How to Choose a Proxitron Proximity Sensor for High-Temperature Environments

Short answer: match the sensor's temperature class to the ambient air around the sensor body, not to the temperature of the object you are detecting. Proxitron splits this into three families — ProxiPolar for cold, ProxiHeat for up to +120 °C, and the ProxiHT stainless-steel series for up to +230 °C. Then check three things most buyers miss: the mounting clamp's own temperature limit, the cable material, and the de-rating of sensing distance in hot air. Get those four right and a high-temperature proximity switch lasts years instead of months.

Why ordinary proximity sensors fail above 100 °C

An inductive proximity switch is a coil, an oscillator and a switching amplifier potted into one housing, and the electronics are the weak point. Standard industrial sensors are rated to roughly +70 °C ambient. Run one at 120 °C and it does not stop instantly — it drifts. Switching points wander as the oscillator's reference shifts with temperature, and the potting and internal joints age far faster than the datasheet's MTBF suggests. That is the practical reason high-temperature sensors are a separate product line rather than a standard part with a higher number printed on it.

The distinction that trips people up: a sensor rated to +230 °C is not detecting a 230 °C object. It is surviving in 230 °C air. The workpiece temperature is a separate problem, handled by sensing distance, exposure time, and shielding such as tubes or furnace windows.

Step 1 — Pick the temperature class

Proxitron's cylindrical inductive range spans −40 °C to +230 °C, and the series names tell you which band you are in.

ProxiHeat is a plastic housing, not a downgrade. Full potting makes it corrosion-free and reliable through fast temperature swings and aggressive ambient — the conditions around a hot rolling mill. It suits most of the 100–120 °C band and is normally the cheaper option.

ProxiHT is stainless steel because of where it has to live. Above 120 °C you are usually in a furnace or casting environment as well: radiant heat, scale, mechanical knocks, washdown. The stainless housing does mechanical and radiant-heat work as much as thermal work.

On the M30 threaded plastic range, Proxitron encodes the class in the part number:

So IKL 015.33 GH1 is a 15 mm M30 sensor, PNP normally open, short-circuit protected, good to +120 °C. The prefix IKL stays constant and the suffix does the temperature work — worth knowing before you ask for a quote, because a GH and a GH1 look identical in a catalogue photo and differ by 20 °C of headroom.

The code table also carries a low-temperature suffix N (from −40 °C), plus connection variants: S4 for an M12 DC plug, S27 for an M12 AC plug, C for an M16 internal thread at the cable outlet, M for a fixed protective hose gland, and F for an offset oscillation frequency. Not every suffix is available on every body size, so confirm the combination before ordering.

The voltage/output matrix is orthogonal to temperature

Within any temperature class, the electrical variant is a separate choice. The M30 plastic family alone offers, at every grade: 20–260 V AC/DC 2-wire; 24 V DC 2-wire; and 10–55 V DC in NPN or PNP, each normally open, normally closed, or a complementary PNP pair in one housing. For a sinking PLC input you want the NPN codes (.30 / .31); for a sourcing input, .32 / .33. Ordering the right temperature suffix with the wrong output type is the most common re-order error.

Step 2 — Work out the real sensing distance you can keep

Nominal sensing distance Sn is measured against a square steel plate whose side length is three times the sensing distance — a 15 mm sensor against a 45 × 45 mm plate. Anything smaller, and you lose range.

Material factor. Reduced distance is normal for non-ferrous targets. Proxitron publishes the multipliers and they are worth carrying in your head:

A 15 mm sensor aimed at brass gives you about 6.75 mm; at copper, 4.5 mm. This is true of every inductive sensor, but it is where high-temperature installations go wrong — you usually choose a hot, awkward mounting point first and then discover you cannot get close enough.

Flush versus non-flush. Flush sensors embed in metal with no clearance; non-flush need a bare metal-free zone around the sensing face. Proxitron's M30 plastic range offers 10 mm flush and 15 mm or 25 mm non-flush. If the drawing forces you into a flush recess you have already accepted the shorter distance — plan target geometry around that, not the other way round.

Thermal de-rating. Leave headroom rather than designing to the nominal figure. If your target sits at the top of the sensor's temperature class, run the sensor 20–25 % closer than the cold-state calculation says, or step up one sensing-distance size. The cost of a longer-range sensor is trivial next to an unplanned shutdown.

Step 3 — Do not forget the parts around the sensor

This is the step that fails in the field: the sensor is rated to +230 °C and the clamp holding it melts.

A +230 °C sensor in an HH6 clamp at 200 °C ambient is a bracket that deforms while the sensor is perfectly happy. Above +120 °C you are generally into fabricated stainless brackets and furnace windows rather than polymer clamps — and this belongs in the RFQ, not on site.

Then the cable, the most common single point of failure. The four materials are not interchangeable:

Standard lengths are 2, 5, 10, 15 and 20 m. Note what this means: a +230 °C sensor with a PVC cable is an +80 °C installation overall. The sensor's rating is not the system's rating, and the weakest component sets the limit. Cable protection hose and adapters (the M16 internal thread option, suffix C, or a fixed protective hose gland, suffix M) exist precisely for this gap.

If you take one thing from this article: the temperature class of your installation is the minimum of the sensor rating, the bracket material and the cable jacket — not the sensor rating alone.

Step 4 — Installation details that prevent nuisance faults

Frequency-tuned sensors for row mounting. Two inductive sensors side by side can couple and interfere, producing phantom switching. Proxitron marks sensors with an offset oscillation frequency with the letter F, so alternating units can be mounted directly adjacent without mutual interference — the standard solution for material tracking, can lines and multi-position detection. If you are replacing sensors in a dense row, check for the F suffix; otherwise expect to space them out or accept intermittent faults.

Connection style. A fixed cable removes a connector that would otherwise sit in the heat; an M12 plug (S4 DC, S27 AC) makes changeout fast. Neither is universally right — the deciding factor is whether maintenance can reach the sensor while the line is hot.

Teach-in. Many Proxitron sensors self-calibrate to ambient conditions at the touch of a button, optimising their own switching threshold for the conditions they find rather than for the bench conditions they were set up in cold.

Which series for which job

If you can move the sensor back from the heat source or shield it — a protective tube, a furnace window with integrated air purging, or simply 100 mm more standoff — you can often step down a temperature class and save real money. Buy the temperature class you need at the sensor body, not the temperature you see on the process display.

Buying Proxitron high-temperature sensors

FINACO supplies Proxitron inductive sensors, optical sensors and hot-metal detectors, and pyrometers to overseas industrial buyers, sourced through original channels with documentation and a traceable supply chain. Browse the Proxitron brand page for series and commonly requested part numbers, or see the full sensor range for matching on temperature class, output type and cable.

To get a correct quote first time, send four things:

1. The ambient temperature at the sensor body, not the process temperature
2. The target material and the closest achievable mounting distance
3. The output type your PLC needs — PNP or NPN, normally open or normally closed
4. The existing part number, including the temperature suffix, if you are replacing one

More selection and cross-reference guidance is in Insights.