Surface temperature is notoriously difficult to measure, as there is a discontinuity between the temperature of the object being measured and its environment, resulting in heat flows which can be significant. A sensor or probe attached to the surface, such as a thermocouple or resistance temperature detector (RTD), will likely be affected by the method of heating, or by the difference between the object and the undulating temperatures of the atmosphere around it.
The problem then is to get the sensor to contact the object without affecting the temperature being measured which, in principle, is impossible.
However, the requirement exists and surface probes have been specifically made for the purpose. A thermocouple wire laid along the surface for instance should make reasonable contact, but this is not always practical. A pointed probe like a skewer is more practical but unlikely to read correctly. Probes with some form of contact pad may also be effective, provided they do not mask the surface, changing the heat flow and temperature.
Perhaps non-contact pyrometers are the best bet, especially for applications in very high or very low temperatures such as furnaces and steel production to flash freezing. AMETEK Land claims to have just the solution.
“We’ve been making non-contact measurement devices since around 1950,” explained Richard Gagg, global IR product manager at AMETEK Land. “And only measure temperature by non-contact means, we don’t do thermocouples or anything like that.”
AMETEK’s latest generation of single point or spot pyrometers, known as ‘SPOT’, feature advanced optical systems so they can be locally or remotely focussed via their motorised focus mechanism. The units use mirror optics as opposed to the more traditional refractive optics meaning that all wavelengths reflect at the same angle as opposed to refracting light and infrared energy at different angles.
Gagg added: “With mirror optics you are precisely measuring what you are targeting rather than taking some of the energy from a slightly bigger area and some of the energy from a slightly smaller area. You’re able to exactly define what’s being measured.”
The SPOT pyrometers use a green high-brightness LED to target the measurement instead of a red laser, which can have safety implications and is hard to see on a red, yellow or white hot surface. They also include a video camera mounted in the same optical axis as the detector allowing users to view exactly what the pyrometer is sensing. Gagg says this is specific to the SPOT sensors.
Another advantage that the non-contact sensors have over surface sensors is that, because of the very nature of not touching the surface, they have a longer lifetime and will avoid tip migration. This is where the tip of a thermocouple will begin to alloy over time, changing the millivoltage of the sensor’s readings.
“I had a call a couple of years ago from a company in California who were complaining that the signals from their IR thermometers were drifting,” Gagg said. “I was quite concerned with this and asked them for serial numbers of these thermometers and they were 28 years old! Changing them out every three months for this long had worn the gold plating off the electrical pins.”
He added that, as well as watching out for the age of the sensors, maintaining the protective window on the pyrometers is also important. “The window on the SPOT pyrometers is made of sapphire, which is nearly as hard as diamond, but less expensive,” said Gagg. “If you get something stuck on it you can clean it off using a wood chisel, you can’t scratch it.”
This is especially useful in applications where a coating is being sprayed onto the surface of the object with a plasma. This is a process where a traditional temperature measurement device could never be used along with plasma nitriding, laser hardening, flame hardening and induction hardening. All of these processes involve the means of heating directly affecting the temperature of the sensor as well as the object it is measuring.
All the pyrometers in the SPOT range feature five selectable operating modes that enable the sensor to operate to its full capacity over specific wavelengths and temperature ranges. This makes them especially attractive to heat treaters of metals as these companies tend to do small batch jobs for customers in different fields. This flexibility means that, rather than have multiple sensors for different processes, the SPOT pyrometers can be used on different materials and processes, eliminating downtime due to switch-out.
“I refer to it as like having a Swiss army knife,” Gagg said.
Of course, all this presupposes that the surface temperature of the required application is significantly higher or lower from ambient. Gagg says that there are some applications where contact sensors are suitable, for example: “If you have a close process that takes hours to come up to temperature and then soak and later ramp downwards slowly, these applications are ideal for a thermocouple.”
Certain suppliers, such as Watlow and Isotech offer what they call 'true surface' temperature probes - TST and Model 944 respectively. These are said to include feed-back heating to compensate for heat losses along the probe, which then more closely show the 'true' surface temperature.
Non-contact sensors - low cost, long wavelength pyrometers, especially - can be prone to showing deviations in accuracy due to emissivity (a radiative property) of the surface, where a 10% change in emissivity may result in an 8% difference in temperature reading. Gagg said that higher quality, short wavelength pyrometers are insensitive, with just a 1% difference in temperature reading from a 10% change in emissivity.
“We have application specific pyrometers like the SPOT AL EQS which is designed especially for very low and uncertain emissivity materials like aluminium,” explained Gagg. “The multi-wavelength design with application specific signal processing algorithms means that these pyrometers actually compute the emissivity value, updated every 15 milliseconds. This makes them extremely accurate.”
The choice of whether to use surface or non-contact sensors depends on the process being carried out. However, it seems that unless the process is one being carried out at room temperature, non-contact seems to have more advantages.
Designing in sensors
Many industrial furnace makers include AMETEK Land non-contact pyrometers in the designs for their furnaces. They have also been designed in to the engines of the Tornado multi-role combat aircraft and the GE90 engines used on Boeing’s 777 airliners to measure the temperature of the turbine blades and the engines’ fuel economies.