18-09-2012, 11:07 AM
Selecting the right infrared temperature sensor
[attachment=33917]
Infrared temperature sensors have been successfully
used for years in process industries for
ongoing temperature monitoring and control.
Although the technology is proven, choosing
among units with different specifications is sometimes
confusing, leaving the process engineer to
rely on more traditional temperature measurement
methods (e.g., those involving contact) or on vendor
recommendations. Recent innovations in
infrared temperature sensor design have provided
process engineers with enhanced functionality, and
more questions about how to integrate and use
infrared temperature sensors in their process.
Determine temperature range
Infrared instruments are available for lowtemperature
applications (from below freezing) to
high-temperature applications (over 5,000°F). In
general, the narrower the temperature range, the
better the resolution of the output signal for monitoring
and controlling process temperatures.
If monitoring start-up or cool-down temperatures
is critical, it is necessary to choose a temperature
sensor with a wider measurement range.
This is critical in heat-treating applications, for
example, where temperature must be held within
a specific temperature range for a period of time
to affect a material’s metallurgical properties.
Establish target size
In infrared temperature measurement, the area
to be measured (i.e., the target) should fill the
instrument’s field of view. Suppliers of infrared
temperature sensors typically recommend that the
measurement target exceed the field of view by
50%. If the target is smaller than the field of view,
background objects (e.g., furnace wall) will influence
the temperature reading. Conversely, if the
target is larger than the instrument’s field of view,
the instrument will not capture a temperature
variation outside the measurement area. An illustration
of field of view is shown in Figure 2.
Determine optical resolution
Optical resolution is specified by the D:S
ratio, which is determined by comparing the distance
from the object to the sensor (D) with the
size (i.e., diameter) of the spot being measured
(S). For example, a 1-inch spot on a target being
measured at a distance of 10 inches has a D:S
ratio of 10:1. Infrared sensors on the market
today have D:S ratios ranging from 2:1 (low
optical resolution) to more than 300:1 (high
optical resolution). The higher the optical resolution,
the more expensive the instrument optics
tend to be. The choice of D:S ratio really
depends on the size of the object to be measured
and the distance the sensor is from the target.
For example, high resolution is needed for hightemperature
applications (e.g., heat treating)
where the sensor must be mounted far away
from the target but must still measure a small
spot.
Fast response time
Infrared temperature sensors reach 95% of the
final temperature reading—a common definition
of response time—much faster than contact temperature
sensors (e.g., thermocouples). This is
particularly important when measuring moving
or quickly heated objects. New infrared sensors
on the market have response times selectable
down to 1 millisecond. However, a fast response
time is not desirable for all applications, especially
in those where a fast sensor may exceed the
capability of existing control instruments. In
addition, when there is significant thermal lag in
heating a process, speed in the instrument may be
unimportant.
[attachment=33917]
Infrared temperature sensors have been successfully
used for years in process industries for
ongoing temperature monitoring and control.
Although the technology is proven, choosing
among units with different specifications is sometimes
confusing, leaving the process engineer to
rely on more traditional temperature measurement
methods (e.g., those involving contact) or on vendor
recommendations. Recent innovations in
infrared temperature sensor design have provided
process engineers with enhanced functionality, and
more questions about how to integrate and use
infrared temperature sensors in their process.
Determine temperature range
Infrared instruments are available for lowtemperature
applications (from below freezing) to
high-temperature applications (over 5,000°F). In
general, the narrower the temperature range, the
better the resolution of the output signal for monitoring
and controlling process temperatures.
If monitoring start-up or cool-down temperatures
is critical, it is necessary to choose a temperature
sensor with a wider measurement range.
This is critical in heat-treating applications, for
example, where temperature must be held within
a specific temperature range for a period of time
to affect a material’s metallurgical properties.
Establish target size
In infrared temperature measurement, the area
to be measured (i.e., the target) should fill the
instrument’s field of view. Suppliers of infrared
temperature sensors typically recommend that the
measurement target exceed the field of view by
50%. If the target is smaller than the field of view,
background objects (e.g., furnace wall) will influence
the temperature reading. Conversely, if the
target is larger than the instrument’s field of view,
the instrument will not capture a temperature
variation outside the measurement area. An illustration
of field of view is shown in Figure 2.
Determine optical resolution
Optical resolution is specified by the D:S
ratio, which is determined by comparing the distance
from the object to the sensor (D) with the
size (i.e., diameter) of the spot being measured
(S). For example, a 1-inch spot on a target being
measured at a distance of 10 inches has a D:S
ratio of 10:1. Infrared sensors on the market
today have D:S ratios ranging from 2:1 (low
optical resolution) to more than 300:1 (high
optical resolution). The higher the optical resolution,
the more expensive the instrument optics
tend to be. The choice of D:S ratio really
depends on the size of the object to be measured
and the distance the sensor is from the target.
For example, high resolution is needed for hightemperature
applications (e.g., heat treating)
where the sensor must be mounted far away
from the target but must still measure a small
spot.
Fast response time
Infrared temperature sensors reach 95% of the
final temperature reading—a common definition
of response time—much faster than contact temperature
sensors (e.g., thermocouples). This is
particularly important when measuring moving
or quickly heated objects. New infrared sensors
on the market have response times selectable
down to 1 millisecond. However, a fast response
time is not desirable for all applications, especially
in those where a fast sensor may exceed the
capability of existing control instruments. In
addition, when there is significant thermal lag in
heating a process, speed in the instrument may be
unimportant.