What Is Calibration?

Simply defined, calibration is the process of adjusting a device to meet

manufacturer’s specifications. Calibration is sometimes also defined as the issuing

of data, including a report or certificate of calibration, that assures an end user of a

product’s conformance with specifications, and perhaps also with external

guidelines, such as those of the International Organization for Standardization,

whose ISO 9001 standards, for example, set worldwide specifications for business

sectors. A company follows these standards to ensure that its products and/or

services gain acceptance among suppliers and customers. This second definition of

calibration is more properly referred to as certification.

A company with

equipment needing

calibration may send it to

a metrology/calibration

laboratory, where a

skilled technician will

either bring it up to specifications or confirm that it already meets them,

using measurement/test instruments that must themselves meet strict

calibration requirements. All or part of the components used in an

industrial process can be calibrated. A temperature calibration, for

example, could involve a probe alone, an instrument alone, or a probe

connected to an instrument (a system calibration).

Typically, the accuracy of a calibration device, or calibrator, is at least

four times greater than the equipment being calibrated. The equipment

usually has a calibration range over which the technician will, at various

points, check specifications. As Mike Cable explains in

Calibration: A

Technician’s Guide,

“the

calibration range

is defined as ‘the region

between the limits within which a quantity is measured, received or

transmitted, expressed by stating the lower and upper range values.’

The limits are defined by zero and span values. The

zero

value is the

lower end of the range.

Span

is defined as the algebraic difference

between the upper and lower range values.”

Adjustments made during calibration must fall within

certain tolerances. Such tolerances represent very

small, acceptable deviations from the equipment’s

specified accuracy.

The manufacturer usually does the initial calibration on

its equipment. Subsequent calibrations may be done

in-house, by a third-party lab, or by the manufacturer.

The frequency of recalibration will vary with the type of

equipment. Deciding when to recalibrate a flowmeter,

for example, depends mainly on how well the meter

performs in the application. If liquids passing through

the flowmeter are abrasive or corrosive, parts of the

meter may deteriorate in a very short time. Under

favorable conditions, the same flowmeter might last for

years without requiring recalibration.

As a rule, however, recalibration should be performed at

least once a year. Of course, in critical applications

frequency will be much greater.

Calibration Primer:

An Omega Engineering White Paper

Page 1 of 5

Introduction

The most sophisticated industrial equipment will not be very useful unless it is

calibrated. Through calibration, adjustments made to a piece of equipment ensure

that it performs as expected—that it can be relied on to deliver predictable,

accurate results that meet quality standards. This white paper from Omega

Engineering explains what calibration is, why it is important, and how it works.

NIST traceability is defined and discussed, and there is a step-by-step description

of a basic calibration. This paper also discusses in-house vs. laboratory

calibration, and it describes major types of calibration devices.

CALIBRATION PRIMER

An Omega Engineering White Paper

OMEGA

offers a full line of calibration devices.

OMEGA’s BB702 high-performance blackbody calibration source

being used to calibrate an OS520 infrared thermometer.

OMEGA’s 5 H x 8 meter L (15' x 25') recirculating temperature-controlled wind

tunnel is used in conjunction with NIST-traceable air velocity standards to provide

accurate flow calibrations.

Patents issued and pending

on many aspects of this

device including but not

limited to laser circle.

PATENTED

What Are Some Types of Calibrators?

Calibrators vary in form and function with the equipment they

are designed to calibrate. A blackbody calibrator, used to

calibrate infrared pyrometers (high-temperature

thermometers), typically has a target plate with very high

emissivity, the temperature of which can be controlled to very

narrow tolerances. To calibrate a pyrometer, readings of the

target plate taken by the pyrometer are compared with the

target plate’s known, controlled temperature. The pyrometer

is then adjusted until any difference is so minimal as to be

insignificant.

The block calibrator, used for temperature probes, contains a

metal block that can be heated to precise temperatures.

These temperatures are compared to those taken by

temperature probes inserted into the block. Temperature

probes generally cannot be adjusted, so this process is one of

verification rather than true calibration.

To calibrate equipment such as panel meters and temperature controllers, a device

known as a signal reference is often used. It is a type of calibrator that can generate a

known electrical signal. There are voltage, current, and frequency signal references. Once

a signal from one of these calibrators is fed into the equipment in question, the display or

output value of the equipment can be adjusted until it matches the known signal. The

simulator, a special kind of signal reference, generates sensor output. Signal references

and simulators can often read as well as generate signals.

Because fluidized baths provide safe, rapid heat transfer and accurate temperature

control, they are useful in calibrating temperature-sensitive instruments. The fluidized

sand bath temperature probe calibrator employs the principle of fluidization that occurs

when a gas—usually low-pressure air or nitrogen—flows up through a partially filled

chamber or retort containing dry, inert particles of aluminum oxide. The gas flows at low

velocity, which sets the particles in motion, separates them, and suspends them to a

stable level. This gives the particles an appearance of turbulence similar to that of boiling

liquid. Not only do fluidization solids circulate and flow like liquids; they also exhibit excellent

heat-transfer

characteristics.

Temperature probes

inserted into the bath

come to a stable

temperature very

quickly, which facilitates

calibration.

The thermoelectric

cooling elements in an

ice point calibration

reference chamber

produce a very precise,

stable temperature of

0°C. Although reference chambers are commonly used to

calibrate or verify temperature probes, their ability to simulate a

thermocouple signal makes them useful for calibration or

verification of instruments that read thermocouples.

Omega Engineering’s metrology lab has a 25-foot

wind tunnel with chillers, pumps, and condensers

that keep recirculating air at a constant temperature

and flow rate. This very large machine is used for

anemometer and vane-type sensor calibrations.

Benchtop wind tunnels operate on the same

principle, producing a highly uniform flow rate across

a greatly reduced test section.

CL1000 mini hot point

shown calibrating a thermocouple

probe, Model KTSS-HH, with the HHM290 handheld thermometer.

Calibration Primer:

An Omega Engineering White Paper

Page 2 of 5

TRCIII-A, ice point

calibration reference chamber,

a high-precision thermoelectric chamber that uses water

to maintain 0°C (32°F) continuously.

FSB-4, 130 lb-capacity fluidized

sand bath temperature probe calibrator.

OMEGA’s benchtop wind tunnel series model WT4401.