Challenging to measure surface temperature

It is difficult to measure temperature. The object to be measured is affected both by the environment and by the thermometer, which can heat or cool the surface. It is best to lower the thermometer into the material to be measured, but this is not always possible.

“Measuring the temperature of a surface sounds simple but is surprisingly difficult. When you put a thermometer against a surface, you cool the surface at the same time. It is difficult to reach very low measurement uncertainties when measuring surface temperature. If you have access to measure the inside of, for example, a pipe, it is much better, but often it is not possible”, says Patrik Broberg, a researcher at RISE who, among other things, works on developing calibration methods for surface temperature measurements.

Great need for quality-assured measurements

Although it is not optimal to measure surface temperature, the need for accurate and quality-assured measurements is great in many industries, for example in the steel industry, medicine, nuclear power, and transport. There are two main methods used in the industry, each with their own challenges. The traditional way is to put a thermometer against the surface, which requires the right technique and comes with the problem that the thermometer simultaneously cools the surface. Patrik Broberg is working on a project that will improve RISE's ability to calibrate equipment for this type of measurement. The principle of calibration is simple: the sensor to be calibrated is placed against a heated plate with a known and uniform surface temperature. The temperature of the thermometer is then compared with the temperature of the surface. At the National Laboratory for Temperature at RISE, there is a surface temperature calibrator that is widely used for calibrations from room temperature up to 600 degrees.

“But we get requests for calibration at higher temperatures than that. With the new equipment we are developing, we will be able to both increase the temperature and halve the measurement uncertainty, which is also requested by the industry”, says Patrik Broberg.

Pyrometers depend on the type of material

The second method is to use a pyrometer, which works in the same way as a thermal imaging camera but only measures one point. All materials radiate and by measuring the intensity of the radiation, the temperature of the material can be determined. Pyrometers are flexible and often good but depend greatly on the type of surface you want to measure. Metals and other surfaces that reflect a lot of light are difficult to measure with a pyrometer.

“When trying to measure the temperature with a pyrometer on a shiny metal surface, it is very much like trying to see the color of a mirror with the naked eye. Since a mirror reflects 95 percent of the light, this is not possible. It works in a similar way with pyrometers and many metals, such as aluminum. There are methods that can reduce the problem, such as painting surfaces with a color that does not reflect as much, but it is not always possible. Pyrometers are also sensitive to the angle you use when measuring. Trying to measure a pipe with a pyrometer can be difficult because of the angle”, says Patrik Broberg.

Patrik Broberg also does research on pyrometers. One example is developing methods to verify that pyrometers measure at the speed they promise. Some new pyrometers can measure 100,000 times per second, but there is a lack of methods to verify that the 100,000 times per second measurements are indeed correct.

But which of the two methods for measuring surface temperature is the best?

“It depends on the conditions. What instrument do you have access to? What material do you want to measure? If it is a shiny metal surface with good heat conduction, the contact method is best, if it is poor heat conduction and a surface with high emissivity, which radiates a lot, then perhaps a pyrometer is best. The pyrometer may also be best if the surface is difficult to access or too hot. But the most important thing in all measurements is that you have sufficient competence to perform the measurement”, says Patrik Broberg.