Emissivity – The Achilles Heel of Pyrometers?
Pyrometers have been used to measure temperature in aggressive thermal processes for many years, and their technical advantages are well-known:
- Pyrometers are an optical measurement of temperature with no physical contact
- Perform well over a high temperature range – up to 2200°C (3200°F)
- They measure the real temperature of the product and not the temperature of the sensor’s housing/mounting – such as with thermocouples (TCs), RTDs, or other contact methods
- Pyrometers can operate in very harsh environments such as in high vibration applications or high ambient temperatures
- The technology allows for extremely fast measurement – down to millisecond response times for fast moving processes
With all these benefits, pyrometers can be an excellent solution compared to traditional “contact” methods, however as with any measurement technique there are typically ‘limitations’ (often based on physical principles) that if not managed correctly, can create significant measurement errors.
So what about the ‘Achilles Heel?’
According to Greek mythology, Achilles was dipped into the river Styx as a baby and all the parts of his body that were exposed to the water became immortal. The only part that wasn’t touched by the water was his heel by which his mother held him – this area becoming his only weakness (or ‘limitation’ if you will).
Strangely enough, we can look at the world of industrial temperature measurement and draw many similarities with Achilles.
Like Achilles, pyrometers have a number of fundamental advantages. Inherently a non-contact optical temperature measurement, they can measure at up to 10,000 times a second, they can be used in harsh environments such as in semiconductor wafer or molten glass manufacturing, and can maintain accuracies of ±1°C up to temperatures of 2000°C or more.
So what’s the catch?
When it comes to pyrometry, there is a ‘limitation’ of the technique that needs to be managed – the effect of changing material emissivity (i.e. how a material radiates thermal energy and how this changes). This physical property is something that every material possesses and when it does change, it drastically affects the measurement performance of any pyrometer.
For a measurement device that can be as accurate as ±1°C, the errors introduced by changes in emissivity can be up to 50 times greater than its spec!
So the challenge is that the emissivity – expressed as “Ɛ” – of a material often changes with temperature, shape, and surface roughness and needs to be accounted for by optical pyrometer techniques – which is not an easy thing to do.
So is Emissivity the Achilles’ Heel of Pyrometry?
In short, the answer is no, it is not, but unless this effect is very well managed, it can be a big headache for achieving measurement accuracy anywhere close to what pyrometry is capable of.
Manufacturers of optical pyrometers have long attempted to solve for this problem – with varying degrees of success. Most have tackled the problem by using so-called two-color or dual-wavelength solutions. Instead of measuring at a single wavelength, these pyrometers attempt to make the system “immune” to any changes in emissivity by calculating the temperature based on the ratio of the two wavelengths being used (the emissivity of the material is still changing – and the pyrometer can’t tell you by how much – but the goal is to enable to pyrometer to ignore the changes in Ɛ).
Unfortunately, this and other passive techniques – such as wavelength tuning – can only work under certain circumstances and are by nature fundamentally limited.
The ‘Active’ alternative…
Recently Advanced Energy introduced an “active” correction method for emissivity changes where not just the temperature of the sample is being measured, but also its emissivity. Using this technique, changes in Ɛ can now be measured and actively corrected for on a frequent basis, allowing the pyrometer to maintain accurate measurement, irrespective of changes in temperature, surface roughness or material properties.
The logic behind this is the technique is to measure the material’s reflectance, R (Emissivity, Ɛ = 1 – R) in real-time so that the pyrometer can maintain measurement accuracy even under varying process conditions.
So, is this ‘the single answer’ for all emissivity related measurement problems? Unfortunately, no, there is no panacea correction for all applications, however working with a vendor that provides both passive and active correction techniques gives the user the greatest possible flexibility to optimize a pyrometer’s performance.
In the next blog in this series, we will continue to review both passive and active emissivity correction techniques and how they can be best applied to achieve reliable temperature measurement.