What Is Instrument Calibration and Why Is it Needed?
Calibration can be a time-consuming process requiring expensive traceable tools, but high-accuracy signals result in higher-quality products, better customer satisfaction, and, ultimately, greater returns.
Calibration is a process of comparing the output of measuring instruments and devices against the input signal of a standard, verified instrument. It detects and identifies any error in the device under test. If the difference between the calibration tool signal and the device under test lies within established limits, the calibration is said to be in a pass condition; otherwise, it will fail.
Figure 1. High-accuracy signals result in higher quality products, better customer satisfaction, and ultimately, greater returns. Image used courtesy of Adobe Stock
If the device passes the calibration process, it can be used for measurement. But if calibration fails, the instrument or device must be rectified or replaced. If repair or replacement is necessary, it must be recalibrated. Only after successful calibration can the device or instrument be returned to the system process.
After the calibration process, its documentation must be filed and made part of the facility record since regulatory and standardization bodies such as the United States FDA and ISO review these documents during their routine inspection.
Calibrator or Calibration Tool
A calibrator, or calibration tool, is a standard reference instrument to which the device under test is compared.
A calibration tool will provide a very accurate output that matches a quantity the sensor or device might receive. For example, if it’s a pressure sensor under test, the calibration tool must have a very accurate pressure. The sensor output signal (perhaps 4-20 mA or 0-10 V) is measured to verify the signal matches with the measured quantity.
Calibration tools themselves must be calibrated regularly to ensure integrity at each level of test hierarchy. This process is known as ‘traceability’.
What Devices Require Calibration?
In principle, every device or instrument that measures any physical parameter would benefit from calibration. Since the process is time-consuming and expensive, it may only be reserved for processes that require only the highest degrees of accuracy or where required by regulation.
Some devices that require calibration include the following.
Figure 2. Multimeter calibration as an example of calibrating an electrical device. Image used courtesy of Adobe Stock
Calibrating Electrical Devices
Electrical device calibration is used for instruments or devices that measure typical electrical quantity, such as current, voltage, resistance, inductance, and capacitance.
Some standard electrical devices that require calibration include digital and analog multimeters, oscilloscopes and scope meters, energy meters, RTD and thermocouple meters, and process instrumentation such as an RPM meter, data logger, and chart recorder.
How Does Electrical Device Calibration Work?
Electrical devices are calibrated by comparing their output with those of precise devices of known performance. These devices can generate measurement functions such as voltage, current, and resistance.
The traceable calibration tools will generate a wide range of the desired quantity, such as a voltage supply for meter testing, which might generate from fractions of a volt up to hundreds of volts.
Figure 3. Thermocouple calibration to determine any deviation in the thermocouple output. Image used courtesy of Adobe Stock
Temperature calibration refers to verifying temperature sensors (RTDs, thermocouples, thermistors, etc.). The calibration determines whether any deviation in the temperature sensor output lies within the acceptable limits.
How Does Temperature Sensor Calibration Work?
Temperature calibration is performed by simulating and comparing a known temperature value with the temperature device under test.
Commonly, there are two types of temperature calibrators: physical and electronic simulating.
Physically simulating tools include a very carefully controlled heat source, such as a hot bath, capable of creating the desired temperature. The temperature sensor under test is placed in this heat source, and its output is compared with the temperature of the heat source.
Electronic simulating tools are handheld devices that simulate the desired temperature value as an electrical signal, such as 4-20 mA or 0-10 V. This calibration might test the signal wires after the physical temperature has been converted to an industrial signal level.
Although humidity is a concern throughout any electrical process or system, humidity calibration is critical in manufacturing industries such as pharmaceuticals, chemicals, and food because uncontrolled moisture or humidity can result in condensation and corrosion.
How Does Humidity Sensor Calibration Work?
The humidity generator inside the calibrator produces a gas stream of known humidity levels, which is compared with the value of the humidity sensors. A display on the humidity generator commonly shows how much humidity it generates.
Based on the humidity sensor's visual display and signal output, the calibration is marked as either pass or fail.
Figure 4. Pressure gauge calibration with portable pump. Image used courtesy of Adobe Stock
Digital and analog pressure sensors can both be calibrated since the digital (discrete) pressure sensors switch on or off at very precise levels, while analog sensors output a current or voltage signal based on the fluid pressure. Based on the fluid medium (liquid or gas), there are many different kinds of pressure calibration tools.
How Does Pressure Device Calibration Work?
For pressure calibration, a portable pump is used to generate a known pressure. The pressure device under test is connected to the pump through appropriate connectors. The pump’s pressure is compared with the device's pressure under test.
Examples of pressure devices that can be calibrated include pressure gauges, digital indicators, pressure transmitters, and transducers.
Mechanical calibration involves calibrating dimensional physical quantities such as force, weight, and torque. It includes measurement devices, torque tools, load cells, balances, and scales.
How Does Mechanical Calibration Work?
Mechanical calibration is performed like other calibration methods. A known value of a particular parameter is applied to the device under test, and the output is compared with the known reference value. Once again, due to each of the parameters being created by different physical forces, many tools may be required for mechanical calibration.
Calibration Certificate Documentation
When records are made following the calibration, a certificate is placed in a secure, traceable location. This document provides information about the calibration process for an instrument or device. The calibration certificate includes the following information
- Device details: model number, serial number, location, etc
- Calibration method
- Date of calibration and due date
- Acceptance criteria
- Result: ‘pass’ or ‘fail’
A small sticker or tag is placed on the device or the instrument. It provides information related to calibration similar to the info on the certificate, such as the calibration date, due date, and the name of the operator performing the calibration.
Figure 5. Regulatory requirements, maintenance, and process efficiency, are all factors in the importance of instrument calibration. Image used courtesy of Adobe Stock
Why Is Calibration Important?
Since it’s a large investment for organizations to create and maintain calibration programs, why is it such a common (and necessary) step in many industries?
The most crucial reason for calibration is the regulatory requirements. Regulatory bodies such as United States FDA and the ISO require affiliated organizations to calibrate their instruments routinely. These bodies can issue warnings–or even fines and penalties–if devices are not calibrated regularly.
The US FDA defines their calibration requirements in 21 CFR 820.72 as "inspection measuring and test equipment". In contrast, the ISO defines its calibration requirement in clause 7.1.5 of ISO 901 2015 as "monitoring and measuring devices for ISO 9001".
It is common and natural for all measuring devices and instruments to drift over time. There can be many reasons for the drift, such as vibration, high or low operating temperature, and humidity. These factors affect the internal mechanical and electronic components and cause deviation in the output. Calibration is performed to detect and remove deviation due to these reasons.
Process Efficiency Improvement
Measuring instruments and devices are an integral part of production processes, and the output of all the intermediate and final stages depends upon these measuring instruments and appliances. A calibrated apparatus works with exact values without any error or drift. It produces efficient output at all stages and results in more reliable manufacturing of high-quality products along with greater revenue, which is ultimately the final goal of any organization.