index Temperature sensors within the process and pharmaceutical industries are important devices in ensuring that the process runs smoothly and product specifications are met on a continuous basis.  The calibration of these sensors on a regular schedule is important to provide confidence in the measurements being taken and thus the quality of the end product being produced.  This blog looks at the various types of temperature sensors, contrasts the value/benefit of dry-block calibrators verses liquid bath calibrators, and the effective process to follow in specifying and procuring temperature calibrators.

Where are the temperature sensors located in process control?

Temperature sensors are found in a variety of processes and instrumentation.  They are often embedded in motors, valves, turbines, bearings and other devices.  They can also be found in so called “smart” devices such as flow meters, pressure transmitters, and level transmitters.  They may also be stand-along temperature devices such as temperature indicators, controllers, recorders and thermometers.

Why Calibrate the Temperature Devices within a Plant?

images1Calibration of temperature sensors helps to ensure the quality control of the entire process and the quality of the end product.  The accuracies of the temperature measurements can ensure that the process is running at the correct temperatures, correct dosages of ingredients are included, and the final product matches the product specifications and leads to ultimate customer satisfaction.  In some industries like the pharmaceuticals and nuclear sites, calibration is one of the requirements in maintaining conformance to the industry and Code of Federal Regulations (CFR).  Often periodic calibration and associated documentation is a critical area of investigation for these regulatory auditors.  Quality systems such as ISO-9000 or ITS-16049 often call for the periodic calibration of all sensors as well.  In addition to the reasons above, some facilities have robust calibration programs to maintain the safety of the plant and the operating personnel.  Finally, general manufacturing practices call for calibration of instrumentation to identify and prevent process defects within the manufacturing process.

Types of Temperature Calibrators

While there are a variety of ways to calibrate temperature devices, this paper will focus on 4 primary methods of calibration:

  • The use of intrinsic standards or fixed point cells
  • Liquid bath calibrators
  • Dry-block calibrators
  • Electronic calibrators

Intrinsic Standards

The use of intrinsic standards such as the triple point of water, melting point of certain metals like Aluminum, Indium, Tin and Zinc are often used to calibrate temperature sensors.  Because these methods are intrinsic and occur naturally, these can be the most accurate form of temperature calibrators.  However, to cover the entire range of typical temperature calibrations, often times multiple standards are required.  These standards can be very expensive and as such are typically only found in calibration laboratories or those plants with high end metrology departments in house.

Liquid Bath Calibrators

imagesLiquid baths incorporate a liquid (typically oil) and a stirring mechanism along with a heating/cooling element.  Because of the constant stirring of the liquid, baths tend to have very uniform specifications throughout the bath as the liquid is circulated throughout the well.  Additionally, baths are useful when calibrating odd shaped sensors such as a large thermo well with large head.  Baths can also be useful in the calibration of very small sensors due to the uniformity of the liquid.  In the calibration of some types of stand-alone sensors such as liquid in glass thermometers, liquid baths are the best method to use in the calibration process.

Liquid baths tend to be fairly stationary (non0portable) and are not easily transported to various areas of the plant.  Because of the oil involved, liquid baths can also be messy and difficult to maintain.  Periodically the baths must be drained and re-filled with the oil and the oil then must be properly disposed of.  The replacement oils can be expsnive as well, with some oils costing upwards of $1000/gallon.  Liquid baths also tend to take longer to reach temperature, thus adding time (and money) to the calibration process.

Please stay tuned for part two of this post discussing Dry-Block calibrators, electronic calibrators and advantages to each.

Written by:  Keith Hadley of Ametek Test & Calibration

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