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Dry block temperature calibrators

Original article date: March 1999

Sensors used in process and production machinery have to be calibrated against known standards from time to time. GAVIN SANDEMAN Marketing Manager of HAWCO explains the latest thinking.

Whichever type of heat sensor is employed within a process line – RTD thermocouple thermistor or thermoswitch – the simple fact remains that in the course of time and under specific process conditions all of these sensor devices will exhibit temperature drift.

The conventional method of detecting errors is to apply instrument calibration techniques to the sensor assemblies and to produce a traceable known temperature standard. The procedure can be carried out within a manufacturer’s own laboratory or by an external calibration service. The temperature instrumentation may then be adjusted according to observed results.

In terms of calibrator design both liquid bath and dry block units are in operation throughout industry. Liquid baths may be preferred for calibrating a multiple number of temperature probes. However generally they are found to offer slow heating and cooling times and are not usually portable. Therefore they are not practical for on-site calibration.

The dry block calibrators which are fully portable ensure probe evaluations to be carried out adjacent to the appropriate process stage enabling easy clean safe reliable and traceable calibration to be achieved with the minimum interruption to process operations.

The traditional Ametek Jofra design of dry block calibrator the 140SE has been produced for the last eight years and has a temperature range of -45 to 160degC. It is claimed to be the only temperature calibrator that can be powered by a 12 battery extending portability to locations where access to mains power is a problem.

Now a new family of Jofra dry block temperature calibrators which are designed for extra portability has been launched by Hawco. The Compact series covers the range -30 to 650 and is lighter more rugged and has faster heat-up and cool-down times. Other standard features include programmable slope rate with programmed steps which makes it possible to repeat accurate temperature settings at precise intervals. In this way operators can calibrate the whole process control loop and ensure conformity with the limits set for a particular production cycle.

In-house calibration is made easier with a menu-driven software program which enables users to recalibrate and adjust the calibration system themselves. Frequent use of the program allows the process operator to adjust for calibrator drift and therefore get more reliable and accurate results. User-defined procedures can be input via an RS 232C interface enabling measurement results to be verified automatically and reports to be generated.

Some processes demand low temperature measurement from installed sensors. Although the calibration unit may operate within a temperature range from -30 to 140degC a separate remote tower incorporating just the dry block assembly may be employed to avoid stressing the on-board electronics of the standard unit. Remote towers can be used where access is difficult or where the probe assembly is very small or very long relative to a typical configuration.

Because electromagnetic interference may influence measuring results calibrator design must fulfil CE regulations including the EMC and Low Voltage Safety Directives.

TRACEABILITY

  • International laboratories: eg NPL NIST BNM PTB
  • National accredited laboratories: eg DANAK NAMAS (*) SIT DK
  • Manufacturer’s laboratory
  • End User

(*) NAMAS the National Accreditation of Measurement And Sampling is overseen by the UK Accreditation Service. Its function is to ensure that measurements and tests done in NAMAS accredited labs are done competently and are traceable (at known levels of uncertainty) to national and international measurement standards.

In practice NAMAS goes considerably further than this (and further than ISO 9001). To be precise NAMAS works to standards laid down in EN 45001/45002 equivalent to ISO guides 25 and 54. What this means is that NAMAS regularly assesses everything from calibration records and staff training to the quality of your intruder alarm (it protects the confidentiality of your results). NAMAS accredited results are recognised by the national standards laboratories of many countries worldwide.

March 1999

Added on 1 May 2009 at 5:49 pm | Read more machine building articles