There has always been a dilemma amongst Instrumentation Tech’s who are starting out on their own as contractors:  what calibration equipment can I buy on a limited budget?  The decision ultimately comes down to a choice of a multifunction calibrator, or a couple of single-function models that will replicate the parameters of a multifunction calibrator.  Let’s spend some time to investigate the features and benefits of using single-function calibrators.  What is the best tool for the job?

A single-function calibrator is most often used when the workload of the technician is predictable – the technician knows that the work he is heading out to do involves a parameter that deals with either temperature, pressure, or electrical.  Whether he knows it or not, the decision to go this route can result in more accurate readings, and better repeatability.  Single-function models are less prone to software glitches due to their simple operation and less interaction between modes of operation.

Typically, single-function calibrators offer a wider range of instrument compatibility, such as intermittent RTD currents and lower system noise.  Most RTD transmitter that are being manufactured today communicate by means of a low, pulsed excitation current, and manufacturers of RTD calibrator are focusing their design toward this new technology.  Make sure your RTD calibrator is able to simulate into HART Smart Transmitters.  If your RTD calibrator doesn’t always work when you are performing a calibration, you may have a compatibility issue, a clash of ‘old world meets new world’.

Some calibrators that perform a dual role of RTD and T/C source and measurement can still be called ‘single-function’, in that their sole purpose is to address the temperature parameter in the instrument loop.  The compact nature of the circuitry can still allow for optimum measurement and temperature stability over time.  Usually, the specifications are identical for each mode when it comes to temperature /time drift.  The ability to automatically detect 2-, 3-, and 4-wire RTD connections is a troubleshooting tool worth looking into.

In the overall picture, a lot can be said about the single-function calibrator.  Being more compact by design, there is less of an internal conflict in design trade-off’s to optimize one feature that hinders another.  This improves calibration and test reliability due to less confusion in option selection and information displayed.  The overall cost of several single-function calibrators will be more than an “all-in-one”, however, the downtime cost of your equipment should be major decision-maker when your calibrator is in for service work – you are only without one parameter with a single-function model, and this can easily be rectified.  The cost of annual calibration or repair is much lower for single-function calibrators as well.

So what features should you look for when you are shopping for the best in single-function calibrators?  We have already discussed the temperature aspect, so let’s look at some of the other notable single-function calibrators on the market today.

Most loop calibrators should be able to accurately source and read mA and VDC, provide 24 VDC loop power, and simulate a transmitter.  Having the ability to display current in mA and as a percent with high contrast graphics to 5-digit resolution and being able to use a ‘hands-free’ auto-stepping function has been invaluable when working with valves.  Most higher-quality loop calibrators have a HART Protocol compatibility mode, where the user can power on a selectable 250 ohm resistor in series with the output when calibrating HART – enabled instruments.  A new feature that has been added to the array of loop calibrators allows the user to view a real-time diagnostic measurement.  This ‘loop diagnostic’ displays loop current, voltage, resistance, and detects any AC transient voltage riding on the DC.  In addition, the loop diagnostic screen will display a measurement of ground leakage current due to faulty wiring, corrosion bridges or moisture in conduit.  As you can see, not all loop calibrators are created equal!

Frequency calibrators are used primarily in flow measurement, and a good one for the job should be able to source and read counts/minute and have a wide range of frequencies, sine and square wave-forms.  Having a frequency calibrator than can also batch and totalize counts will make flow meter proving a ‘one-instrument job’.

So, there you have it.  Single-function calibrators have the versatility to cover a very broad scope of instrumentation applications combined with manufacturers who continue to develop models with even better measurement resolution and features.

Written by:  Stephen J. Kelly, B.Sc., C.E.T. – Cameron Instruments Inc.


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