Dwyer application note: Dwyer Sensors: Choosing and Verifying the Right Instrument for the Job

There Is No One 'Best' Dwyer Sensor

I work in quality for an instrumentation-focused company, and I review every sensor order before it reaches the floor. That's roughly 200 unique items per year. The first thing I tell our engineers is simple: don't start with 'which Dwyer sensor is best?' Start with 'what am I actually trying to measure?'

A Dwyer submersible level transmitter is the right tool for a storage tank or a lift station. A differential pressure switch is the right tool for monitoring a filter or an airflow alarm. And a pipette aid is the right tool in a lab when you need to move small volumes by hand. They serve different physical problems, and no single brand or model is a universal answer.

Scenario 1: Continuous Liquid Level, Like a Tank or Sump (Dwyer Submersible Level Transmitter)

If you need to know the level of water, wastewater, or a compatible liquid at all times, a Dwyer submersible level transmitter is a practical option. It sits below the liquid surface and measures hydrostatic pressure from the column of liquid above it. The transmitter converts that pressure into a typical industrial signal, usually 4-20 mA.

According to Dwyer's product documentation (dwyer-inst.com, accessed January 2025), submersible level transmitters are designed to be submerged and use a vented cable to reference atmospheric pressure. That venting matters: if the cable is pinched, flooded, or cut, you will get a drifting zero point.

Here's where prevention pays off. Before I order a transmitter, I check three things:

• The pressure range in feet or inches of water, not just the tank height.
• The maximum submersion depth and cable length.
• Whether the liquid is compatible with the wetted materials.

I don't have hard data on how often the wrong range gets ordered, but I can tell you anecdotally that it's one of the top causes of delivery delays in our facility. In Q1 2024, we received a batch of ten transmitters where the specified range was twice the actual tank depth. The vendor said it was 'within industry standard.' We rejected the batch and expedited the correct units. A 20-minute spec review before ordering would have saved about ten days of schedule.

Scenario 2: A Threshold Alarm or Control Signal (Dwyer Differential Pressure Switch)

A submersible level transmitter gives you a continuous number. But sometimes you don't need a number. You need a switch to tell you when a filter is clogged or when airflow has dropped below a limit. That's a different application, and a Dwyer differential pressure switch is usually the more direct answer.

A differential pressure switch compares two pressures and changes its contact state when the difference crosses an adjustable set point. It's common on HVAC filter banks, pump protection, and air flow proving. It is not a replacement for a transmitter, and it is not a gauge. It's a decision point.

The part people oversimplify is deadband. It's tempting to think a switch simply opens or closes at one number. But the reset point can be very different from the set point. If you ignore the deadband, you might have an alarm that won't clear, or one that cycles on every pressure surge.

My advice: bench test the switch with a manometer before you install it. I've seen switches set so close to the normal operating pressure that they alarm every morning when pumps start. A five-minute test in the workshop would have caught it. This is the whole 'prevention over cure' argument—the test is boring, but the retrofit isn't.

Scenario 3: Verifying a Sensor or Loop (Fluke 117 True RMS Multimeter)

Now you're in troubleshooting territory. Before you replace a transmitter or a switch, you need to verify the electrical side of the loop. This is where a true RMS multimeter earns its place.

Why true RMS? In many plants, sensors share panels with variable frequency drives and solenoids. Those loads create distorted voltage waveforms. A basic averaging meter can give you a voltage reading that looks fine but is actually wrong. A true RMS multimeter like the Fluke 117 measures the heating equivalent of the waveform, so you get a number you can trust.

According to Fluke's published specifications for the 117 (fluke.com, accessed January 2025), the LoZ mode is designed to help eliminate ghost voltage readings caused by capacitive coupling. That is not a gimmick. It has saved us from unnecessary sensor replacements more than once.

How to Use the Fluke 117 True RMS Multimeter for Sensor Checks

1. Turn the dial to the auto voltage position. The 117 can detect AC or DC automatically, which saves time when you're checking an unfamiliar terminal strip.
2. If you're near other live cables, use LoZ mode. This loads the circuit and helps ghost voltage from adjacent cables discharge. Without LoZ, a floating wire can read 24 V when it's actually dead.
3. Measure the supply voltage at the sensor terminals. For most Dwyer transmitters, you're looking for a stable DC supply voltage. Record the actual reading, not the label value.
4. For a 0-10 V output sensor, measure across the signal positive and signal common. Compare that to the expected output at the known process condition.
5. For a 4-20 mA loop, understand the Fluke 117's limits. It's an electrician's meter, not a mA loop meter. I prefer a mA clamp meter or a loop calibrator for current checks. Trying to measure current by jumpering across the loop is the fastest way to blow a fuse or short a line.

I wish I had kept a log of every false alarm that traced back to wiring rather than the sensor itself. Based on my experience, it would be a majority. In one case, our technician saw 24 V on a disconnected signal cable and assumed the sensor was live. LoZ showed less than 1 V—ghost voltage. That one check avoided an unnecessary replacement.

How to Know Which Scenario You're In

Ask yourself three questions:

• Do I need a continuous measurement value? That points toward a transmitter.
• Do I need an on/off decision based on a pressure difference? That points toward a differential pressure switch.
• Do I need to verify an existing signal before buying a replacement? That points toward a meter and a wiring check.

And if you're in a lab and the task is manual liquid transfer, don't over-specify the solution. A pipette aid is the right tool for that job. The point isn't to use a brand name wherever possible; it's to match the measurement principle to the physical problem.

This approach works for us because we're a mid-size operation with predictable water and HVAC systems. If you're dealing with aggressive chemicals, high vibration, or hazardous area ratings, your selection rules will be different. Your mileage may vary.

One last thing. The most expensive mistake I see isn't choosing the wrong instrument category. It's skipping the verification step after the instrument arrives. Five minutes of checking before installation beats five days of rework later. That's not a slogan—it's the reason I still have a job. And as always, verify current specifications at the manufacturer's site before you order.