which consists of two thermometers. One has a dry bulb and the other a bulb that is kept wet to measure wet-bulb temperature. This is a method often used in laboratories and humidity and temperature test chambers.
  2. A mechanical hygrometer measures and records humidity using an instrument that expands and contracts with humidity changes, such as human hair.
  3. A lithium chloride dewpoint indicator is a measurement principle based on the hygroscopic characteristic (ability of a substance to attract water molecules) of lithium chloride.
  4. A resistance type hygrometer utilizes the principle that electrical resistance varies in a material that absorbs moisture.
  5. A capacitance type hygrometer measures humidity by detecting the change in capacitance of a thin polymer film.
  6. A mirror dewpoint indicator utilizes the occurrence of dew at dewpoint temperature when air containing water vapor is cooled. This type of sensor is often used in research institutes.

Sensors 1-6 can measure general humidity levels. Sensors 5 and 6 are also used for low dewpoint measurement.  Sensors mostly used in cleanrooms include the resistance type hygrometer, capacitance type hygrometer (dew indicator) and mirror dewpoint indicator.

Regular traceable calibration is important

One should always make sure that the data produced by the measurement equipment is reliable and accurate. Periodic calibration is absolutely essential.

From a global perspective, all measurements are based on the globally agreed International System of Units (SI). This ensures that we use the same quantities, and that measurements performed with various types of equipment in various locations are comparable.

National laboratories are responsible for maintaining and developing traceability and for providing the highest accuracy calibrations. The calibration services of the National Measurement Standard Laboratories may be limited to calibration of the highest grade primary standards.

Commercial calibration services provide calibration services for lower level standards and measurement equipment. These may be manufacturer services providing calibration services for their own products, or laboratories providing calibration services for any measurement equipment.

Non-accredited calibration services are the majority service providers, including most of the measurement equipment manufacturers’ calibration services and a considerable amount of commercial calibration services. Without accreditation the competence of these services is not proven. Before use, the competence should be confirmed by auditing the service.

Each calibration service provider must maintain an effective traceability chain. At the very least, the primary standard must be calibrated at an outside laboratory and then used for calibrations.

Laboratory calibration is preferred to field calibration. In a laboratory, the effects caused by the environment can be minimized, and the number of factors influencing the calibration are reduced significantly. Field calibration is a quick and easy way of checking measurement equipment without having to remove it from the process or process area.

Visit www.vaisala.com for application notes, infographics, animations and product information related to humidity measurement in cleanrooms.

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