Low cost measurement of high temperature applications

Written by: Chris Shaw | Published:

Micro-Epsilon has expanded its range of non-contact infrared temperature sensors to include a new infrared ratio temperature sensor (ratio pyrometer).

According to Micro-Epsilon, the new thermoMETER CT ratio pyrometer is suitable for OEMs that need to measure the temperature of metals, metal oxides, ceramic or semiconductor materials in harsh, high temperature environments, including metal casting, welding, forming and sintering.

The thermoMETER CT ratio is a fibre optics ratio pyrometer that measures temperatures from 700 up to 1800°C. The sensor functions according to the ratio principle and is therefore insensitive to contaminants such as dust, smoke or steam that may partially obstruct the target. Even in cases where the infrared radiation beam is weakened to 95% of its original intensity due to smoke or steam, Micro-Epsilon says that the ratio pyrometer still provides faultless measurements.

The fibre optics ratio pyrometer operates on the two channel measurement principle. Two integrated infrared detection channels with relatively close spacing, but with mutually different measuring wavelengths, produce two independent electrical signals. These signals depend on the infrared radiation emitted from the target and signal components, which are influenced by the optical transmission path between the object and the pyrometer, or from changes of the object if for example, it is moving or changes its size.

By taking the ratio of the two signals, the ratio pyrometer principle eliminates (or averages out) any signal parts that are similar in both channels. The measured temperature of the object therefore only depends on the signal ratio and the emissivity ratio, or 'slope value'.

When it comes to measuring the temperature of objects with unknown or changing emissivity, the ratio thermometer principle provides extremely accurate results, assuming that the emissivity ratio is not changing over time or temperature.

The compact sensor head of the ratio pyrometer has a diameter of 25mm and a length of 70mm, enabling it to be mounted into restricted spaces in production lines and machines. A fibre optic cable connects the optical sensor head to the electronics. The optics and the cable can be used in ambient temperatures of up to 250°C without the need for additional cooling. By using a fibre optic cable, the device is insensitive to electromagnetic interference normally found in induction based heat treatment processes.

Micro-Epsilon says that rapid temperature measurements with response times of 5ms can be achieved and an optical resolution of 80:1 with a selectable focus, in combination with built-in laser aiming, enables measurements from a spot size of 1.3mm.

In order to adapt the sensor to different measurement tasks, the pyrometer offers a choice of a one or two channel (ratio pyrometer mode) device, designed to enable users to select the most appropriate one without having to purchase additional infrared sensors.

Typical applications for the ratio pyrometer include the temperature control of inductive heating coils, temperature measurement of the pouring stream on casting machines; during the reheating of steel ingots and steel billets in forging processes; in general soldering and welding processes; and in process steps in semiconductor production and in photovoltaics.

The electronics enable the programming of the sensor, display of temperature measurements and wiring of the analogue outputs 0/4-20mA and 0-5/10V for the transmission of signals into industrial control cabinets. To facilitate network integration, Micro-Epsilon offers an optional integrated Profibus DP interface or other plug in digital interfaces including USB, RS232, RS485 or Ethernet. Programmable signal processing ensures complete flexibility for different production processes.

Micro-Epsilon also offers software that provides graphical interface and display, the logging of temperature readings of all output channels and signal characteristics over time. The user can also configure parameters and set up alarms such as 'dirty lens' alarms.

The standard focus of the lens is 60mm. However, an additional, variable focus lens can be provided, which enables the user to set the focal point of the lens between 65mm and 300mm.

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