Extech Instruments 42525 User Manual page 6

Infrared Measurement Considerations
1. The 42525 automatically compensates for ambient temperature deviations, however it
may take up to 30 minutes to adjust to extremely wide ambient temperature changes.
2. When low temperature measurements are taken followed by high temperature
measurements, several minutes are required for stabilization before the high
temperature measurements can be made accurately.
3. Measurement Field / Distance: The object under test should be larger than the spot
size shown in the diagram on the previous page. For optimum accuracy, the object
should be 1.5 to 2 times larger than the spot size shown in the diagram on the previous
page.
4. Measurement Interference: Objects having low emissivity or objects with low
temperature yet high emissivity emit little IR energy. Such objects are adversely
affected by IR energy radiated from nearby objects having high emissivity and
temperature. For example, when such objects are measured in sunlight, erratic
readings occur because of the powerful radiation (sunlight) reflected off of the object's
surface into the 42525 sensor.
5. If the surface of the object under test is covered with frost, oil, grime, etc. clean before
taking measurements.
6. If an object's surface is highly reflective, apply masking tape or flat black paint
(emissivity 0.95) before measuring.
Manually Adjusting Emissivity
The emissivity is factory set to 0.95, which covers 90% of all applications. However, the
closer the meter's emissivity setting is to the actual emissivity of the object under test, the
more accurate the temperature measurements will be. To calculate the emissivity of an
object: Measure an object's temperature using a Type K Thermocouple and note the
reading. Measure the object again using the IR sensor. Now, adjust the emissivity pot
(located under the rubber grommet on the meter's side) using a small screwdriver until the
IR measurement matches the thermocouple reading. Note that the use of a surface-mount
thermocouple works best for this purpose. Turn the pot counter-clockwise to lower the
emissivity value. With the pot set fully clockwise the emissivity will be 0.95, which is the
factory default setting.
Offset Adjustment Potentiometer
The OFFSET function allows the user to adjust the temperature readings in fine detail to
match an external calibrator, to compensate for display drift, or for other diagnostic uses.
Turn the pot (located under the rubber grommet on the side of the meter) using a small
screwdriver to adjust the displayed temperature to the desired setting.
Data Hold
Press the HOLD button to freeze the displayed measurement. The "HOLD" icon will appear
on the LCD to indicate that the Data Hold mode is activated. Press the HOLD button again
to exit the Data Hold mode and return to normal operation.
Relative Measurements
While measuring, press the "%" button to activate the Relative Mode. The measurement
on the LCD at the time of the button press will be displayed as 100%. Subsequent
readings will be displayed as a percentage of the reference value. Press the "%" button
again to return to normal operation.
Data Recording (Maximum/Minimum Readings)
The Data Recording feature offers maximum and minimum reading record/recall. Press the
RECORD button once to activate this feature (the REC symbol will appear on the LCD).
Press the RECALL button once to view the maximum reading (MAX). Press the RECALL
button again to view the minimum reading (MIN). Press the RECORD button again to
return to normal operation.
Model 42525 Version 2.2 October 2003
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