Test and calibration: Differences defined
08 September 2014
Test and calibration in the instrumentation sector are sometimes confused. However, test is not calibration (although, for low spec instruments, it can be) and calibration involves more than mere connection to a sensor or instrument, as Mike Shelton, GE Measurement & Control, explains
In its basic form, testing relies on connecting a test instrument to the device to be tested to establish that there is a reading and that that reading is what should be expected. A multimeter is a test instrument, which is regularly used by electricians to test circuits and electrical parameters. And many test devices in the process instrumentation sector have multimeter functionality. However, instrumentation calibration involves much more than checking voltage, current and resistance.
So what then is the difference between testing and calibration? Well, basically, it comes down to the level of accuracy of the testing instrument. If a multimeter with 0.5% accuracy is used to check a transmitter with a 4-20mA output, then the actual accuracy of the multimeter is not important. However, when a device needs adjustment, a test/calibration instrument of suitable accuracy is required. For example, a 0.5% accuracy multimeter cannot be used to adjust and calibrate a transmitter of 0,1% accuracy. On the other hand, if the transmitter has just 2% accuracy, then the 0.5% multimeter may well be good enough.
Calibration itself involves the comparison of two instruments or measuring devices, one of which is of known accuracy, to establish the accuracy, or otherwise, of the device to be calibrated. Calibration is necessary because all measuring devices drift over time and calibration is carried out to ensure we are measuring to the accuracy required, accuracy set on installation or accuracy set later to some agreed value. Both testing and calibration are carried out to ensure process or product quality but calibration provides the precise data essential to meet regulatory standards and improve operational efficiency and safety.
Test and calibration instruments
There are test and calibration instruments and accessories to suit every situation, regime and budget. Simple portable test instruments include pressure indicators and digital pressure test gauges, which are used with ancillary hand pumps, and smart loop testers to check the electrical parameters of pressure devices. Some of these instruments are very accurate and can also be used in calibration and can source the relevant parameters.
Typical instruments are RTD-, thermocouple, electrical and frequency loop calibrators. Dedicated pressure calibrators incorporate a pressure generating capability and, for example, the DPI611 from the Druck product line of GE, can create 95% vacuum or generate maximum pressure of 20bar in just 30 seconds by means of a mechanical pressure-generating system, while holding the instrument in one hand or on a table top. Some instruments include HART communications, which means they can be used with digital instrumentation increasingly found in control systems which have historically been analogue.
However, today’s instrumentation engineer has a choice of increasingly sophisticated portable calibration instruments. GE’s Genii, for example, combines multi-function and pressure calibration functionality into an advanced, hand-held indicator/calibrator. It can measure and simulate the electrical parameters, temperatures and frequencies associated with the majority of process instrumentation and offers a modular and highly flexible pressure system, which can generate pneumatically from 95% vacuum up to 20 bar and up to 100 bar and hydraulically up to 1000 bar. It can make life easier and productive for the instrumentation engineer with its intuitive swipe screen technology and TASK menu which allows single touch configuration for a range of devices.
In addition, the HART and Fieldbus communication capabilities of Genii work seamlessly with the calibration functions, allowing configuration and commissioning as well as calibration of digital devices. Moreover, it integrates with calibration and maintenance software, such as GE’s 4Sight, so that calibration and configuration data is available from the field devices through to Enterprise Resource Planning (ERP).
Decisions regarding the suitability of measurement devices for the purpose of calibration are complex and procedures are needed to determine their suitability in different applications. This can be a minefield, so one method is to select test equipment that has accuracy suitable for calibration across all devices on the plant. A test tool cannot always be a calibrator, but a carefully selected calibrator can always be a test tool.