The key to trusting infrared temperature measurement is proper installation, operation and maintenance of the device.

This dirty window detection device adds a “bill” to the sensing window of the infrared thermometer. The bill contains an external mirror that is used to help detect the presence of buildup on the sensor window.

For infrared thermometers to be relied upon, plant operators must have absolute trust in the validity of the readings they offer. Without such trust, the temperature measurement will remain of secondary importance to industrial process control.

Despite the reliable accuracy of infrared thermometers, certain environmental factors must be accounted for that can have a significant impact on their performance. For example, failure to maintain the integrity of the power supply to the thermometer, the air supply to its air-purge unit, or the water supply to its water-cooling jacket, will result in misleading temperature readings that can affect manufactured product quality and even risk catastrophic plant breakdown.

Advances in infrared thermometers and thermal imaging systems provide the operator with the means to conduct a total system “health check” that is designed to give the operator confidence in the target temperature measurement. This usually includes diagnostic data that allows an operator to check on sensor window contamination, calibration, core temperature, case temperature and the analog output.

Should any of the health checks reveal that the thermometer is operating outside of acceptable tolerances, then an alarm relay will be triggered that can be utilized by the process. In addition, detailed error codes can be accessed either locally through the thermometer's integral keypad and LED display or remotely via an RS485 connection.

Window Contamination. All single-wavelength infrared temperature sensors are affected by any contamination of the lens window -- by soot, water droplets and scratches for example -- that causes loss in infrared signal transmission and inaccurate readings. The traditional way to avoid contamination issues is to use an air-purge device mounted on the end of the infrared sensor. The air purge provides positive pressure to keep contamination away from the sensor lens. While this approach is effective, dirty plant air and larger particulate matter can still find their way to the lens of the infrared thermometer and cause loss of accuracy. In those applications where maintaining consistent accuracy is paramount, infrared manufacturers have started to offer dirty window detection options to provide continuous monitoring of the lens window.

Using the option, the operator has the ability to adjust the sensitivity of an alarm triggered by the signal attenuation caused by any buildup of contamination on the detector's lens. As a result, even the effect of wiping the thermometer lens with a dirty cloth can be signaled and maintenance procedures amended and scheduled accordingly.

Calibration.Blackbody calibration sources are the most common way to calibrate infrared thermometers. These perfect emitters have an emissivity of 1.0, which allows the user to calibrate the thermometer to match the blackbody at a given temperature. The downside of blackbodies is that they require the user to remove the temperature sensor from operation to perform the calibration.

More recent designs of infrared thermometers allow for online verification of calibration. Set by operator command or on a timed basis, an electronic flag within the thermometer causes the infrared energy arriving from the target to be blocked and instead, an internal light reference is energized and picked up by the sensor. The resulting measurement is compared with a known reference to verify that the thermometer calibration remains correct.

Sensor Core and Case Temperature. Changes in ambient temperature at the sensor can also introduce errors by affecting the performance of the electronics and skewing infrared detector readings. To ensure that thermometer readings are not in any way drifting in response to the ambient temperature conditions, modern infrared thermometers provide the operator with the means to check that the temperatures of both the core sensor and that of the inner thermometer casing and supporting electronic circuitry remain within certain limits.

Analog Output. The accuracy of the temperature reading also will depend on the correct functioning of the 4 to 20 mA current loop output. At a particular temperature reading and point on the current loop scale, a comparison can be instigated with the temperature reading being supplied by the core sensor; any divergence is flagged by the alarm relay output.

New technology has added features to infrared sensors. These products now have features that allow them to monitor the health of the sensor without taking the unit out of service. This results in better accuracy and less maintenance cost to the end user. PH

SIDEBAR:
Applications That Can Benefit from Dirty Window Detection

If you require noncontact temperature measurement and your application or plant has any of the following characteristics, consider an infrared thermometer with dirty window detection:
  • Airborne dust or film that can coat over time.
  • Contaminated plant air (when using an air purge on the infrared thermometer).
  • Unreliable plant air quality.
  • Airborne splash, spray or particulate matter that can reach the infrared thermometer lens.
  • Inability to tolerate drift due to loss of signal.


SIDEBAR:
Give Your Thermometer a Checkup

  • Is the internal temperature of the thermometer within spec?
  • Is the sensing window dirty?
  • Are the sensor electronics functioning properly?
  • Is the infrared detector within spec?
  • Is the analog output within spec?


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