Process instrumentation and sensors require periodic calibration and maintenance, and change over time and/or usage due to drift, environmental factors, process changes, output loop changes and what not. It is important that the information we get from measurements is “reliable and accurate”.
Electrical calibrators are designed to source and measure electrical signals such as voltage, current, resistance, and frequency, and pulses so as to verify the accuracy of sensors and measuring instruments.
Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device. The device with the known or assigned correctness is called the standard. The second device is the unit under test, test instrument, or any of several other names for the device being calibrated.
The calibration procedure depends on the type of sensor or instrument that is to be calibrated. The calibrator generally sources or simulates a signal with known accuracy which is read by the unit under test (UUT). Any discrepancy is the error of the UUT, and can then be adjusted until it displays the correct value. Typically, the UUT is checked at several points throughout its calibration range.
To communicate the quality of a calibration the calibration value is often accompanied by a traceable uncertainty statement to a stated confidence level. This is evaluated through careful uncertainty analysis.
In many countries a National Metrology Institute (NMI) will exist which will maintain primary standards of measurement (the main SI units plus a number of derived units) which will be used to provide traceability to customer's instruments by calibration. The NMI supports the metrological infrastructure in that country (and often others) by establishing an unbroken chain, from the top level of standards to an instrument used for measurement. Examples of National Metrology Institutes are NPL in the UK, NIST in the United States, PTB in Germany and many others. Since the Mutual Recognition Agreement was signed it is now straightforward to take traceability from any participating NMI and it is no longer necessary for a company to obtain traceability for measurements from the NMI of the country in which it is situated.
Multifunction Calibrators can be nicknamed the "do everything instrument" and are typically portable and can measure and source multiple DC signals such as voltage, current, frequency, pulse, resistance and power. Often times they can be fitted with special probes to give them the capability to measure AC current and voltage.
Some examples of their capabilities are:
- VOLTAGE: Very common in process applications, used in petrochem, water, level, humidity, aerospace (airspeed) and many other applications.
- CURRENT: The most common process signal, and integral to 4-20 mA current loops used in most industrial automation applications.
- FREQUENCY: Used in a variety of process applications such as counters and flow meters.
- RESISTANCE: Commonly used in temperature applications such as aResistance Temperature Detector (RTD) and thermistor.
- DIGITAL SIGNALS: HART and Field Bus Communications are the most common process digital signals used with sensors and instrumentation.
- The HART Communications Protocol (Highway Addressable Remote Transducer Protocol) ia a digital industrial automation protocol which communicate over legacy 4-20 mA analog instrumentation wiring.
Fieldbus is the name of a family of industrial computer network protocols used for real-time distributed control, standardized as IEC 61158. SENSORS and PROCESS TRANSMITTERS: A multifunction calibrator can simultaneously power up and measure a variety of sensor types. For example, a pressure transmitter can be powered up and have its signal output measured against a pressure reference built into the multifunction calibrator.
Once the device under test has been tested, it can be adjusted to bring it into tolerance. This is called, calibration!
Loop calibrators are specifically designed for 4-20 mA current loops. These instruments are capable of measuring and sourcing current and commonly used as portable instruments in the field for troubleshooting sensors and transmitters.
What is a Current Loop?
For industrial process control instruments, analog 4–20 mA current loops are commonly used for analog signaling, with 4 mA representing the lowest end of the range and 20 mA the highest. The key advantages of the current loop are that the accuracy of the signal is not affected by voltage drop in the interconnecting wiring, and that the loop can supply operating power to the device. Even if there is significant electrical resistance in the line, the current loop transmitter will maintain the proper current, up to its maximum voltage capability. The live-zero represented by 4 mA allows the receiving instrument to detect some failures of the loop, and also allows transmitter devices to be powered by the same current loop (called two-wire transmitters). Such instruments are used to measure pressure, temperature, level,flow, pH or other process variables.
Depending on the source of current for the loop, devices may be classified as active (supplying power) or passive (relying on loop power). For example, a chart recorder may provide loop power to a pressure transmitter. The pressure transmitter modulates the current on the loop to send the signal to the strip chart recorder, but does not in itself supply power to the loop and so is passive. (A 4-wire instrument has a power supply input separate from the current loop.) Another loop may contain two passive chart recorders, a passive pressure transmitter, and a 24 V battery. (The battery is the active device).
Panel mount displays and chart recorders are commonly termed 'indicator devices' or 'process monitors'. Several passive indicator devices may be connected in series, but a loop must have only one transmitter device and only one power source (active device). The relationship between current value and process variable measurement is set by calibration, which assigns different ranges of engineering units to the span between 4 and 20 mA. The mapping between engineering units and current can be inverted, so that 4 mA represents the maximum and 20 mA the minimum.
Questions to ask yourself when deciding on which Electrical Calibrator to Choose:
- What type of signals are used by my equipment?
- What measurements are critical to my process and that need to be calibrated?
- Where will I calibrate? In the field or in the lab?
- What level of accuracy is needed?
- Are any communication protocols needed (HART, Field Bus etc)?
- What type of calibration certification is needed: As Left / As Found and As Left Data, NAVLAP Certification etc?
If you have any questions regarding electrical calibrators please call to speak with one of our engineers