Application Bulletin IR3 Flame Detector – Metal Recycling Cabinet

Application Bulletin IR3 Flame Detector – Metal Recycling Cabinet

We were approached by a customer who required assistance with the following problem:

I would like to install an IR3 Flame Detector within a metal recycling cabinet, will this unit require daily cleaning due to the build up of dust which would cover the lens? There is also aluminium foil and other reflective metals in the smasher drum and these would reflect light as it rotates in the drum, would these reflective surfaces cause the flame detector to alarm or would the detector recognise that this is not a fire?


An IR3 flame detector is pretty good at looking through smoke, dust, and dirt.

SENSE-WARE have carried out experiments with grease and dirt during the development of our flame detectors, in which we saw that for a thick layer, there is a measurable but not a dramatic attenuation.

For flame detectors almost exclusively the detection distance determines the size of fire that is needed to get the detector to respond. In a confined cabinet the distances are very limited, so that is an advantage of this application.   

For placing the detector, I would recommend the same method as in fume hoods for laboratories: don’t use a swivel mount and place the detector in one of the corners of the ceiling.

This way, build up of dust is limited as you can see from the diagram above, for this application we would recommend that the window should be cleaned once every 2 months.

IR3 flame detectors have two reference sensors for supressing unwanted alarms caused by hot reflective surfaces, vibrating at the flicker frequency of 1 – 20 Hz.     

Detailed information

A single frequency IR flame detector measures an IR light signal (either the CO2 band 4.3 micron or the H2O band 2.7 micron) and the flame flicker frequency (typically 1 – 20 Hz). Only if the signal is strong enough AND if the flicker frequency is present, a fire alarm will be activated.

However, for applications with hot, reflecting surfaces, vibrating at 1 – 20 Hz a single IR flame detector may not be good enough in supressing unwanted alarms. If the light signal is strong enough in the CO2 band or the H2O band AND the hot reflective surface is vibrating at 1- 20 Hz, for the single frequency IR flame detector both criteria for giving a fire alarm are fulfilled.      

This is the reason why IR3 flame detectors are developed. Next to the main sensor (CO2 band sensor or H2O band sensor) there are two reference sensors. The signals from both reference sensors are subtracted from the signal from the main sensor. If the light comes from a hot, reflective surface, the signal from the main sensor goes up, but that from the reference sensors as well. The net effect is, that it will not result in an alarm signal, because of the subtraction.

In case of a fire, in the IR3 the reference signals will go up, but the hydrocarbon emits a lot of hot CO2. So, in a case of a fire, the main signal is much stronger than the reference signals. Thus, in case of a fire the net signal is higher than the alarm threshold.

 So, indeed, an IR3 flame detector can identify that there is not a fire in the smasher drums, despite the reflective metals.

IR3 flame detectors have a very good unwanted alarm suppression if no significant amounts of hot CO2 are emitted, that means that even IR3 flame detectors can be fooled. For example, by hot CO2, coming from an exhaust of a generator or from a helicopter in a hangar application.

If you require assistance with your Flame Detection application, please contact us

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