Image Credit: Badger Meter | Buck Research | OMEGA
Calibration instruments use electrical signals or physical quantities to calibrate sensors and meters.
Calibrators can be broadly classified based on their function, and further classified by the type of instrument they calibrate.
Precision measuring calibrators can test or compare the output of an instrument, sensor, or element to a known standard or reference.
Precision source calibrators produce a standard output signal which is used to calibrate an instrument or sensor input.
Calibration standards and reference sources are used when a particular instrument cannot be calibrated using another instrument or calibrator. In this case, a reference standard of known properties is used to calibrate the instrument. Reference standards include weight and mass standards, chemical standards, calibration gases, hardness test blocks, and time and frequency standards.
Calibration instruments are specific to the type of instrument requiring calibration. For example, a pH meter requires a pH meter calibrator, and a current meter requires a current meter calibrator. Calibrators can be grouped into three groups based on this principle:
Electrical instrumentation calibrators calibrate instruments that measure or sense attenuation, capacitance, voltage, charge, inductance, power, or other related electrical specifications.
Physical property instrumentation calibrators are used with devices that measure acceleration, vibration, acoustics, displacement, position, force, torque, or other physical properties.
Process instrumentation calibrators are used to calibrate instruments that measure conductivity, flow, environmental factors, water quality, pH, and other properties.
Calibrating two types of tachometer using a tachometer calibrator (a physical property calibrator).
Video credit: gweecy / YouTube
Calibration Signal Specifications
Common calibration signals include:
AC Voltage - Devices produce AC voltages that can be used to calibrate AC voltage sources or other sensors or instruments.
DC Voltage - Devices produce DC voltages that can be used to calibrate DC voltage sources or other sensors or instruments.
AC Current - Devices produce AC currents that can be used to calibrate AC current sources or other sensors or instruments.
DC Current - Devices produce DC currents that can be used to calibrate DC current sources or other sensors or instruments. The most common instruments are transmitter calibrators, which produce mA currents (e.g. 4 - 20 mA).
Calibration instruments that produce a pulse stream, pulse range, or frequency range are also available.
Signal and waveform generators are designed to produce a sine wave, square wave, triangular wave, or other predefined waveform.
- Sine wave is the best waveform, as it is the shape of an (ideal) AC electrical signal from the wall. The highest-quality inverters produce a true sine wave output, which requires fairly expensive components in the inverter. True sine wave outputs are normally found only in higher-end models.
- Square wave is a flattened version of a sine wave. Instead of the voltage smoothly increasing from the negative maximum to the positive maximum and back again, it shifts suddenly from negative to positive, stays there for half a cycle, and then jumps to full negative and stays there for half a cycle, then repeats.
- Stepped wave is a quasi-square wave or sine wave - they are typically inexpensive power inverters that mimic a sine wave using a stepped waveform. The output waveform signal changes abruptly from zero volts to a maximum value, then abruptly again to a zero value. After a short off time, the signal then switches abruptly again to a maximum negative voltage and then abruptly again back to zero.
- Triangular wave is sometimes referred to as a saw tooth wave for its approximation to a saw blade.
Wave Examples. Image Credit:Wikipedia
Most manufacturers specify calibration instruments with a range of signal values that includes minimum and maximum amounts.
Selecting calibration instruments requires an analysis of physical, electrical, process, environmental, and thermodynamic properties.
- Physical properties indicate whether instruments can be used to calibrate devices that sense or measure acceleration, vibration, acoustics, displacement, position, force, load, or torque.
- Electrical properties indicate whether calibration instruments can be used to calibrate devices that sense or measure attenuation, capacitance, charge, inductance, power, resistance or impedance.
Calibration instruments that sense and measure process, environmental and thermodynamic properties are used with many variables. Examples include conductivity, liquid flow, gas concentration, humidity, moisture content, dew point, oxygen reduction potential (ORP), potential hydrogen (pH), and resistivity. Specialized products are used to calibrate resistance temperature detectors (RTD), thermistors, and thermocouples.
There are several form factors for calibration instruments.
- Handheld devices are designed to be operated manually, while held in one hand.
- Portable devices are not necessarily hand-held, but may include wheels, handles, or a carrying case for ease of movement. These calibrators, along with handheld ones, are well-suited for field or remote use.
- Fixed instruments are designed to be mounted and/or used in one place.
- Benchtop and panel-mounted devices are also available.
- Modular calibration instruments can be interfaced to different sensors or input ranges.
- Batteries can be used for full operation in some calibration devices, while others have batteries as backup.
Modems or RF transmitters can be included in the calibration instrument.
Calibration instruments provide many different user interface options.
- Analog displays consist of a meter or simple visual indicator such as a needle or light emitting diode (LED).
- Analog front panels provide user inputs such as potentiometers, dials, and switches.
- Digital devices display numeric values and can be setup or programmed with a digital keypad or menus.
- Cathode ray tubes (CRTs), liquid crystal displays (LCDs), or other multi-line displays provide visual output of controls and results for the user.
- Computer interface can controll or monitor a calibration instrument.
- Integral application software runs on a host computer for use with the instrument or its data.
Common serial interfaces include RS232, RS485, and universal serial bus (USB).
Popular parallel interfaces include the general-purpose interface bus (GPIB), a standard also known as IEEE 488.
Devices that produce physical quantities are often traceable to standards from the National Institute of Standards and Technology (NIST), Physikalisch-Technische of Bundesanstalt (PTP), National Measurement Accreditation Services of the U.K. (NAMAS), or Bureau Nationale Metrologie of France (BNM). Regulated standards are especially important because calibration readings must be consistent and accurately comparable to readings from other institutions or laboratories.