Process instrumentation, from the simplest to the most complex instruments, requires periodic calibration and maintenance to ensure that it’s operating correctly. Drift,
environmental factors, electrical supply, the addition of components to an output loop, and process changes can all cause process instruments to lose accuracy. Only by calibrating
those instruments can accuracy be assured. Electrical calibrators are the class of calibrators designed to source and measure electrical signals such as voltage, current,
resistance, and frequency as a way to verify the accuracy of sensors and measuring instruments.
Calibration
Calibration is a comparison between two devices. The first device is the unit to be calibrated, often called the unit under test. The second device is the calibrator, which has been
fixed to a standard of a known accuracy.
The exact calibration procedure depends on the type of instrument being calibrated. For displays and controllers, the calibrator generally sources or simulates a signal which is read
by the unit under test. As the signal sent by the calibrator is of a known accuracy, any discrepancy is the error of the unit under test. The unit under test can then be adjusted until
it displays the correct value.
For sensors and transmitters, the calibrator generally reads the signal sent from them while they are under test conditions, such as a pH sensor being immersed in a calibration solution
of a known value. With the test conditions being exactly known, any discrepancy between those conditions and the measured value is the error of the unit under test. Adjustments can then
be made to account for that error.
Typically, calibration of an instrument is checked at several points throughout the calibration range of the instrument.
Not all standards are created equally. While all standards have a known accuracy, there are some—known as primary standards— that are the highest level of accuracy for a specific parameter.
Primary standards achieve their high accuracy by relying upon measurement technologies using fundamental physical constants that do not drift. For example, the value of the volt is defined
by the Josephson Effect which has an accuracy of 1 part per billion. These fixed values minimize uncertainty, making primary standards the most accurate calibration tools.
Though the accuracy of calibrators varies pretty widely depending upon the model and the measurement parameters, they can be roughly categorized into three groups :
- Industrial standards, also known as field standards ideally have an accuracy 4 times greater than the instrument being calibrated. These are
useful for spot checking sensors at the point of use rather than a laboratory environment.
- Secondary standards, also known as laboratory standards provide greater accuracy than field standards and are used to calibrate field standards.
- Primary standards provide the highest calibration accuracy. Primary standards are used to calibrate secondary standards.
Traceability
To improve the quality of a calibration to levels acceptable to outside organizations, it is generally desirable for the calibration and subsequent measurements to be traceable to
internationally recognized standards. Establishing traceability is accomplished by a formal comparison to a standard which is directly or indirectly related to national standards
(such as NIST in the USA), international standards, or certified reference materials.
Multifunction Calibrators
Multifunction calibrators are the do everything calibration instrument. They include a sophisticated array of electronics that allow them to very accurately read and source voltage,
current, frequency and resistance signals. This versatility enables multifunction calibrators to calibrate any device that uses those signals as either input or output.
Some examples of calibration applications involving different signal types:
- Voltage: Voltage signals are extremely common in process applications. They are found in pressure, level, water quality, humidity, airspeed and many other
applications.
- Current: Like voltage, current is a very common signal. It is also integral to current loops found in a wide range of recorder and data logger applications.
- Frequency: Frequency signals are often used in applications which utilize pulse output. This includes flow meters and counters.
- Resistance: Resistance signals are commonly found in temperature applications involving RTDs and thermistors.
Due to the versatility of multifunction calibrators, they can easily calibrate both sensors/transmitters as well as meters and display devices. For meters and displays, a multifunction
calibrator can source a highly accurate signal to be read by the meter or display, easily indicating the accuracy of the meter or display.
Sensors and transmitters, on the other hand, can be subjected to test conditions—for example a pressure sensor having pressure applied to it via a hand pump—and the multifunction calibrator
set to read the signal sent under those conditions. Again, the accuracy is easily determined by comparison between the pressure applied and the signal sent by the sensor.
Once the accuracy of the device has been tested, adjustments can be made to bring it to within tolerance.
Loop Calibrators
Loop calibrators are a type of electrical calibrator specifically designed to troubleshoot 4-20 mA current loops. These versatile instruments are capable of measuring current, sourcing
current to unpowered devices in a loop, as well as simulating the operation of loop-powered 4-20 mA transmitters.
An analog current loop is an electrical signaling scheme that uses a current source and a current receiver. Current loops are extremely common in industrial measurement and control applications
due to their ease in setting up, wide power supply requirements, low noise output, and ability to be transmitted over great distance without loss—even if there is significant electrical
resistance in the line.
With current loops, the sensor draws current from its power source in direct proportion to the mechanical property it measures. For example, a 0 to 100 psi pressure sensor with a 4-20 mA current
loop draws 4 milliamps from its power supply at 0 psi and 20 milliamps at 100 psi. The relationship between current value and the corresponding process measurement value is determined by
calibration by which measurement values are assigned ranges within the span of 4-20 mA.
A primary benefit of the current loop is its simple wiring. Generally, a current loop consists of a power supply, a transducer and one or more pieces of instrumentation all connected together
in a ring with the supply voltage and measuring current supplied over the same two wires. Current loops are used to measure pressure, temperature, flow, pH, or other process variable. A
current loop can also be used to control a valve positioner or other output actuator.
Depending on the source of current for the loop, devices may be classified as active or passive. Some instruments have an active output which includes both the control of the current in the
loop as well as provide the supply voltage. Passive loops, on the other hand, require an external power supply.
Things to Consider When Selecting an Electrical Calibrator:
- What type of signal is used by the equipment to calibrate?
- What measurement parameters do the units to be calibrated have?
- Where will calibrations take place? The lab? The field?
- What level of accuracy is needed?
- Are any communication protocols needed?
- What types of adapters are needed to attach the unit to be calibrated to the calibrator?
- Is a calibration certificate is needed? Which one?
If you have any questions regarding electrical calibrators please don't hesitate to speak with one of our engineers by e-mailing us at sales@instrumart.com or calling 1-800-884-4967.
