Product category:
Temperature measurement: IR techniques
News Release from: Calex Electronics | Subject: EL sensors
Edited by the Processingtalk Editorial
Team on 31 August 2004
IR Thermometry: from Herschel to modern
times
Dr F S Ritchie surveys the development of IR temperature measurement technology from the original experiments to modern Calex EL sensors used in many process industries
In 1800 William Herschel, the great astronomer and telescope maker, tried an experiment Like Newton in 1666, he passed a beam of white light through a glass prism and found, spread out before him, the familiar spectrum of colours, from violet to red
This article was originally published on Processingtalk on 2 Feb 2005 at 8.00am (UK)
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Unlike Newton, he placed a thermometer at various points along the spectrum and, as he suspected, found that the temperatures differed.
The lowest were at the violet end of the spectrum and they rose toward the red and then, beyond the end of the spectrum, they kept rising.
Not only had Herschel proved what he had set out to prove (that light and radiant heat are identical, itself a momentous finding) but he had discovered infrared radiation extending beyond the end of the visible spectrum.
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As the great man noted, radiant heat consists of "invisible light".
We know now that light can be interpreted in terms of electrical and magnetic radiation and that infrared (IR) radiation is just one manifestation of this phenomenon.
IR has numerous uses in electronic devices such as remote controls, rangefinders, and lasers.
Infrared sensors are also used in security systems and heat-seeking missiles.
However, since IR radiation is emitted by heated bodies, it can be used to measure their temperature quickly, without contact, and without disturbing or contaminating them.
One company, which has been successfully providing IR temperature measurement sensors since it was founded in 1974, is Calex.
Examples in the use of these products are many, it says, with their EL range of IR sensors being particularly in demand for industrial applications including food, paper, rubber, and chemical processes amongst many others.
And now the EL range has been extended in order to offer users more choice in setting up their control systems whilst retaining the core attributes for which the range has become justly well known.
One big advantage of IR sensing is its ability to monitor temperatures remotely without contact, such as in food processing, where contamination of the product must be avoided.
The food mix is often prepared under conditions of high ambient temperature with steam being present.
Not only has Calex developed accessories to shield the EL sensors from such harsh environments, but they have built into the sensors patented technology that minimises the effects of changing ambient conditions, thus maintaining the accuracy of the measurements.
Of course, the EL are small and their rugged stainless steel housings make them ideal for applications where cleanliness and hygiene are paramount.
IR thermometers use optical lenses to gather energy from a spot on the target object and focus it onto a detector in the sensor.
It is important to ensure that the target completely fills the measurement spot if accurate readings are to be obtained - otherwise the IR sensor will receive radiation from the background.
Distance-to-size ratio (D:S) enables the user to calculate the size of the measurement spot at a given distance.
For example, if a sensor has a D:S ratio of 10:1, then a target at a distance of 600mm must be at least 600/10 mm or 60mm diameter in order to fill its measurement spot.
The EL range is now available with D:S ratios of 2:1, 10:1, and 30:1.
For most applications involving fairly large targets, a D:S of 2:1 or 10:1 should suffice.
However, small objects require optics with higher resolution and this is now available in the EL301 with 30:1 optics.
Interfacing with PLCs and control systems is possible due to a large range of outputs.
The EL sensors can be supplied as either 2-wire units with a 4-20mA output, or 4-wire units with voltage or thermocouple outputs.
If the J or K thermocouple output is selected, the user must remember to connect the EL sensor to its controller or recording device by means of the appropriate thermocouple cable or compensating cable.
This choice of outputs makes the EL series compatible with almost any indicator, controller, recorder or data-logging instrument.
It is obvious that the temperature range of an IR thermometer should be matched to that of the process being monitored and controlled.
In this way, the sensitivity of the output to changes in process temperature is optimised.
For example, if the process is to be controlled at 50C, a sensor with a temperature range from -20 to 100C is preferable to one with a range of 0 to 500C.
If the output of both sensors is 4-20mA, the sensitivity of the first is 133 microamps per degree compared with the 32 microamps per degree of the second.
The three standard options available in the EL series cover the range from -20 to 500C.
However, sub-ranges (minimum span 100C) can be supplied to order.
These sensors have an outstanding response time of 200ms, which not only provides the means for tight control of most processes but makes them ideal for measuring moving objects.
This paper was supplied by Dr F S Ritchie on behalf of Calex Electronics Limited.
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