Back to Basics: How Emissivity Affects Temperature Measurement

by John C. Voitl
Predictive Maint. Tech.
Coors Brewing Co.
Golden, Co

I have a VFD salesman friend who was having trouble with a customer. The customer has a drive in a hot area that occasionally trips with an overheat indication. The customer was taking the temperature of the aluminum heat sink with an inexpensive non contact thermometer, and obviously, it was reading low. He asked me for an explanation. This article was created to demonstrate low emissivity and high reflectivity, and their effects on temperature measurement with infrared thermometers.

Empty aluminum can filled with warm water, low emissivity

This is a beer can before the label is printed on it, filled with warm water. It is shiny aluminum, with very low emissivity. To compensate for the emissivity, I placed a piece of Scotch 33 electrical tape with an emissivity of 0.95 on the can. The tape reads the actual temperature of the can, about 139 degrees. As can be seen on the graph of the temperatures of the line across the can, where there is no tape the can itself is reading less than 80 degrees. The circle, AR01, indicates the approximate area an infrared thermometer with a 6:1 field of view would cover at this distance. As can be seen in the table the average reading in this area is 83.1. That's what the IR thermometer would read. This is a simple demonstration of how low emissivity and field of view affect the indicated temperature of a material when checked with an IR thermometer.

Sheet of Aluminum - Low emissivity, high reflectivity

This is a sheet of aluminum. The emissivity of this material is quite low, thus the reflectivity is high. It's hanging on the wall in my office, and most of the sheet reads about ambient temperature. There appears to be a warm spot on the sheet. The warm spot appears to be wearing glasses. Look, it's me. The glasses look cooler than my face as glass does not transmit IR through it so the temperature shown there is the temperature of the glass, rather than my face. There also appears to be a cold spot on the sheet. This is the end of a delicious glass of Coors Lite beer. These high and low temperatures are not accurate because the aluminum has some emissivity, therefore the reflectivity is not 100 percent. This is a simple demonstration of high reflectivity when the emissivity is low, and how it can adversely affect indicated temperatures.

Editor: John receives an InfraMation executive attaché case for his article contribution.