Ultrasonic & Clamp-On Flow Meters for Water, Thermal and Compressed Air

The range below spans the two families we specify most: the EES ultrasonic series — inline and clamp-on water meters, BTU meters and a handheld survey meter — and the Keyence FD-R and FD-G clamp-on series for liquid and compressed air. Both integrate with the wider water submetering range, BTU and thermal energy meters and compressed air flow meters.

Clamp-on vs. inline vs. insertion

• Clamp-on (external transducers). Transducers strap to the outside of the pipe and couple through the wall. Nothing is cut, nothing is drained, and the system stays in service during installation — the single strongest argument for ultrasonic on a retrofit. There is no pressure drop, no wetted part to corrode, and the meter can be relocated later if the metering plan changes. The trade-off is that accuracy depends on installation quality: pipe wall condition, couplant, and correct entry of pipe material, wall thickness and liner.
• Inline (spool-piece). The transducers are factory-mounted in a calibrated body, so geometry is known and the meter is wet-calibrated as a unit. This is the tightest-accuracy option and the right choice for tenant billing on a new line, a planned replacement, or any branch that can be broken during a scheduled shutdown.
• Insertion. A probe enters through a hot tap or an existing port. It is the middle ground for large-diameter mains where a spool piece is impractical and the wall is too thick or too irregular for reliable clamp-on coupling.

The retrofit argument is usually decisive: an occupied building rarely tolerates draining a chilled water riser or shutting a plant air header for a meter. Background reading: clamp-on vs. inline flow meters.

Transit-time vs. Doppler

Transit-time meters send a pulse diagonally upstream and downstream and measure the difference in travel time. They need a clean, acoustically transmissive fluid — potable water, chilled and condenser water, glycol mixtures, condensate, most process liquids — and they are what almost every building application should use. Nearly all the meters on this page are transit-time.

Doppler meters rely on reflectors in the fluid: bubbles, solids or slurry. They suit dirty, aerated or heavily loaded streams — wastewater, sludge, some cooling tower blowdown — and they fail in genuinely clean liquid because there is nothing to reflect from. If your line swings between clean and aerated (a partially drained loop, for example), say so at the quoting stage; the choice of technology, not the choice of brand, is what will decide whether the reading is trustworthy.

Pipe size, material and straight run

• Size range. The EES clamp-on and inline units cover roughly 1" to 48"; the Keyence FD-R series covers common building line sizes from around 1" to 8", and the FD-G air series covers 1" to 8" plant air headers. Below about 1", inline is usually the only sensible option.
• Material matters for clamp-on. Steel, stainless, copper, ductile iron, PVC and HDPE all transmit well. Cast iron with heavy graphite flake, badly corroded or scaled pipe, concrete-lined pipe, and any pipe with an air gap behind the liner are marginal to unusable. Wall thickness and liner thickness must be measured, not assumed — an incorrect entry biases every reading.
• Straight run. Plan on roughly 10 pipe diameters upstream and 5 downstream as a working minimum; more after a pump, a partially open valve or two out-of-plane elbows. Straight run exists to let the velocity profile settle. Metering into a disturbed, swirling profile is the most common cause of a meter that reads consistently — and consistently wrong — because the meter infers volumetric flow from velocity across the acoustic path.
• The line must be full. A partially full pipe or entrained air near a high point will drop out the signal. Mount on a horizontal run at the 3 and 9 o'clock positions, or on a vertical run with upward flow.

Sizing and placement walkthroughs: meter selection help for water and meter selection help for BTU and thermal metering.

Portable & Handheld Ultrasonic Flow Meters

Not every measurement needs a permanent meter. A portable clamp-on unit answers a question and moves on, which makes it the cheapest way to survey a building before committing to a metering scope.

• Survey work. The EES-501 portable handheld ultrasonic water meter clamps onto a riser, logs for a day or a week, and tells you the actual flow range on that line — which is what a permanent meter should be sized to, rather than the pipe diameter.
• Temporary verification. Use a portable meter as an independent check against an installed meter or a BAS point before a billing dispute becomes an argument, and as the reference reading during commissioning and M&V.
• Leak investigation. Overnight or weekend flow on a line with no legitimate demand is a leak, and a portable meter finds which branch it is on without instrumenting the whole building. The same method works on compressed air with the VPFlowScope probe start kit, whose insertion probe drops into an existing port to quantify off-shift air load.

Permanently installed meters earn their place when the data feeds something recurring — tenant invoices, a BTU energy balance, a leak alert, an ENERGY STAR submission. Portables answer one-off questions. Most portfolios need both.

Accuracy classes and tenant billing

Typical figures: inline ultrasonic water and BTU meters land around ±1% of reading over their calibrated range, wet-calibrated with a certificate; clamp-on liquid meters land nearer ±1–2% of reading when installation conditions are good and degrade beyond that when straight run, coupling or wall data are compromised; clamp-on compressed air meters sit around ±3% of reading. Note that percent of reading and percent of full scale are not comparable — a full-scale spec looks tight at design flow and is useless at 10% of it, which is where an oversized meter spends its night.

For tenant billing, specify an inline, wet-calibrated meter with a traceable certificate and check what your jurisdiction requires for meter approval; several states restrict which meter types may be used to raise a tenant charge. Clamp-on is excellent for allocation, diagnostics, load profiling and M&V, and it is the right tool where cutting the pipe is not on the table — but where the reading becomes an invoice line, put the calibration certificate behind it. See meter requirements for tenant billing.

Outputs and integration

• Modbus RTU (RS-485) and Modbus TCP — the default for BTU meters, since you want flow, supply and return temperature and calculated energy as separate registers, not one pulse train.
• BACnet MS/TP and BACnet/IP — where the meter is being handed to a building automation system and the mechanical contractor owns the point list.
• Pulse output — the simplest and cheapest path into a data logger or an existing pulse-counting input. Fix the pulse weight (gallons or cubic feet per pulse) in writing before install; a mismatch between the meter's scaling and the logger's is the most common commissioning fault we see.
• 4–20 mA analog — an instantaneous rate signal for control, trending or process interlocks. It carries rate, not total, so pair it with a pulse or register read if you need consumption.

Where meters are scattered or the building has no usable network, add a gateway from connectivity or a protocol controller from integration components. For the Keyence clamp-on approach specifically, see Keyence clamp-on water flow metering and Keyence clamp-on compressed air flow metering.