The standards and accuracy levels achieved are identical in all cases.

The ALARMLOG uses a Pt100 Platinum Resistance Thermometer element to BS EN 60751 (IEC751): 1996, Class A, for temperature measurement.

This is used in a four terminal configuration, with an accurately controlled reference current of 1 mA being passed via two terminals, and the resulting developed voltage measured across the other two terminals. By using a four terminal configuration, errors caused by connecting cable lengths can be completely eliminated. The standard Contronics probe can be used with cable lengths in excess of 30 metres without any measurable errors.

The voltage developed across the Pt100 element, which is directly proportional to its resistance, and hence directly related to the measured temperature is converted, within the ALARMLOG, to a digital value using a 12 bit analogue to digital converter (ADC) which is capable of measuring 4096 discrete levels. The ALARMLOG is designed so that these 4096 levels span a temperature range slightly greater than the 200 ºC measuring range of the ALARMLOG. The temperature resolution of each ADC step is thus approximately 0.05 ºC.

Each of the 4096 steps of the ADC output is directly converted to an equivalent digital temperature value in units of 0.1 ºC by a 4096 entry look-up table stored within the ALARMLOG.

This table is recalculated individually for each ALARMLOG during the calibration process and loaded into non-volatile memory within the ALARMLOG.

As described above, the calibration accuracy of an ALARMLOG depends primarily on the generation of a 4096 entry look up table which accurately converts ADC output, representing Pt100 resistance, to a digital value of temperature.

Calibration is performed by accurately measuring the ADC output of the ALARMLOG at three equivalent temperatures, at range minimum, centre and maximum, and computing the values of each look-up table entry using curve fitting equations so that each of the three calibration points translate to the correct temperatures. The accurate temperature reference levels are derived from precision fixed resistors with resistance values equal to those of an ideal Pt100 resistance thermometer element at the required reference temperatures. These reference resistors are provided by a Contronics Pt100 Temperature Reference Unit, which is described below.

The curve fitting equations used are the Callendar - Van Dusen equations for an ideal Pt100 temperature element, with coefficient values defined in BS EN 60751 (IEC 751): 1996.

The calibration process is semi-automatic, and is controlled by a custom PC Calibration Program, ALARMLOG Calibration for Windows™ 32.

As can be seen from the description above, the ALARMLOG is calibrated, assuming an ideal Pt100 temperature probe. To the measurement uncertainties of the actual calibration must therefore be added any probe errors. All Contronics ALARMLOG installations use Class A probes, which have a maximum error of +/- 0.15 ºC at 0 ºC, rising to +/- 0.55 ºC at +/- 200 ºC.

All probes are purchased to this specification, and can be supplied with certificates of conformity thereto . All probes are individually serial numbered.

The Contronics Pt100 Temperature Reference Unit, Type EU281, used for all ALARMLOG temperature calibrations, contains 9 manually switched precision temperature reference values at 50 ºC steps between -200 ºC and +200 ºC.

Each temperature reference has been individually checked and certified by an accredited test laboratory using standards traceable to national standards, to an equivalent temperature accuracy of better than +/- 0.05 ºC, under the same measurement conditions present when used on an ALARMLOG.

The ALARMLOG Calibration for Windows™ 32 program has been custom written by Contronics to permit ALARMLOG calibration to be performed by users to the same accuracy standards used within the factory. The program requires no skill to operate, other than the ability to read instructions and set switch positions on the Pt100 Temperature Reference Unit.

The program automatically detects the model of ALARMLOG being calibrated and selects appropriate temperature calibration and verification values. Following calibration, the calibration accuracy is checked at five different temperatures over the full operating range of the ALARMLOG, and a calibration certificate printed automatically listing the full results of the calibration.

A calibration will be considered to have failed, if any of the five validation measurements, following calibration, differs from the true measurement by more than +/- 0.1 ºC.

Following a successful calibration, the calibration date is automatically stored within the ALARMLOG, and can be inspected at any time using the normal ALARMLOG for Windows™ 32 or SCANLOG for Windows™ 32 data analysis programs.

This section describes the steps used by the ALARMLOG Calibration for Windows™ 32 program to perform an ALARMLOG calibration.