ifm Ultrarobust Sensors, Load Cells, and Moisture Meters: A Maintenance Engineer's Guide to Testing Before Replacing

The short version

If you've ever replaced a sensor, sent the old one back, and watched the new one do the same thing, you know the problem. You didn't have a sensor failure. You had a system failure. And system failures don't show up in the spare parts budget until after the second or third replacement.

I'm the person who keeps our maintenance checklist. Not because I'm a genius. Because I've made enough expensive mistakes to fill a binder. Since 2017, I've personally documented 14 significant sensor-related screw-ups. The total wasted budget is roughly $46,000, and most of that wasn't parts. It was downtime, rushed shipping, extra labor, and the rework that happened while I blamed the wrong component.

My experience is mostly in food, packaging, and machine tools. If you're in a different industry, your mileage may differ. But the logic of testing before replacing tends to travel well.

Here is the rule I now use on every quote and every repair: total cost of ownership, not price. The $200 sensor that fails in 18 months in a washdown area is more expensive than the $320 Ultrarobust sensor that survives five years. I didn't always think that way. Now I do. Bottom line: the price tag is only the beginning.

The three situations that matter

Most sensor decisions fall into one of three boxes:

• Scenario A: You're choosing a sensor for a new application, or replacing a failed one in an existing machine.
• Scenario B: You're staring at a reading that doesn't make sense and trying to figure out why.
• Scenario C: You're wondering whether the load cell or sensor is actually bad, or whether you can test it and reuse it.

These need different approaches. Don't use Scenario C thinking in Scenario A. That's the mistake I made for years.

Scenario A: New install or replacement

If you're buying a sensor for a machine that gets washed down daily, look at ifm Ultrarobust sensors first. The Ultrarobust line is designed for exactly that: high-pressure washdown, chemical cleaners, and physical abuse. According to ifm's published technical data, Ultrarobust sensors carry an IP69K rating (per ISO 20653), which means they can handle water sprayed at close range.

But here's the part that goes against common sense: don't automatically pick the toughest sensor. If the sensor is on a dry conveyor in a temperature-controlled area, a standard ifm inductive sensor will do the same job at a lower price. In my first year, 2017, I ordered the heavy-duty version for a whole line because stronger is better. It worked fine. But it cost 35% more, and we never needed that extra protection. That was money I could have spent on a spare sensor and cable sets (note to self: pay attention to the environment, not just the price).

One thing I've realized: people think a higher price means a sensor lasts longer. It's actually the other direction. Sensors that are designed to survive harsh environments are priced accordingly. If your environment is benign, that extra cost doesn't buy reliability. It buys unneeded armor.

The same logic applies to an ifm compressed air flow meter. These are great for finding leaks and monitoring consumption, but they're not magic. They need a straight pipe run before and after the meter. I installed one in September 2022 without checking the recommended straight-run distance. The readings were all over the place, and I spent a weekend convinced the meter was faulty. It wasn't. The pipe layout was wrong. Installation conditions are part of the total cost.

Scenario B: Weird readings

When a sensor reading is unstable, most people's first instinct is to replace the sensor. Stop. That instinct is a relic from an era when sensors were simpler and diagnostic tools were expensive. Today, the first thing to test is the environment around the sensor.

I can't count the number of times a 'dead' sensor was actually a loose M12 connector, a damaged cable, or water sitting in a junction box. That's where an MR176 moisture meter earns its place in your toolbox. It measures moisture content in materials, and it's perfect for checking whether the mounting surface, conduit, or cable jacket is actually wet before you blame the electronics.

One example: We had a pressure transmitter on a mixer that kept jumping. Everyone wanted to replace it. Before ordering a new one, I used the MR176 to check the cable gland area. It was saturated. The sensor was fine. The way the conduit entered the cabinet was trapping water. Dried it out, re-sealed the gland, and not another issue for 14 months. The replacement would have cost about $380 plus lost production. The moisture meter had paid for itself.

If you're in this situation, here's what to do before replacing anything:

1. Check supply voltage at the sensor, not just at the PLC.
2. Inspect the connector and cable for damage, corrosion, or moisture (the MR176 makes this fast).
3. Look at the mounting position. Is there a risk of water pooling, vibration, or heat?
4. Check the signal at the sensor output with a multimeter. If the signal changes correctly, the sensor likely isn't the problem.

I don't have hard data on how many sensor replacements are actually cable or environment failures. But based on the 200-ish jobs I've documented, my sense is more than a third. Wasted parts buyers love that stat. Maintenance people just nod.

Scenario C: How to test a Rice Lake load cell

Rice Lake load cells are common in weighing systems. The question 'how to test a Rice Lake load cell' comes up more than most people admit. The answer depends on whether you're testing a complete system or a single cell.

If you're testing a single cell, start with a resistance check. Disconnect the cell from the indicator and meter to avoid interference. Measure the resistance across the excitation wires, and then across the signal wires. You're looking for a stable bridge resistance, often around 350 ohms. Check the datasheet for your exact model. If the resistance is open or shorted, the cell is bad. If it's stable, the load cell may be fine.

Then do a live zero and span check. Reconnect the cell, power it up, and read the output with no load. It should be near the zero setpoint. Apply a known weight, or something close to it, and see if the output moves proportionally. A load cell can have perfect zero but no span. That usually means a mechanical overload broke a strain gauge. An open resistance reading is a red flag.

The common mistake is skipping the resistance test and jumping straight to calibration with the indicator. I did that on a $1,200 Rice Lake cell in 2019. The calibration failed, so I assumed the cell was dead. Turned out a cable was pinched inside a cable chain. Continuity was intermittent. The cell itself was fine. We re-ran the cable and it worked for years. That mistake cost us a week of downtime and a very awkward call to our distributor.

The sensor is rarely the first thing to fail. The cable, the connector, the environment, and the installation are the usual suspects.

This is also where the MR176 moisture meter helps again. Moisture inside a load cell junction box is a classic intermittent failure. Before replacing a load cell, check the junction box for water entry. If it's damp, the fix may be as simple as drying it out and replacing the desiccant.

How to tell which scenario you're in

Here's a simple way to decide where to put your effort:

• If you haven't installed anything yet, you're in Scenario A. Spend time on spec and total cost.
• If you have a reading you don't trust, you're in Scenario B. Test the environment before touching the part.
• If you've already checked cables, connectors, and moisture, and the sensor output is still wrong, you're in Scenario C. Test the sensor itself with a meter before replacing it.

That sequence alone would have saved me most of the $46,000 I mentioned at the start.

There's one more thing I go back and forth on even now: standard ifm vs Ultrarobust. On paper, standard sensors look like the smart financial choice. But in a washdown environment, the slightly higher price of the Ultrarobust version is a no-brainer when you include the cost of a mid-line failure. On a dry machine, it's just wasted money. There's no universal answer. Only the answer for your situation.

I wish I had tracked all of this from day one. What I can say anecdotally is that since I started using this checklist, and the MR176, we've caught 47 potential sensor replacements before they turned into purchases. That's not me bragging. That's me counting the mistakes I didn't make.

If you're in the middle of a sensor problem right now, don't order a new sensor yet. Take ten minutes to test the system around it. The answer might be sitting in a cable tray, not in the part catalog.