LumaSense Technologies M67 - Manual (Page 18)

M67, M67S Manual
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6.5 Important information about cooling and purging your infrared thermometer
Just as a thermocouple measures temperature in relation to its reference junction, an infrared thermometer measures temperature
in relation to the internal temperature of the infrared thermometer (IRT) its self. An ambient temperature compensation circuit
is built into the IRT to maintain calibration accuracy. It will not compensate effectively for very rapid changes in ambient
temperature or for ambient temperatures which are outside the compensation limits (usually 0 to 60°C, 32 to 140°F). Please
refer to the specification of the model you are using to determine the “Operating Ambient Temperature Limits”.
Rapidly changing ambient temperature, such as could be caused by intermittent hot targets or opening and closing of furnaces
or oven doors near to the sensor, can cause measurement problems. The mass of the IRT housing reacts slowly to these
changes, resulting in compensation action which is out of phase with ambient changes and which never reaches a state of
equilibrium. This could be incorrectly attributed to sensor instability.
If your application is likely to expose the IRT to rapidly changing ambient temperature or temperatures beyond the operating
ambient temperature limit, the IRT must be installed in a M,.521 Mounting Jacket. When determining the need for a jacket do
not forget to take into account radiant and conducted heating from the process, especially if this is intermittent; and also
seasonal extremes. For extremely cold conditions /XPD6HQVH offers a Winterization Package. The mounting jacket selected, may be
either an un-cooled version for moderate conditions or one with cooling passages for higher ambient temperatures. If you select
a jacket with cooling connections, it is important that you read the section devoted to “Good Cooling Practice”.
KEEPING THE LENS AND SIGHT PATH CLEAR
The accuracy of an IRT will be seriously affected if the lens is not clean and the sight path is obscured by dirt, process material,
smoke or steam. If any or all of these conditions are present in your application it is necessary to install a Purge Assembly. The
air purge assembly includes a short tube extension, but in very severe conditions it may be necessary to install a longer tube to
keep the process environment away from the IRT lens. The air purge assembly also incorporates separate connections for
cooling water or air, and this will often provide adequate cooling for the sensor, allowing a plain mounting jacket to be used in
place of a cooling jacket.
A flow of filtered, plant air, or inert gas, of 75 CFH(2CMH) is adequate for keeping the lens clean under most circumstances,
and for maintaining a positive pressure in the sight tube against process atmosphere. The purge should not be relied upon to
clear away fumes, dust and steam etc., beyond the end of the extension tube. If these latter conditions exist, other methods must
be applied to keep the sight path clear, such as the use of fan to blow the dust, steam, etc. clear or by using a sight tube long
enough to span the distance between the sensor and the target material. When deciding on this step, select a tube internal
diameter, which will not cut-off the IRT field of view. Under no circumstances exceed the recommended purge flow rate, as
this will create turbulence in the purge assembly, and may cause contaminants to be conducted into the lens. It is prudent to
install a rotameter type flow meter to maintain the correct flow. When deciding whether or not a purge is required, do not over
look the presence of gaseous contaminants from resins, sizings etc. which may not be apparent to the eye, but will coat the lens
over a period of time.
GOOD COOLING PRACTICE
Infrared thermometers should be operated within the specified operating temperature limits to prevent ambient temperature
compensation errors and to avoid the malfunctions and damage that will occur if the sensor is heated beyond its maximum
limits. Where possible, steps should be taken to reduce the amount of heat reaching the sensor through conduction, radiation
and convection. Such steps include the use of a spacer tube to distance the mounting jacket from hot furnace walls and roofs;
placing an insulating spacer flange between the IRT mounting flange and the process; and by the use of reflective shields such
as aluminum sheet. These steps will not usually eliminate the need for cooling, but may permit the use of air rather than water as
a medium, or reduce the demand on cooling water. Air is usually inadequate as a cooling medium in very hot environments, but
can be enhanced by utilizing a vortex cooler in combination with an Insulation Enclosure.
When applying cooling to an IRT it is most important not to overcool the jacket. If the jacket is overcooled, moisture in the
atmosphere or from the process will condense on all the cool surfaces. At best it will condense on the IRT lens and seriously
affect the accuracy, and in the worst case it will condense in the interior of the IRT causing component damage. The M,.521
mounting jacket is classified as a NEMA 4 enclosure, and the IRT is sealed, but this will not be proof against severe
condensation due to overcooling. The easiest method for determining correct cooling is to sample the coolant outflow
temperature, or the jacket exterior temperature. An ideal temperature is around 90°F (32°C). More exact data regarding coolant
supply temperature and relative humidity can be found in Table 1. In addition to these precautions, it is necessary to ensure that
moisture cannot enter the jacket through the electrical conduit or by flowing down the wiring. This can be achieved by
following the example in the installation drawing in Figure 12.
PROTECTING THE COOLING AND PURGING SYSTEM