How to Use a Multimeter to Verify Pressure Instruments: A Six-Step Checklist

I'm a maintenance planner who handles instrumentation orders for a food processing plant. Eight years in, I've personally made (and documented) 14 significant mistakes, totaling roughly $13,000 in wasted budget. Now I maintain our team's checklist. This is the list I wish I had in 2017—use it when you're ordering, installing, or troubleshooting pressure instruments.

This is not the full theory. It's the checks that catch expensive errors. It's tempting to think you can compare model numbers and prices and be done. But identical specs from different vendors can result in wildly different outcomes. Here are six checks I run before I trust any pressure reading.

1. Verify the Range Before You Install It

I once ordered 30 Ashcroft vacuum pressure gauges for a vacuum drying line. On the screen, they were perfect: 0-30 in. Hg, 1/4 in. NPT, stainless case. The process could swing positive during startup. The needle hit the inner stop on day three. The gauges weren't faulty. They were the wrong type—vacuum-only instead of compound.

Before you mount anything, answer three questions:

• What is the normal operating range?
• What is the worst-case excursion, including startup and shutdown?
• Does the gauge need to read both vacuum and positive pressure?

If the answer to that last one is yes, do not buy an Ashcroft vacuum pressure gauge with a 0-30 in. Hg scale. You need a compound gauge that goes from 30 in. Hg to +30 PSI. The model number looks close. It is not close enough. (Note to self: I still check this twice.)

2. How to Use a Multimeter on a 4-20 mA Loop

New transmitters are calibrated, so why check? Because a loop is more than the transmitter. I've installed an Ashcroft differential pressure transmitter that was correct on the bench, but the wiring termination was bad and the current did not follow pressure. A multimeter can catch that in ninety seconds.

Here's how to use a multimeter for a basic loop check:

1. Set the meter to mA. On most meters, the red lead must go into the mA jack, not the V/Ω jack.
2. Break the loop and connect the meter in series. Do not put the ammeter in parallel across the loop. You'll short the signal and probably blow the meter fuse.
3. Power the loop. At zero differential, the output should be 4 mA. If you can apply a known pressure, the current should track the pressure up toward 20 mA.

I once had a spreadsheet tell me to buy a rebuilt transmitter for 14% less than the new Ashcroft unit. Same range, same output, same specs. My gut said no. I did not buy it. Later, the rebuilt batch had zero-drift problems after heat cycling. The data was not wrong; it just was not complete.

If you are using a multimeter on a live loop, check the meter's fuse first. I have blown more than one mA fuse by measuring current with the lead in the voltage jack. It happens.

3. Check Encoder Outputs Before You Trust the Direction

If the flow meter has an encoder output, do not assume the direction is correct just because the counter shows a number. I wired a meter's encoder to a PLC and saw counts going up. The direction register was wrong. The A channel was dead.

Set your multimeter to DC volts. Connect across the encoder output and common. Rotate the shaft or paddle slowly. The output should toggle between low and high. Do the same for both A and B channels. This catches a stuck channel, the most common field failure.

What a multimeter can't do is measure phase offset between A and B. If you need quadrature timing, use an oscilloscope. But before you get to that level, check the basics: supply voltage, output wire, and pulse count. The pulse count can be wrong if the encoder disc and the electronics do not match. That mistake caused a 3-day production delay in September 2022 (ugh).

4. Look at the Process Tube Before You Blame the Transmitter

I used to troubleshoot by staring at the display and assuming the sensor was bad. Then I missed a partially blocked impulse line for two days. The transmitter was fine. The impulse line was the problem.

Now I take a thermal image before I pull anything off the line. A 20°F difference between two impulse lines is a clue. It is not the transmitter's fault.

If you're looking at the Topdon vs FLIR thermal camera debate, do not base the decision only on the side-by-side videos. Check three things:

• Can you adjust emissivity? Fixed emissivity is fine for electrical panels, not for process pipes with different paint and insulation.
• What is the spot size at the distance you'll work from? A tiny spot at ten feet can miss the problem.
• Does the software export the image with temperature data? A screenshot without scale is not a report.

We use a FLIR for electrical surveys and a Topdon for quick mechanical scans. Both have a place. Neither fixes a poorly designed installation.

5. Ask 'What's Not Included' Before You Approve the Quote

This is the step most people skip. The quote looked like the cheapest option until the extras arrived: calibration certificate, NIST-traceable data, mating connectors, rush handling. I've learned to ask 'what's NOT included' before 'what's the price.'

The vendor who lists all fees upfront—even if the total looks higher—usually costs less in the end. That's true whether you're ordering an Ashcroft gauge or a replacement encoder. I do not mean a small difference. I mean the $890 redo plus a one-week delay on a $3,200 order because the thread type was not caught before approval.

Before you approve any PO, ask these:

• Is the calibration certificate included?
• Is it NIST-traceable? As of January 2025, our quality system requires it for anything tied to a lot record.
• Are connectors/cables in the quote?
• Is delivery ex-works or delivered to site?
• Are all options in the model number?

6. File the Documentation Before You Need It

After the third rejection in Q1 2024, I created a two-minute paperwork step. Every instrument gets a work order with four things: serial number, calibration date, order number, and the engineer's name. That's it. It is not a complex system. It stops a question at 4:55 PM on a Friday.

Even after choosing the right vendor for the next large order, I kept second-guessing. What if the delivery slipped? What if the calibration data did not arrive? The two weeks until delivery were stressful. That paperwork did not make the wait shorter, but it made the arrival easier to verify.

Common Mistakes I Still Find

A few things I do not see disappearing:

• Assuming a new instrument is already correct. New units fail final calibration too. It's rare, but it happens.
• Using a multimeter on the wrong setting. Reading 24 V in the voltage jack and then reading 0 mA because the lead is still in the voltage jack. I've done both.
• Buying a thermal camera with better resolution but no emissivity adjustment. Resolution is not the same as a report.

That's the list. It's not elegant. It's the difference between a part you install and a part you trust.