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3-IN-1 HAND-HELD TEMPERATURE CALIBRATION

Even in this digital world of ours, most primary measurements in process plants are still analogue by nature – they only become digital down-line. Temperature starts out as voltage derived from joining two dissimilar metals together or from a charge in resistance from metals like platinum, nickel or semiconductors or measuring infrared radiation. Pressure, flow and force are measured by physical deformation with strain gauges (i.e. Wheatstone bridges or piezo-electric effects). There are myriad ways in which Level can be measured and most start with an analogue measurement. It’s for this reason then, that in plant maintenance, commissioning and instrumentation workshops, process calibrators remain indispensable.

Even in this digital world of ours, most primary measurements in process plants are still analogue by nature – they only become digital down-line. Temperature starts out as voltage derived from joining two dissimilar metals together or from a charge in resistance from metals like platinum, nickel or semiconductors or measuring infrared radiation. Pressure, flow and force are measured by physical deformation with strain gauges (i.e. Wheatstone bridges or piezo-electric effects). There are myriad ways in which Level can be measured and most start with an analogue measurement. It’s for this reason then, that in plant maintenance, commissioning and instrumentation workshops, process calibrators remain indispensable.

Accuracy

Because most field transmissions have error limits of less than 0,5%, calibration equipment needs to be an order of one magnitude higher (0,05%). Modern single chip voltage references, 24 bit serial A/Ds and 20 bit D/As can be within full-scale limits of 0,01%. The trick is to design input circuits with low-drift amplifiers for voltage and high stability shunts for current and resistance, for minimum additional error. In linear or rotary measurement, most inputs are already digital by nature from encoders, whether inductive, optical or magnetic. The calibrator’s function is then to measure or source frequency or counts. This is referenced to the quartz time-based used for the microprocessor which is extremely accurate and, in some case - where absolute ‘realtime’ is needed - from a GPS satellite.

Practical considerations

When a precision instrument like a process calibrator is going to be used in the field it had better be rugged, both physically and electrically and have an acceptable battery life. The housing needs to be thicker than normal, have a rubber protection cover to withstand knocks and drops and be reasonably splashproof. Besides the normal CE requirements, for electrical emission and noise immunity, inputs need additional surge protection with heavy duty MOVs and high speed fuses.

Instrotech’s locally designed and manufactured Calog range of portable process control calibrators incorporate the most modern component technology, comprehensive feature capabilities and tough package design, making the units ideal for use in servicing, repairs in workshops and in tough plant environments.

The hand-held CALOG-TEMP multifunctional temperature calibrator is a high-precision calibrator for the calibration and troubleshooting of process control instrumentation. It has selectable automatic, manual or internal cold junction compensation. Temperature can be displayed digitally or in the form of a trend, ideal for monitoring temperature over time or optimizing PID controllers. Inputs and outputs are isolated from each other and can be displayed simultaneously making this unit ideal for measuring temperature or calibrating 2, 3 or 4-wire transmitters. The mA output can be set to track the temperature for use as a transmitter replacement in emergencies.

In RTD mode the calibrator is suitable for 2, 3 or 4-wire measurement or sourcing of platinum and nickel RTDs as well as ohms and mA. RTD, ohms and mA can be measured or sourced and as they are isolated from each other, and can be displayed simultaneously making this unit ideal for measuring temperature or calibrating RTD transmitters. Data logging (SD card) and downloading via the on board USB port is standard. It measures milliamp, millivolt, thermocouple, ohm and resistance temperature detector (RTD) in a single device. It simultaneously measures and sources ten commonly used types of thermocouples (K,J,T,B,R,S,E,N,U,L) and seven (Pt50, Pt100, Pt200, Pt500, Pt1000, Ni100 and Ni120) resistance temperature detectors (RTD). It measures milliamps (passive or active) with a 24 V power supply whilst sourcing or simulating millivolts, TCs, RTDs or ohms to calibrate transmitters. The thermocouple ranges have automatic internal cold junction or manual or external temperature compensation. The temperature ranges are isolated from the mA and can be displayed simultaneously. Injecting thermocouple millivolts (or RTD ohms) into a transmitter while measuring the output (mA) simplifies field or workshop calibration.

A plug-in extender is supplied with spring terminals for bare wire connection directly to thermocouple wires or compensating cables. The cold junction correction is built into this unit.

The backlit LCD display can be selected for measurement in a clear, precise numeric display or as a graph for use when logging temperature or milliamps over longer periods. This is ideal for when monitoring ovens or optimising PID control.

For more information on the Calog-TEMP multifunctional temperature calibrator, Hugh du Plessis at Instrotech on 011 462 1920 or visit sales@instrotech.co.za



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