The most basic design consists of a lens to focus the infrared (IR) energy on to a detector, which converts the energy to an electrical signal that can be displayed in units of temperature after being compensated for ambient temperature variation. This configuration facilitates temperature measurement from a distance without contact with the object to be measured. As such, the infrared thermometer is useful for measuring temperature under circumstances where thermocouples or other probe type sensors cannot be used or do not produce accurate data for a variety of reasons. Some typical circumstances are where the object to be measured is moving; where the object is surrounded by an EM field, as in induction heating; where the object is contained in a vacuum or other controlled atmosphere; or in applications where a fast response is required.
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When selecting noncontact
temperature measurement instruments, it
is necessary to take into account not only
the target and its emissivity, but also the
surroundings and the invtervening atmosphere.
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Common Questions When Using an Infrared Thermometer:
Infrared pyrometers allow users to measure temperature in
applications where conventional sensors cannot be employed.
Specifically, in cases dealing with moving objects ( i.e., rollers,
moving machinery, or a conveyor belt), or where non-contact
measurements are required because of contamination or
hazardous reasons (such as high voltage), where distances
are too great, or where the temperatures to be measured are too high for thermocouples or other contact sensors.
The critical considerations for any infrared pyrometer include
field of view (target size and distance), type of surface being
measured (emissivity considerations), spectral response
(for atmospheric effects or transmission through surfaces),
temperature range and mounting (handheld portable or
fixed mount). Other considerations include response time,
environment, mounting limitations, viewing port or window
applications, and desired signal processing.
The field of view is the angle of vision at which the instrument
operates, and is determined by the optics of the unit. To
obtain an accurate temperature reading, the target being
measured should completely fill the field of view of the
instrument. Since the infrared device determines the average
temperature of all surfaces within the field of view, if the
background temperature is different from the object
temperature, a measurement error can occur.
OMEGA offers a unique solution to this problem. Many OMEGA infrared
pyrometers feature patented laser switchable from circle to dot. In the circle mode
a built-in laser sighting creates a 12-point circle which clearly indicates the target area being measured.
In the dot mode
a single laser dot marks the center of the measurement area.
Emissivity is defined as the ratio of the energy radiated by
an object at a given temperature to the energy emitted by
a perfect radiator, or blackbody, at the same temperature.
The emissivity of a blackbody is 1.0. All values of emissivity
fall between 0.0 and 1.0.
Most infrared thermometers have
the ability to compensate for different emissivity values,
for different materials. In general, the higher the emissivity
of an object, the easier it is to obtain an accurate temperature
measurement using infrared. Objects with very low emissivities
(below 0.2) can be difficult applications. Some polished, shiny
metallic surfaces, such as aluminum, are so reflective in the
infrared that accurate temperature measurements are not
always possible.
There are five ways to determine the emissivity of the
material, to ensure accurate temperature measurements:
- Heat a sample of the material to a known temperature,
using a precise sensor, and measure the temperature using
the IR instrument. Then adjust the emissivity value to force
the indicator to display the correct temperature.
- For relatively low temperatures (up to 500°F), a piece of
masking tape, with an emissivity of 0.95, can be measured.
Then adjust the emissivity value to force the indicator to
display the correct temperature of the material.
- For high temperature measurements, a hole (depth of which
is at least 6 times the diameter) can be drilled into the object.
This hole acts as a blackbody with emissivity of 1.0. Measure
the temperature in the hole, then adjust the emissivity to force
the indicator to display the correct temperature of the material.
- If the material, or a portion of it, can be coated, a dull
black paint will have an emissivity of approx. 1.0. Measure
the temperature of the paint, then adjust the emissivity to
force the indicator to display the correct temperature.
- Standardized emissivity values for most materials are
available (see pages 114-115). These can be entered into
the instrument to estimate the material’s emissivity value.
The pyrometer can be of two types, either fixed-mount or
portable. Fixed mount units are generally installed in one
location to continuously monitor a given process. They
usually operate on line power, and are aimed at a single
point. The output from this type of instrument can be a local
or remote display, along with an analog output that can be
used for another display or control loop.
Battery powered, portable infrared ‘‘guns’’ are also available;
these units have all the features of the fixed mount devices,
usually without the analog output for control purposes.
Generally these units are utilized in maintenance, diagnostics,
quality control, and spot measurements of critical processes.
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