Understanding the thermal issue

4 min read

Temperature is a key measurement parameter in controlling the quality of glass or glass products. Conventional thermal imagers with spectral wavelengths of 8-14µm do not measure accurately the temperature of the glass.

The thermoIMAGER G7 from Micro-Epsilon is a thermal imaging camera specifically designed for measurements on glass object. With a spectral range of 7.9µm, the thermoIMAGER G7 accurately measures the temperature of glass.

At longer wavelengths, glass allows transmission of temperature from objects behind or near to the glass, which gives inaccurate measurements, normally lower than the true temperature. However, using the G7 camera at a wavelength of 7.9µm eliminates any transmission from other objects, resulting in very accurate glass temperature measurement.

Chris Jones, managing director at Micro-Epsilon UK said: “When measuring at 7.9µm, the glass object becomes a solid body and only emits its own temperature. This results in very accurate temperature measurements, even on very thin glass or thin walled objects such as glass tubes, bottles and substrates.”

Even if the glass has a protective (reflective) coating applied to it, an additional reference pyrometer can be set up to provide an adjustment factor to the camera, which corrects for this reduction in transmission of infrared temperature from the glass – a unique feature of the thermoIMAGER software supplied as standard with the G7.

Powered and operated via a USB 2.0 interface, the G7 provides temperature images and profiles of a target area. This plug-and-play unit enables the real time capture at 80Hz full frame rate and storage of images or video of an event for slow motion play back or snapshots at a later date – a key feature in many quality, inspection, R&D and failure diagnostics work.

An alternative method of measuring the temperature of glass and glass products is to use a high precision infrared temperature sensor. Micro-Epsilon also has a thermoMETER CT family of infrared temperature sensors that include the CTLaserGLASS, a non-contact infrared thermometer specifically designed to measure the temperature of glass surfaces.

The CTLaserGLASS uses a 5.0µm wavelength detector to accurately measure temperatures from 100°C up to 1650°C. The average measuring wavelength of 5.0µm provides a low depth of penetration and enables reflection effects to occur for the infrared measurement of glass. Using shorter wavelengths than this would mean the sensor would measure through the glass rather than measuring the true temperature of the glass itself.

In container glass production, for example, the operator must obtain the temperature of the glass gob (molten glass that is poured into a blow mould) to observe the ratio between glass viscosity and gob weight. The mould temperature measurement is therefore critical for balancing the cooling levels of mould shells.

In the production of flat glass, automotive glass and construction glass, homogeneity of the complete glass panel is important, particularly when it comes to bending, annealing and tempering zones.

The double laser aiming of the thermoMETER CTLaserGLASS marks the real spot location and spot size up from 1mm at any distance. The 70:1 (or 45:1) optics with selectable focus, provide a very small spot size of just 1mm.

ThermoMETER CTLaserGLASS has a stainless steel sensor head and can be used in ambient temperatures of up to 85°C without cooling and to protect the laser aiming optics, has an automatic laser switch off at 50°C. Cooling and protection accessories are also available for harsh environmental conditions. For example, a water-cooled version is available for ambient temperatures of up to 175°C.

The thermoMETER CT series operates with specific wavelengths and are ideal for measuring the temperature of virtually all materials. These include metals, glass, ceramics and composites, from -50°C to +2,200°C, using laser sighting to easily locate the target to be measured and also define measurement spot size.

The thermoMETER CTLaser M3, for example, has a start temperature of 50°C and so fulfils the demands of end users who need to measure the temperature of metals, ceramics and composites, while processing at room temperature. The short wavelength also enables measurements to be taken through glass or transparent plastic windows, a common task in the latest laser welding or lighting systems.

In applications where the emissivity of a target is unclear or varies, the thermoMETER CTRatioM1 offers an alternative solution where the ratio between two different short wavelength detectors is compared.

The ratiometric principle, sometimes referred to as a ‘2-colour pyrometer’, minimises measurement errors caused by objects in the optical path blocking the path. For example, scale build-up on hot and molten metals, steam or smoke, which block up to 90% of the measurement spot do not affect the measurement accuracy. The CTRatioM1 measures temperatures from 700°C to 1800°C and the use of glass fibre optic cables means the sensor can withstand ambient temperatures up to 250°C without the need for additional cooling.

Another temperature sensor has recently come to light from the University of Tokyo. There, researchers have produced a flexible, lightweight sensor that is said to respond rapidly to changes in body temperature. The sensor’s resistance is said to change by up to 100,000 times over a range of 5°C, allowing accurate temperature measurement without additional complicated display circuitry.

The sensor is composed of graphite and a semicrystalline acrylate polymer formed of two monomers. The temperature at which the sensor is most precise can be selected by altering the proportions of the two monomers. The research group measured temperatures between 25 and 50°C, with response times of less than 100ms and a temperature sensitivity of 0.02°C. The device was also found to be stable, providing repeated readings up to 1800 times.

“By printing an array of these sensors, it is possible to measure surface temperature over a large area,” said Professor Takao Someya from the University’s Graduate School of Engineering. “Because the huge response of the sensor to temperature change allows us to simplify the circuitry, we could print our sensors onto adhesive plasters that could then monitor body temperature.”

Other possible applications include wearable electronics, where the sensor could be applied beneath fabric to measure temperature during sporting and other activities.

Another product of similar ilk to come to market is from Fluke Process Instruments. It has introduced the Endurance Series of high-temperature ratio pyrometers. These instruments are said to enable continuous visual process monitoring and are designed to meet the demands of harsh industrial environments, including primary and secondary metals manufacturing, carbon processing and silicon production.

The Endurance Series pyrometers provide a robust solution for manufacturers seeking to improve product quality and uniformity, reduce reject rates, maximise throughput, and minimise energy costs. They offer optical resolution of up to 150:1, for viewing critical process operations. Multiple lens, sighting and focus options are available for different mounting distance and sighting needs.

The units feature galvanically-isolated inputs/outputs, as well as an IP65 rated stainless steel housing able to withstand ambient temperatures up to 65°C or up to 315°C using cooling accessories.

The pyrometers are also said to be versatile and easy to install. The sensors operate with either PoE or DC power, and interface to various bus systems. A rear-panel user interface is claimed to simplify navigation. PC-based Endurance setup and monitoring software is provided for configuration and deployment, and a built-in web server enables archiving of historical data for traceability and process troubleshooting.