Most sensor problems are specification problems, not quality problems. Work through these seven questions before you order and the sensor you receive will do the job first time.
1. What is the target made of?
This decides the technology before anything else.
- Metal → inductive sensor. Note that ferrous metals give the full rated range; aluminium, copper and brass reduce it to roughly 30–50%.
- Non-metal at close range → capacitive sensor.
- Non-metal at longer range, or anything opaque → photoelectric sensor.
- A magnet, or a target behind a non-magnetic wall → magnetic reed or Hall effect sensor.
2. How big is the sensing gap?
Measure the worst-case gap, not the nominal one. Include mechanical tolerance, vibration, thermal growth and any wear that will open the gap over time — then choose a sensor whose rated distance comfortably exceeds it.
Design to about 70–80% of the rated sensing distance, not 100%. The rated figure is measured on a standard mild-steel target under laboratory conditions. Real targets, real temperatures and real alignment all eat into it, and a sensor running at the very limit of its range is the one that starts missing parts six months later.
3. Which barrel size?
Range scales with size, so the gap you measured largely picks the barrel for you:
| Size | Rated distance (mild steel) | Design for a gap up to |
|---|---|---|
| M8 | 2 mm | ~1.5 mm |
| M12 | 4 mm | ~3 mm |
| M18 | 8 mm | ~6 mm |
| M30 | 15 mm | ~11 mm |
| M50 | 25 mm | ~19 mm |
| M80 | 50 mm | ~38 mm |
If the barrel that gives you the range will not fit the space, consider a rectangular housing — it mounts from the front face and often fits where a barrel and its lock-nuts cannot.
4. Shielded or unshielded?
- Shielded (flush) — mountable flush in a metal plate. Choose this if the sensor sits in a metal bracket or machine face. Range is shorter.
- Unshielded (non-flush) — longer range, but needs free space around the sensing face. Mount one flush in metal and it will latch on permanently.
5. What output does your controller need?
- PNP or NPN — must match your PLC input card. See our guide to PNP vs NPN.
- NO or NC — normally open for straightforward detection, normally closed for fault-tolerant and interlock circuits.
- 2-wire, 3-wire or 4-wire — 3-wire is the default; 2-wire saves a conductor at the cost of a voltage drop; 4-wire gives both NO and NC at once.
- Analog — if you need a measurement rather than a switch, specify 0–10 V or 4–20 mA.
- 230 V AC — if there is no DC supply in the panel and you are switching mains devices directly.
6. What is the environment?
- Coolant, oil or wash-down → IP68 sealing and a chrome-plated brass or stainless housing.
- Weld spatter → brass barrel, and consider mounting position to keep the face out of the spatter path.
- Explosive atmosphere → NAMUR intrinsically safe sensing with an appropriate barrier.
- High vibration → avoid arrangements that depend on optical alignment; slot and fork sensors remove that failure mode.
- High temperature → tell us the actual figure, as standard sensors have limits that are easily exceeded near ovens and furnaces.
7. How will it be mounted and replaced?
- Cable or connector? A plug-in M12 connector version turns a sensor replacement into a one-minute job with no electrician.
- Is the position accessible? If not, connector versions repay their small extra cost the first time one fails.
- Cable length — standard is 2 m. Specify longer at order rather than adding a joint in the field, which is a common ingress point.
The short version
If you would rather not work through all of that, send us four things and we will specify it for you:
- What the target is made of, and roughly how big it is.
- The sensing gap, including worst case.
- Your PLC or controller model, or the supply voltage available.
- A photograph of where the sensor has to fit.
That is normally enough for us to recommend a part number and quote it the same day.
Related reading
Need help specifying a sensor?
Send us your application — target material, sensing distance, supply voltage and housing constraint. We will recommend a part, quote it, and ship from Mumbai.