High-Temperature Proximity Sensors: Specifying for Heat

Heat kills more proximity sensors in Indian plants than water does, and it almost always traces back to the same error: the sensor was specified against the plant ambient temperature rather than against the temperature it actually experiences.

A sensor mounted two metres from hot stock is not at ambient. It is receiving radiant heat continuously, and its case temperature can sit far above the air around it. Before you buy anything rated higher, it is worth understanding the three ways out, because they are cheaper in the order given.

First, measure what the sensor actually sees

Not the plant ambient. Not the design figure. Put a thermocouple or a thermal camera on the intended mounting position during normal production, in the hottest part of the shift, in the hottest month you can manage.

It takes ten minutes and it routinely produces a number nobody expected. It also tells you which of the three fixes below you need, and whether you need a higher-rated sensor at all.

Fix one: move it

Distance is the cheapest heat shield there is. Radiant heat falls off sharply, so a mounting position half a metre further back often solves the problem outright and costs a bracket.

This is worth exhausting before anything else. It needs no consumables, nothing to maintain, and nothing that can fail.

Fix two: shield it

A simple steel plate between the sensor and the radiating source blocks the line of sight. Radiant heat travels in straight lines, so interrupting the path is remarkably effective, and a shield costs very little.

Leave an air gap between the shield and the sensor so the shield itself does not conduct heat straight into it, and make sure the shield does not sit in the sensing field if the sensor is unshielded, or it will detect the plate.

Fix three: cool it

Air purge or water cooling, where the first two are impossible. Effective, and the standard approach around furnaces, kilns and rolling mills.

The caution is that you have now added a service that can fail. A cooling supply that stops will destroy the sensor quickly and quietly. If you go this route, plan the air supply at the same time as the sensor rather than retrofitting it, and consider whether loss of cooling should be alarmed.

What heat actually does to a sensor

Worth knowing, because the symptoms get misdiagnosed.

Sensing distance drifts. The oscillator circuit is temperature dependent, so the effective distance changes as the sensor heats. A sensor set at the edge of its range on a cool morning becomes intermittent by afternoon. This is why you design to the assured operating distance rather than the rated one, and why 70 to 80 percent of rated is the sensible mounting gap.

Seals age. Elastomer seals and cable sheaths harden and crack with sustained heat. The sensor then fails on ingress during the next washdown, months after the heat caused it, which makes the cause hard to see.

The cable fails before the sensor. In our experience of hot installations the cable is usually first, cooked against a hot surface or at the gland where it leaves the sensor. Route it away from the heat, support it so the gland carries no weight, and leave a service loop.

Where this comes up

• Steel plants and rolling mills, where radiant heat from stock is the dominant problem and hot metal detectors handle the stock itself
• Cement plants, around the kiln hood and clinker cooler
• Sugar mills, in the boiling house and around the boilers
• Forging, heat treatment and foundry, where the heat is intermittent but severe
• Plastics processing, around heated barrels and moulds, where the temperature is lower but constant
• Engine and vehicle test rigs

Getting a recommendation

Send us the measured temperature at the mounting point rather than the plant ambient, whether the heat is constant or cyclic, how close the sensor has to be to the target, whether purge air is available, and what the target is. A photograph of the position is worth more than a description.

We manufacture proximity switches in Mumbai and will tell you plainly whether the standard range covers your temperature, whether a special build is the answer, or whether the right fix is a bracket rather than a sensor at all.