Standards and Calibration

STANDARDS AND CALIBRATION

Standard

A standard is a physical representation of a unit of measurement. The term 'standard' is applied to a piece of equipment having a known measure of physical quantity.

Types of Standards

i) International Standards (defined based on international agreement)

ii) Primary Standards (maintained by national standards laboratories)

iii) Secondary Standards (used by industrial measurement laboratories)

iv) Working Standards (used in general laboratory)

i) International standards:

International standards are standards developed by international standards organizations. International standards are available for consideration and use worldwide. The most prominent organization is the International Organization for Standardization (ISO).

Purpose of International Standards:

International standards may be used either by direct application or by a process of modifying an international standard to suit local conditions. The adoption of international standards results in the creation of equivalent, national standards that are substantially the same as international standards in technical content, but may have (i) editorial differences as to appearance, use of symbols and measurement units, substitution of a point for a comma as the decimal marker, and (ii) differences resulting from conflicts in governmental regulations or industry-specific requirements caused by fundamental climatic, geographical, technological, or infrastructural factors, or the stringency of safety requirements that a given standard authority considers appropriate.

International standards are one way of overcoming technical barriers in international commerce caused by differences among technical regulations and standards developed independently and separately by each nation, national standards organization, or company. Technical barriers arise when different groups come together, each with a large user base, doing some well-established thing that between them is mutually incompatible. Establishing international standards is one way of preventing or overcoming this problem.

ii) Primary Standard:

A primary standard in metrology is a standard that is sufficiently accurate such that it is not calibrated by or subordinate to other standards. Primary standards are defined via other quantities like length, mass and time. Primary standards are used to calibrate other standards referred to as working standards.

iii) Secondary Standard:

Secondary reference standards are very close approximations of primary reference standards.

iv) Working standards:

Although the SI definition of the "meter" is based on a laboratory procedure combining the speed of light and the duration of a second, a machine shop will have a physical working standard (gauge blocks for example) that are used for checking its measuring instruments. Working standards and certified reference materials used in commerce and industry have a traceable relationship to the secondary and primary standards.

Working standards are expected to deteriorate, and are no longer considered traceable to a national standard after a time period or use count expires.

Laboratory Standards:

National organizations provide calibration and private industrial laboratories with items, processes and/or certification so they can provide certified traceability to national standards. Laboratory standards are kept in controlled conditions to maintain their precision, and used as a reference for calibration and creating working standards. Sometimes they are (incorrectly) called "secondary standards" because of their high quality and reference suitability.

Calibration

Calibration in measurement technology and metrology is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of known accuracy, a device generating the quantity to be measured such as a voltage, or a physical artefact, such as a metre ruler. The outcome of the comparison can result in no significant error being noted on the device under test, a significant error being noted but no adjustment made, or an adjustment made to correct the error to an acceptable level. Strictly, the term calibration means just the act of comparison, and does not include any subsequent adjustment. The calibration standard is normally traceable to a national standard held by a National Metrological Institute.

Importants of Calibration

Calibration defines the accuracy and quality of measurements recorded using a piece of equipment. Over time there is a tendency for results and accuracy to 'drift' particularly when using particular technologies or measuring particular parameters such as temperature and humidity. To be confident in the results being measured there is an on-going need to service and maintain the calibration of equipment throughout its lifetime for reliable, accurate and repeatable measurements.

The goal of calibration is to minimize any measurement uncertainty by ensuring the accuracy of test equipment. Calibration quantifies and controls errors or uncertainties within measurement processes to an acceptable level. So if you know that a particular food product needs to be kept above 68°C and the instrument system you are using displays a figure of 68.8°C then provided the system is calibrated to be accurate within 0.5°C at 68°C you can be confident the food is safe, if the system has an accuracy of 1°C though then you cannot be certain that the food's temperature has been correctly controlled. Food is, of course, only one example of why it is essential to have a confirmed calibrated level of accuracy. Manufacturing processes that require specific controlled curing temperatures are another in fact the list goes on. In summary, calibration is vitally important wherever measurements are important, it enables users and businesses to have confidence in the results that they monitor record and subsequently control.

Types of calibration:

The process of testing calibration can be performed on a number of products and types of equipment, across multiple sectors. Following are some of the most common types of calibrations service used today;

Pressure calibration

This is a widely used calibration process in which gas and hydraulic pressure are measured across a broad spectrum. A number of pressure balances and calibrators are generally used, along with a variety of pressure gages. Examples of pressure equipment that can be tested for calibration include;

i. Barometers

ii. Analogue Pressure Gauges

iii. Digital Pressure Gauges

iv. Digital Indicators

v. Transmitters

vi. Test Gauges

Electrical Calibration

This calibration service is used to measure voltage, current frequency and resistance. The process also monitors resistance and thermocouple simulation covering process instrumentation. Examples of electrical equipment that can be tested for calibration include;

i. Multi-meters

ii. Counter timers

iii. Insulation Testers

iv. Loop Testers

v. Clamp Meters

vi. RCD

vii. Data Loggers

Mechanical calibration

Generally speaking, mechanical calibration housing facilities will be temperature controlled. A number of dimensional, mass, force, torque and vibration elements will be calibrated during the testing process. Examples of mechanical equipment that can be tested for calibration include;

i. Weight & Mass Sets

ii. Torque Wrenches & Screwdrivers

iii. Scales/Balances

iv. Micrometers, Verniers, Height Gauges

v. Accelerometers

vi. Load Cells & Force Gauges

Temperature and humidity calibration

Temperature calibration usually takes place in a controlled environment. A number of different types of equipment can be tested using temperature calibration, including the following;

i. Thermometers/Thermocouples

ii. Dial Thermometers

iii. PRTS and Thermistors

iv. Thermal Cameras

v. Infrared Meters

vi. Chambers/Furnaces

vii. Weather Stations

viii. Data Acquisition Systems

Again, humidity calibration will usually take place in a controlled environment and will generally cover a range of 10 - 98% RH. A variety of instruments can be tested for humidity calibration, including the following;

i. Humidity Recorders

ii. Humidity Generators

iii. Digital Indicators and Probes

iv. Transmitters

v. Psychrometers

vi. Thermohygrographs

vii. Tinytag Sensors

The calibration processes listed above are perhaps the most commonly-used and more widely-know about methods. However, calibration is used on a much wider scale in many industries. A few additional examples of calibration types are;

i. Waterflow Calibration

ii. Oilflow Calibration

iii. Air Velocity Calibration

iv. Air Flow Calibration

In the majority of cases, an industry or company will require a combination of calibration techniques to fully test all of their equipment and devices - to this end, one piece of equipment may undergo a number of strict calibration tests to be deemed safe for use and performing at optimal standards. The standards used to verify calibration levels differ somewhat, depending on a number of factors, including; the industry sector, the location (Country), the type of equipment involved and the specification of the calibration testing equipment used.