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LINK: APPLICATION NOTES Equipment/Design:
The choices of available equipment from even a few manufacturers can be bewildering to someone unfamiliar with the field of temperature measurement, so bewildering that many users just let the manufacturer’s representative suggest the equipment to use. There are, to be sure, some good reps but there are also a lot of poor reps who really don’t understand the limitations of their equipment or know how to correctly design an installation. I’ve personally witnessed many cases where reps have given absolutely incorrect advice to hapless users. So, how can you tell if advice is good? That’s why people like me exist! But, I’ll give a bit of free advice: Most manufacturers have similar product lines, selling RTs of various wavelengths and of various styles. Unless there are compelling reasons to do otherwise, it is usually best to select a manufacturer with whom your maintenance people are most familiar. You’ll make them happy. Of course, this is not always possible as there are some devices made by only one manufacturer. Before even thinking about selecting a piece of equipment, carefully examine the proposed site. Is sight path obscuration likely? If so, how much, and how can it be reduced or overcome? Are reflections a possible problem? (Don’t forget sunlight shining on the area during certain times of day or sodium vapor lights). Is the ambient temperature likely to exceed the equipment specifications, and is air or water cooling (or heating) necessary? Is there a lot of vibration at the mounting point? Does the product vary in shape, angle or emissivity? Properly taking all these (and other points) into consideration and carefully selecting the best design and location will pay off in terms of best accuracy. It’s better to spend a few extra thousand dollars on a good installation than to waste far more than that on poor product that would result from inaccurate temperature readings. In planning any installation, you must understand any factors that affect emissivity settings or, if using a ratio unit, affect the emissivity ratio. (Generally, ratio units are ten times more sensitive to emissivity ratio errors than brightness units). You should always select a wavelength where emissivity is a maximum as this will give you the least error if emissivity of the product varies slightly. Most of the RT manufacturers will be able to give you an idea of what this wavelength should be for the material you are measuring, but don’t always trust them for an exact emissivity to use. As I discussed under my emissivity notes, many different factors can affect the emissivity number you should use. Remember that the longer wavelength that you use, the greater any errors will be for emissivity variations and reflections. In general, I try to avoid using the general purpose hand held RTs using the 8-14 micron range (which claim to measure from room temperature to thousands of degrees) because they are notorious inaccurate on anything but a blackbody. They are okay for checking for hot spots on a relative basis, but they are merely a toy for any other use where accuracy is required. To some extent, each RT installation is unique if it is carefully planned. Plan carefully. Among the more common infrared instrument manufacturers are:
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