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Improving glass temperature measurement accuracy

29 December 2015

Inline (fixed) thermal imaging cameras or non-contact infrared temperature sensors are currently limited in the accuracy they can achieve, but recent developments by Micro-Epsilon could change that.

The thermoIMAGER G7 thermal imaging camera is one of Micro-Epsilon's solutions to a longstanding problem for the glass manufacturing industry - how to achieve accuracy in the measurement of glass temperature. Conventional thermal imagers with spectral wavelengths of 8-14µm do not provide an accurate measurement because at longer wavelengths, glass allows transmission of temperature from objects either behind or near to it, introducing inaccuracies and measurements that are generally lower than the true glass temperature.

With a spectral range of 7.9µm, the thermoIMAGER G7 accurately measures the temperature of the glass - even very thin glass.  Using a wavelength of 7.9µm eliminates any transmission from other objects, resulting in very accurate glass temperature measurement, as Micro-Epsilon UK's managing director, Chris Jones explains:

“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 the corresponding reduction in transmission from the glass – a novel feature of the thermoIMAGER software supplied as standard with the G7.

The G7 is also supplied with full, licence-free operating and analysis software, which includes a variety of process control features such as ‘line scan mode’ – a feature that enables the camera to view the glass through a narrow slit (restricted field of view), yet which still provides a complete thermal image of the entire sheet.

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 playback or snapshots at a later date – a feature benefiting many quality, inspection, R&D and failure diagnostics work.

Alternative methods for glass
An alternative method of measuring the temperature of glass and glass products is to use a high precision non-contact infrared temperature sensor. Micro-Epsilon’s thermoMETER CT family of infrared temperature sensors includes the CTLaserGLASS, a non-contact infrared thermometer specifically designed to measure the temperature of glass surfaces or products such as solar panels, flat glass lines, light bulbs, car glass finishing and glass containers.

The CTLaserGLASS uses a 5.0µm wavelength detector to provide accurate measurement of temperatures from 100°C to 1,650°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 and 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 at 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.

Other industrial applications
Micro-Epsilon’s thermoMETER CT series of compact, inline (fixed) infrared temperature sensors are suitable for use in a wide range of other industrial applications, from maintenance and process monitoring tasks, to R&D and test laboratories.

The thermoMETER CT series operates with specific wavelengths and are capable of measuring the temperature of a wide variety of material, including metals, glass, ceramics and composites, from -50°C to +2,200°C, using laser sighting to locate the measured target and define the measurement spot size.

The thermoMETER CTLaser M3, for example, has a start temperature of 50°C and so fulfills 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 method where the ratio of two different short wavelength detectors is compared. The ratiometric principle, sometimes referred to as a ‘two-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 percent of the measurement spot do not affect the measurement accuracy.

The CTRatioM1 measures temperatures from +700°C to +1,800°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.

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