Infrared Sensor Interview Questions & Answers

  1. Question 1. Can I Look Inside Of An Oven And Measure The Target Temperature?

    Answer :

    Yes, but you have to consider reflection. Usually the oven is hotter than the target and the target has some reflectivity, so the infrared thermometer measures the emitted energy of the hot target, as well as the reflected energy of the oven walls or heaters, causing the indication to read too high.

    This problem may be eliminated using a system called background compensation where the instrument and software will subtract the reflected energy and provide temperature based on the real target energy. The second choice is to measure the target at the exit of the oven. For glass, plastic films, food and paper applications, selection of the right thermometer can eliminate the reflection problem without background compensation or viewing at the exit of the oven.

  2. Question 2. Can Ir Sensors See Through Dust And Smoke?

    Answer :

    Infrared thermometers cannot see through these interferences. However, in most industrial applications, the dust and smoke are rising from the hot object. If our eyes were as fast as an IR thermometer, we would see openings where the instrument has a clear line of sight. With the aid of a function called a peak picker, the IR instrument can detect the target temperature and ignore the cold readings caused by dust and smoke.

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  4. Question 3. Does The Sensor Have To Be Aimed Perpendicular To The Surface?

    Answer :

    No, often looking at a low angle allows you to avoid obstructions in the optical path or high ambient conditions. When viewing rough surfaces like hot strip steel, the sensor can be aimed 15º from the horizontal. When viewing glass, plastic or paper, the lowest angle allowed is 45º.

  5. Question 4. How Critical Is Focusing The Ir Instrument?

    Answer :

    All infrared thermometers resolve a specific target size based on the model of the instrument and the distance from the sensor to the target. Ideally the target should be 2 times larger than the spot size in order to indicate the correct temperature. If the target is smaller than the resolved spot, then the instrument will measure everything that is filling the remainder of the target. These principles do not apply to two-color ratio thermometer.

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  7. Question 5. What Can I Do If The Ambient Temperature Requires Water-cooling, But I Have No Water Available?

    Answer :

    Consider the use of an instrument with a fiber optic sensor. The fiber can be as long as 22 m (72 ft) and can operate in an environment up to 600ºF (315ºC) without cooling.

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  9. Question 6. What Are The Smallest Target Ir Sensors Can Measure?

    Answer :

    IR sensors measure targets as small as .43mm (.017 in). The limitation is the temperature and emissivity. As the temperature goes lower or the emissivity is lower, then the instrument cannot go as small in spot size. The sensor will have to be placed closer to the target and a close focus lens will have to be utilized.

  10. Question 7. How Does Distance To Target Affect The Ir Sensor?

    Answer :

    As the IR sensor is placed further from the target, the spot size measured by the sensor becomes bigger. The target has to be large or small enough for the instrument to view it and measure correctly, depending on the application.

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  12. Question 8. What Is A Two-color Pyrometer?

    Answer :

    A two-color pyrometer an instrument that measures the energy in two different wavelength bands (colors) in order to determine temperature. A two-color pyrometer is often called a ratio thermometer because the temperature is measured by calculating the ratio between the two detector signals. The two color technique used in this instrument has been shown to be effective for correcting errors due to partial blockage of the target caused by smoke and dust particles, a dirty lens, or the target not filling the resolved spot size.

  13. Question 9. What Is The Effect Of Scale On The Surface Of Steel?

    Answer :

    If the scale is tightly bonded to the steel it will usually have no effect because it is the same temperature as the hot target. However, if the scale breaks loose, it will cool and cause a cold spot on the target. If the instrument is a single wavelength instrument, it will indicate a low temperature. If the instrument is a two color and only a portion of the spot is filled with the cold scale, it usually will not affect the temperature indication.

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  15. Question 10. Will Color Of The Measured Object Affect The Measurement?

    Answer :

    For instruments that operate from the visible to 2.6 microns, the color will usually change the emissivity. For wavelengths longer than 3 microns, the color will not affect the emissivity. However, color does affect heating. Dark objects will get hotter than light colored objects.

  16. Question 11. Can An Infrared Sensor Be Overcooled?

    Answer :

    Yes, the IR sensor can get overcooled because the air inside the sensor contains moisture. If overcooled, the moisture will condense; fill the sensor with water eventually destroying it.

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  18. Question 12. How Critical Is Water On The Surface Of The Target I’m Measuring?

    Answer :

    Usually infrared thermometers cannot see thru water so the temperature indication may be very erratic. If the water layer is intermittent then consider the use of the peak picker. The peak picker will allow the instrument to detect the real target temperature when the water is not present and hold the real temperature when there is water in the line of sight. This provides a continuous and accurate temperature indication.

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  20. Question 13. How Do I Calibrate My Instrument?

    Answer :

    Infrared thermometers are calibrated on a standard called a Blackbody. If a blackbody is not available, consider the use of a transfer standard. This is an infrared thermometer that is calibrated and certified, and is used to compare to the online instrument. This instrument should be calibrated and certified each year.

  21. Question 14. How Often Should Ir Sensors Be Calibrated?

    Answer :

    It is common practice for IR instruments that have ISO calibration to be checked on a blackbody once a year.

  22. Question 15. My Portable And Online Ir Sensors Indicate Different Temperatures. Which One Is Right?

    Answer :

    Usually different temperature readings are caused by emissivity. Most likely, the portable thermometer has a different wavelength and emissivity settings from the online instrument. You need to set the correct emissivity for both instruments and also verify correct distance to spot ratio.

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  24. Question 16. What Happens If The Ir Sensor Is Overheated?

    Answer :

    Every sensor has an upper ambient temperature limit that is allowed without cooling. If this temperature is exceeded, the instrument will begin to provide erratic or incorrect temperature indications. If the sensor gets extremely hot, it will be destroyed. For hot environments above sensor’s standard ambient operating temperatures, air or water-cooling cooling accessories are available.

  25. Question 17. What Is A Peak Picker?

    Answer :

    Most infrared thermometers have peak picker function. There are many applications where the target is intermittent or there are intermittent interferences, such as steam and smoke. As the hot target fills the spot momentarily, a peak is obtained. The peak picker holds this peak until another peak comes along. This allows the instrument to provide a continuous indication even though the target is intermittent.

  26. Question 18. What Is E Slope?

    Answer :

    On a two color thermometer, the instrument relies on a ratio between two detectors. Often this ratio is affected by interferences that affect one wavelength and not the other, as for example, when Pyrex is used for a window instead of the recommended quartz. To correct the ratio, a control is used to add a signal to one of the detectors to make it indicate the correct temperature. This correction is only needed to be set once for a specific application.

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  28. Question 19. Why Does The Instrument Indicate Too Low Of Temperature?

    Answer :

    If you suspect too low temperature readings, you need to check for: Incorrect emissivity Dirty lens or window incorrect focusing of the instrument blocked line of sight by an obstruction.