Dwyer application note: Dwyer Pitot Tubes & Flow Sensors: 7 Questions From 15 Years of Emergency Orders

What This FAQ Covers

When I first started as an applications specialist at an instrumentation distributor, I assumed rush orders were simple: take the part number, check stock, ship it. Then I sent a customer the right Dwyer pitot tube with the wrong mounting flange — and their commissioning schedule went sideways. Three mis-specified orders later, I learned to ask questions before I promised anything.

Since then, I've triaged roughly 200+ urgent orders for measurement equipment. Some are lab emergencies — multichannel pipettes for a research team, digital micrometer sets for a QC line down in Detroit. But the majority are flow-related: Dwyer pitot tubes, flow sensors, indicating flow meters. So if you're in a hurry right now — a TAB report's due, a furnace won't certify, a dead sensor is holding up a crew — here are the seven questions that actually matter, answered the way I'd answer them on a phone call.

1. What's the actual difference between a pitot tube and an indicating flow meter?

A pitot tube is basically a probe that measures velocity pressure at a single point in a duct or pipe. It doesn't show flow by itself. You pair it with a gauge like the Magnehelic, or a transmitter, and then convert pressure to velocity and flow. The math is simple — velocity equals the square root of two times dynamic pressure divided by air density — but it's still math, and it assumes the flow profile is clean where you're measuring.

An indicating flow meter, on the other hand, is self-contained. Dwyer's Rate-Master and Visi-Float series use a float in a tapered glass tube, and the scale on the tube reads directly in CFM or GPM. No secondary instrument, no calculation. Look at it, and you know the flow.

They solve different problems. Pitot tube plus gauge is usually the right call for air in ducts when you want low pressure drop and don't mind a little calculation. An indicating flow meter wins for clean, smaller lines where you want a direct visual reading. Neither one belongs on dirty gas, steam, or liquid service — and yes, I've seen pitot tubes ordered for liquid lines before. Wrong tool.

2. How do I know which Dwyer flow sensor I need?

This is the question I wish more people would ask before ordering. The selection process isn't complicated, but it does require five answers:

• What's the medium — air, water, chemical, steam?
• What's the expected flow rate or velocity range?
• What's the duct or pipe size?
• What are the process temperature and pressure?
• What output do you need — a visual indicator, a 4-20 mA signal, or a switch?

If you can answer those five, you're most of the way there. The rest is mechanical fit: duct size determines pitot tube insertion length, line size determines the indicating flow meter connections, and material compatibility rules out brass in corrosive service. In my experience, about 70% of wrong orders trace back to people skipping the medium and range questions.

3. How accurate are Dwyer indicating flow meters, really?

Honestly? For what they are, the specs are solid. Dwyer publishes around ±2% of full scale for the better Rate-Master models, and a similar figure for the Magnehelic. Some of the budget indicating meters run looser, maybe ±4-5%. That's not laboratory precision — a Coriolis meter or a magnetic flow meter crushes those numbers — but those cost 10 to 20 times more. (Should mention: I haven't personally verified every datasheet claim, but in 15 years I've never caught Dwyer publishing a number they couldn't back up. The FTC's advertising substantiation rules keep manufacturers honest, and it shows.)

But here's the thing that matters more than any datasheet: installation error swamps instrument error. A ±2% meter installed badly is a 20% reading. The biggest accuracy upgrade you can make isn't buying a better meter — it's installing the one you have correctly. That's just reality.

4. Why does my Dwyer flow reading look wrong?

When a client sends me a "this thing is garbage" email, the culprit is almost always one of these:

• Installation too close to an elbow, transition, or damper — short straight run on the upstream side.
• Pitot tube not aligned with the flow direction. I've seen one mounted 90° off. The impact port was facing sideways. That's not even a measurement, that's a guess.
• Blocked pressure ports on the pitot tube, or water in the lines between the probe and the gauge.
• Wrong range. If your Magnehelic is 0-10" WC and your actual velocity pressure is 0.4" WC, the reading sits at the bottom and looks like nothing's happening.

Back in February 2024, a plant in Wisconsin called about a brand-new Dwyer flow sensor that read 40% low. We went through the checklist over the phone. The install crew had mounted the probe with the alignment arrow pointing up, treating it like a "this side up" sticker instead of an alignment indicator. Once oriented properly, the reading was right on. So glad it was a simple fix — the alternative was re-tapping the duct, which would have shut down their process for most of a day.

5. I need a flow meter NOW. How fast can this actually happen?

This is my territory. The short answer: if it's a standard catalog item — Series 160 pitot tube, Magnehelic gauge, common Rate-Master size — and it's sitting in a distributor's stock within courier range, you can have it today or tomorrow morning. Dwyer's North American distribution network is genuinely a competitive advantage. People get used to waiting weeks on offshore orders; they forget that domestic stock exists.

To give you a concrete example: in March 2024, a TAB contractor called me at 2 PM on a Thursday. Their pitot tube kit had been stolen out of a truck the night before, and they had a hospital airflow certification starting at 7 AM Friday. The normal cost of a Series 160 pitot tube plus Magnehelic is maybe $250-$300. We found one in stock 200 miles away, sent a courier to meet halfway, and added about $80 in freight. The contractor lost a couple of hours of prep time, not two days. Their alternative was telling a hospital accreditation team to reschedule — that would have been ugly.

Part of me still bristles at rush fees. Another part has watched the warehouse crew drop everything to make a same-day delivery happen, and I get why the premium exists. I just don't have to like it. For genuinely tiny replacement parts — O-rings, small valves — USPS is fine. As of January 2025, a one-ounce First-Class letter runs $0.73 per USPS. But when it's a $300 pitot tube and the client's schedule is measured in hours, I don't gamble on 2-3 day shipping. Pay the rush fee. It's cheaper than the delay. At least, that's been my experience in deadline-critical work.

6. How do I clean and maintain a pitot tube?

Pitot tubes are low-maintenance, but they're not no-maintenance. The pressure ports are tiny, and anything in the air stream can plug them: dust, fiber, moisture, insects. I once pulled a pitot tube out of a duct that had a spider living in it, perfectly content. The reading had basically become a barometer.

Here's my practical schedule:

• Dirty or industrial air: inspect every 3 months.
• Normal HVAC: once a year is enough.
• Condensing environments: check monthly until you know what you're dealing with.

Cleaning is straightforward. Pull the probe, inspect the ports, blow them out with compressed air, and wipe down the shaft. If there's an oily film, use isopropyl alcohol and let it dry completely before reinstalling. Also check the pressure lines and the gauge side — a water trap is often necessary and equally often forgotten. (And I get cleaning questions about all kinds of instruments, by the way. The number of people searching "how to clean an Eppendorf pipette" is genuinely high. Same principle everywhere: follow the manual, don't force anything, and never soak something that wasn't meant to be soaked.)

7. What's the question nobody asks? (But everybody should)

Not "what does it cost?" The question is: "does my duct have enough straight run for the reading to mean anything?"

For a single-point pitot tube, industry practice suggests at least 7-10 duct diameters of straight, unobstructed run upstream, and a couple of diameters downstream. Most real-world ducts don't have that. There's an elbow, a transition, a damper, a filter bank — everything that messes up the velocity profile.

If you're in that situation, you've got options: do a multi-point traverse (the ASHRAE 111 approach), switch to an averaging flow sensor, or pick an instrument more tolerant of distorted profiles. Or accept the reading as an approximation and document the limitation. What I'd really try to do is design the installation with straight run in mind before the duct gets built — moving a probe after everything's welded and wrapped is a lot more expensive.

So that's the advice from someone who's untangled a lot of urgent orders. The right Dwyer product, installed thoughtfully, is as reliable as this category gets. Skip the straight run, skip the setup questions, and no meter in the world will save you.