Clean room humidity measurement

Clean room humidity measurement
To get the best moisture measurement data, it is important to choose the right measurement instrument. At the same time, calibration should be carried out on a regular basis in accordance with traceability standards.
Products in the clean room range from pharmaceuticals to semiconductors. Normally, humidity, temperature, pellets and pressure are controlled because these parameters have a very important impact on product quality and productivity.
Relative humidity
Relative humidity (%RH) is used to describe the amount of water vapor present in the gas mixture of air and water. It refers to the percentage of water vapor actually contained in the air and the amount of saturated water vapor at that temperature. Problems occurring at the production site, such as expansion and contraction of materials, hardening and softening, changes in viscosity of liquids, proliferation of microorganisms, increase in static electricity, corrosion and rust, are largely affected by humidity.
Dew point
Dew point (Td) is the temperature at which dew or condensate forms on the cooling gas. The dew point temperature is used to describe the very small moisture content of a gas such as air. In the micromachining process, semiconductors are extremely dry due to the fact that water molecules are considered to be contaminants. In this case, the relative humidity is almost maintained at 0% RH, but the dew point temperature is still very sensitive to changes in the moisture content of the gas to be measured.
Different applications, different needs
A pharmaceutical factory is usually equipped with a large number of clean rooms. Temperature and humidity control and recording are performed in strict accordance with GMP (Good Manufacturing Practice) requirements. The most important characteristic of a humidity sensor is that the error is very small. Accurate calibration is critical to the sensor so you can check for drift over time.
In food processing plants, the production site must be maintained at or below a certain humidity. Usually a humidity value of 40% or less. This helps to inhibit the growth of microorganisms and bacteria that may cause food poisoning.
In semiconductor and electronics factories, product updates are getting faster and faster. Therefore, the control of humidity and dew point in the production process has become more stringent. In the micro-environment of the production site, it is often required to achieve an ultra-high precision of +/- 1% RH. Humidity control in LCD manufacturers and paint manufacturers is equally important. In this case, the durability and accuracy of the humidity sensor are very important. These plants produce a variety of gases that affect the sensor components.
Humidity and dew point sensor technology
Humidity sensors for measuring the moisture content in the air can be roughly classified into two types. One is used to measure humidity and the other is used to measure dew point. Humidity measurement is usually required in air with a humidity level higher than 10% RH, and in a low humidity environment, it tends to measure the dew point. Dew point measurement is more convenient under certain high humidity conditions.
Humidity and dew point sensors include:
Dry and wet bulb hygrometer
2. Mechanical hygrometer
3. Lithium chloride dew point meter
4. Resistance type hygrometer
5. Capacitive hygrometer (condensation meter)
6. Mirror dew point meter
Sensors 1 - 6 measure general humidity. Sensors 5 and 6 can also be used for low dew point measurements. The principle of each technology is briefly described as follows :
1. Dry and wet bulb The hygrometer is a simple type of hygrometer and consists of two thermometers. One thermometer has a dry ball and the other ball is kept wet to determine the wet bulb temperature. The wet bulb is cooled by the evaporation of water. The amount of evaporation and the cooling state of the thermometer depend on the humidity of the air. This data, along with the humidity checklist or calculation results, is used to determine the partial pressure and relative humidity of the water in the surrounding air. This method is usually used in laboratories as well as temperature and humidity test chambers.
2. Mechanical hygrometers measure and record humidity with an appliance that can stretch or contract with changes in humidity , such as human hair. This measurement method has been used for a long time. However, the accuracy of this method is not ideal.
3. According to the hygroscopic characteristics of lithium chloride (the ability of substances to absorb water molecules), a lithium chloride dew point meter can be used as a measuring means. The sensor consists of a reel that is covered with an absorbent cloth and an inertial electrode double-stranded coil (two insulated wires, the current is reversed through). The reel is coated with lithium chloride. The alternating current is passed through the coil and the lithium chloride solution to achieve the effect of resistance heating. After the reel is heated, moisture evaporates from the lithium chloride solution, and its evaporation rate is controlled by the moisture pressure in the surrounding air. When the spool begins to dry, the resistance of the lithium chloride solution increases, at which point the current through the coil is small. This will allow the reel to cool. When the heating and cooling of the reel reaches an equilibrium point, it neither absorbs nor discharges moisture, and its equilibrium temperature is proportional to the dew point temperature of the surrounding air.
4. Resistive hygrometers use a change in material resistance to absorb moisture. It uses a special sensor to measure the current resistance through the wire. These sensors are suitable for mass production and are used in most household appliances and other electrical products. However, in extremely low or extremely high humidity environments, the measurement accuracy may be affected.
5. Capacitive hygrometers measure humidity by detecting changes in the capacitance of the polymer film. These sensors can easily reach fairly high precision and are mostly used in the industrial sector. The HUMICAP® humidity sensor, a patented product from Vaisala, uses this technology.
6. The mirror dew point meter uses the characteristics of water vapor-containing air to produce dew at the dew point temperature during cooling. The mirror is cooled after reaching the dew point temperature of the gas. When the dew condenses, it changes the reflected light on the mirror surface. When the mirror reaches equilibrium, the rate of evaporation and condensation is the same, and the temperature of the mirror is equal to the dew point temperature of the gas being measured. Such sensors are often used by research institutions.
Most of the sensors used in cleanrooms include resistive hygrometers, capacitive hygrometers (condensation meters), and mirror dew points. When choosing an appropriate instrument, you should not only pay attention to its price and product specifications, but also its measurement accuracy, the manufacturer's application experience, and the services available. All of these factors help to improve the user's actual experience and help to achieve a successful run.
Regularly traceable calibration is very important
Make sure that the data provided by the measuring device is reliable and accurate when calibrating. Regular calibration is absolutely necessary. See Table 1 for commonly used calibration cycles. Table 2 gives an example of a traceable information chain for installed humidity and temperature devices. Globally, all measurement data is based on the global system of international units (SI). This ensures that we use the same amount and that the data measured on various types of devices is comparable in a variety of locations.
Table 1 Typical calibration interval period of the measuring device

Table 2 Example of Traceability Chain of Humidity and Temperature Measuring Devices

The National Laboratory is responsible for retaining and developing this traceability record and providing the highest precision calibration services. The calibration services provided by the National Metrology Standards Laboratory may be limited to the highest level of original standard calibration.
Commercial calibration services are available for lower standards and measurement equipment. These can be calibration services provided by the manufacturer for their own products, or the laboratory can provide calibration services for any measuring device. Unapproved calibration services refer to calibration services and a large number of commercial calibration services from most service providers, including most measurement equipment manufacturers. Unqualified services are not reliable. Prior to use, calibration qualifications should be reviewed accordingly. Each calibration service provider must have an effective traceability information chain. The minimum requirement is that the original standard must be calibrated in an external laboratory before being used for calibration services. The calibration certificate provided by some commercial calibration services does not include an uncertainty estimate for this service if not separately requested. Some calibration services simply cannot calculate uncertain data. Therefore, please be sure to consider the qualifications of these services.
Having an internal calibration system is sometimes feasible. This is effective if the measuring device is difficult to transfer (on-site calibration) or if the number of calibration devices is large. To establish an internal calibration system, an appropriate organization should be established. This institution can have only one person or a department with managers and calibration staff.
Laboratory calibration is usually better than field calibration. In the laboratory, the environmental impact can be minimized and the adverse effects affecting the calibration can be significantly reduced. Field calibration is a quick and easy way to inspect a measurement device without removing the device from the processing area. Field calibration requires an operating standard as a baseline. Such operating standards may dictate the use of handheld devices or other devices to calibrate instruments installed at the process site. Operating standards should be at a higher level
The laboratory is calibrated.
Vaisala has been certified for calibration services for Vaisala pressure, temperature, dew point and humidity instruments. Users can get the corresponding calibration service through the regional service center. These services can be used for both installed devices and new devices delivered.
The Vaisala Calibration Manual is available free of charge at book. This manual contains all the useful information about calibration.
For details, please see:
references:
Arun S. Mujumdar; Handbook of Industrial Drying (2006)
Arun S. Mujumdar; Industrial Drying Handbook (2006)
Vaisala Calibration Book (2007)
Vaisala Calibration Manual (2007)

Neurosurgical Operating Table

Electrical Surgical Table,Mechanical Operating Table,Neurosurgical Operating Table,Mechanical Surgical Table

NINGBO TECHART MEDICAL EQUIPMENT CO.,LTD , https://www.techartmed.com

Posted on