Flow Meters & Gas Detection

A flow meter measures how much fluid moves past a point — either the velocity (which, with the pipe area, gives volumetric flow) or the volume directly. There is no single "flow meter"; there are several physical principles, each suited to different fluids: magnetic meters use Faraday's law and a magnetic field to read the velocity of a conductive liquid; turbine & impeller meters spin a rotor whose speed tracks flow; ultrasonic meters use transit time or doppler sound waves traveling with and against the flow; vortex meters count eddies shed off a bluff body; positive-displacement meters (gear, oval-gear, nutating-disc) trap and count discrete volumes; differential-pressure elements (orifice, Venturi, averaging pitot, cone, wedge) infer flow from a pressure drop; radar, velocity and ultrasonic based meters measure partially filled & open channels; and variable-area meters (rotameters) float an indicator against a tapered tube. The right choice depends on the fluid, the accuracy you need, and the pipe — which is the next FAQ.

Start from the fluid and the pipe, not the brand:

• Magnetic (ModMAG M-Series) — conductive liquids: water, liquid food ingredients, chemicals, wastewater, slurries. No moving parts, no pressure drop, no straight-run-hungry. Will not work on hydrocarbons or pure water (see conductivity, below).
• Turbine (Blancett, Cox) — clean, low-viscosity liquids and gases where you need high accuracy and custody-transfer-grade repeatability.
• Clamp-on ultrasonic (Dynasonics TFX) — retrofit on existing pipe with no shutdown, no penetration; water, glycol, hydronic, BTU energy.
• Vortex (VN2000) — steam, compressed air, and gas; rugged, no moving parts.
• Positive displacement / oval gear (Blancett B1750, IOG) — viscous fluids, fuels, lubricants, chemical injection, where viscosity changes would defeat an inferential meter.
• Differential-pressure elements (Preso) — very large pipes, high temperature/pressure, or where a primary element on a big line makes sense.
• Variable-area (Hedland) — local visual indication and flow switching on hydraulic, water, and gas lines.
• Impeller / paddle-wheel (Data Industrial) — water, insertion and hot-tap into large pipes economically.
• Water sub-metering (RecordALL Disc, Turbine, and Compound; Dynasonics clamp-on) — potable-water consumption metering and tenant or process sub-billing.

Tell us the fluid, pipe size, accuracy target, and whether the line can be shut down, and the field narrows fast.

A magnetic flow meter measures the voltage a conductive fluid generates as it moves through a magnetic field, so the fluid has to conduct. The ModMAG M-Series needs a minimum of about 5 µS/cm — and about 20 µS/cm for demineralized water. Below that threshold the meter cannot read reliably. That rules magnetic meters out for hydrocarbons, deionized/ultrapure water, and other non-conductive fluids — for those, use a turbine, ultrasonic, vortex, or positive-displacement meter instead. For ordinary water, wastewater, and most chemicals, conductivity is not a concern.

Both time ultrasonic pulses to measure flow; they differ in how they couple to the fluid. A clamp-on meter (Dynasonics TFX) straps transducers onto the outside of an existing pipe — no cutting, no penetration, no process shutdown, and it can be moved. That makes it the tool for retrofits, energy audits, and verification. Its accuracy depends on knowing the pipe wall and on a full pipe. An inline meter (E-Series G2, U500w) is a spool piece plumbed into the run, with wetted transit-time sensors — for lower price point ultrasonic measurement.

Accuracy spans a wide band by technology: Cox precision turbine meters reach ±0.1%; magnetic meters run ±0.2–0.3%; oval-gear and positive-displacement ±0.5%; vortex around ±0.7–1.0%; clamp-on ultrasonic and impeller meters ±0.5–1.5%; variable-area rotameters ±2% of full scale. But the published number is only achievable if the installation cooperates. The big accuracy killers are: not enough straight pipe run (see below), a partially full pipe on a meter that needs a full one, viscosity outside the calibrated range, air or gas entrainment, and operating near the bottom of the meter's range. Specify the meter for the actual worst-case conditions, not the nameplate ideal, and the rated accuracy holds up.

Inferential meters read fluid velocity, and velocity is distorted by anything that swirls or skews the flow profile — elbows, valves, pumps, reducers. Most need straight run upstream and downstream to let the profile settle: turbine and orifice meters are the hungriest (commonly 10–20 pipe diameters upstream and around 5 downstream); vortex and ultrasonic meters need a moderate amount; magnetic, positive-displacement, oval-gear, and cone-type DP elements need little or none because they either don't care about profile or actively condition the flow. If your piping can't give a turbine meter its straight run, that's a reason to pick a different technology rather than accept the error. In some cases (Cox) flow straighteners are available where space is tight.

Often, yes — there are three live-install paths. Clamp-on ultrasonic meters mount entirely on the pipe exterior, so there's nothing to break into. Hot-tap insertion meters (impeller Series 225/226, Vortex insertion, averaging-pitot hot-tap elements) go in through a tapping valve under pressure and once originally installed, can be removed the same way for service. Insertion meters generally need only a single small tap rather than a full spool-piece cutout. A full-bore spool meter — magnetic, inline ultrasonic, turbine — does require the line open. If continuous operation is non-negotiable, say so up front and we'll steer the selection toward a clamp-on or hot-tap design.

Most precision meters are wet-calibrated at the factory against a NIST-traceable standard — Blancett turbine meters ship with a water calibration and a unique K-factor; Cox meters carry a 10-point MIL-PRF-7024 calibration (others available); positive-displacement meters get an oil calibration; custom-fluid and multi-viscosity calibrations are available when the process fluid differs from the calibration fluid. In service, you don't always have to pull the meter to check it: magnetic meters have an in-line field verification tool that confirms the amplifier, coils, electrodes, and I/O are still in spec without removing the meter, and a meter's K-factor can be re-scaled against a reference. For billing or custody transfer, recalibrate on the interval your standard or regulator requires.

Turndown — a/k/a rangeability — is the ratio between the highest and lowest flow a meter measures at its rated accuracy. A 10:1 turndown meter sized for 100 gpm holds accuracy down to 10 gpm; below that it degrades or reads zero. It matters because real processes rarely run at one steady rate — they ramp, batch, and idle. If you size a meter only for peak flow and the process spends half its time at low flow, you're measuring that half badly. Turndown varies a lot by technology: turbine meters run about 10:1, oval-gear meters reach into the hundreds-to-one on viscous fluids, and DP elements are limited by the square-root relationship between flow and pressure drop. Tell us the full flow range and we size for it.

A standard closed-pipe meter assumes a full pipe; sewers, storm drains, flumes, and channels are not full, so they need open-channel instruments. There are two approaches. With a primary device — a flume or weir of known hydraulics — a non-contact level sensor (Dynasonics IS-4000) measures depth and a built-in table converts it to flow; this is deterministic and well-suited where a flume already exists. Without a primary device, an area-velocity meter measures both the flow velocity and the level directly and multiplies them — radar Doppler (Telog Raven-Eye 2), submerged ultrasonic Doppler (Telog Beluga, Dynasonics IS-6000) — which is the retrofit answer for existing pipes and channels and avoids building a structure. These pair with recorders and cellular RTUs for CSO/SSO event capture and inflow-and-infiltration studies.

Gases and steam need meters that tolerate compressibility and don't rely on a conductive or liquid medium. Vortex meters (VN2000) are the workhorse for saturated and superheated steam, compressed air, and process gas — no moving parts, and with an integral temperature element they output mass flow and BTU/energy. Differential-pressure elements (Preso averaging pitot, Venturi, cone) suit large gas and steam lines and high temperatures. Gas-specific turbine meters (Blancett Gas QuikSert) handle clean gas where high accuracy is needed. For steam and gas, specifying mass flow rather than volumetric flow — and accounting for pressure and temperature — is what makes the number meaningful, so tell us the line pressure and temperature along with the flow range.

A turbine meter is calibrated on a fluid of a particular viscosity, usually water or a light oil. Run a thicker fluid through it and the rotor sees more drag, spins slower than the calibration predicts, and the meter under-reads — especially toward the low end of its range, where viscous effects dominate. The fix is not a bigger meter; it's viscosity-aware compensation. Cox precision turbine meters are characterized across a range of viscosities (the Universal Viscosity Curve), and a flow computer applies that curve to hold accuracy as viscosity shifts with temperature. The remedies, in order: pair the meter with a UVC-aware flow computer such as the FC-5000; have the meter re-characterized at the actual process viscosity and temperature; or, if it's chronically below its linear range, drop to a smaller-bore model. For genuinely viscous fluids, a positive-displacement or oval-gear meter avoids the problem entirely.

Match the output to whatever is going to receive it. 4–20 mA analog is the universal industrial choice — long cable runs, good noise immunity, one signal per loop, reads straight into a PLC. HART superimposes digital data on that same 4–20 mA loop — use it when the host or asset-management system is HART-capable and you want remote configuration and diagnostics over the existing pair. Pulse / frequency output is the right one for totalizing and batching, since each pulse is a discrete volume. An open-collector (transistor) output is the simple, low-cost digital version of that — scaled pulses or on/off status into a counter or PLC digital input, with no relay to wear out. Modbus (RTU over RS-485, or TCP over Ethernet) is the digital workhorse for SCADA and multi-meter networks — many meters daisy-chain on one bus. BACnet (MS/TP or IP) is the choice when the data goes to a building-automation system, common for campus BTU and energy metering. EtherNet/IP, Profibus DP, and M-Bus cover plant networks built on those standards. For cloud-hosted remote reads, AquaCUE cellular endpoints upload meter data with no fixed network to build. Tell us the host system and we'll set the output card accordingly — the ModMAG M2000, for example, takes plug-in daughterboards for Modbus RTU/TCP, BACnet MS/TP and IP, EtherNet/IP, HART, Profibus DP, and M-Bus.

Cost scales with line size, accuracy class, materials (a stainless or exotic-alloy wetted path costs more than bronze), hazardous-area certification, and electronics (a local indicator is lower cost vs a multi-protocol transmitter with datalogging). A small impeller meter for an irrigation application and a large custody-transfer turbine meter in a hazardous area are an order of magnitude apart. For the lines we sell on our webstore — RecordALL and the nationwide-distribution meters — unit prices are published there, so the number is in front of you; engineered, configured, and hazardous-area builds are by quote only.

It depends on whether the meter has moving parts. No-moving-part meters — magnetic, vortex, ultrasonic, DP elements — have very little to wear out and run for many years with little more than occasional electrode or sensor inspection; magnetic meters can be verified in place without removal. Meters with moving parts — turbine, positive-displacement, oval-gear, impeller — last well too, but their bearings and rotors are wear items, and life depends heavily on fluid cleanliness; grit and debris are the enemy, which is why upstream filtration (often a 200-mesh strainer) is recommended. Across all types, the practical maintenance items are keeping the fluid clean, keeping electronics in their ambient range, and recalibrating on your quality interval.

Gas detection monitors (ATi, GasSens) sense a target gas and alarm before it reaches a dangerous concentration. Most use electrochemical sensors for toxic gases (chlorine, H₂S, ozone, ammonia) or catalytic/infrared (NDIR) sensors for combustibles and CO₂ — NDIR has the advantage that it can't be poisoned and works in oxygen-free atmospheres. The fixed-vs-portable split is about the job: fixed transmitters mount permanently to watch a defined space continuously — a pump room, a chlorine building, a semiconductor tool — and tie into alarms, ventilation, and shutdown logic, with multi-channel controllers managing up to dozens of sensors. Portable detectors (D16 PortaSens III) go with a person for spot checks, confined-space entry, and leak hunting. Many facilities need both: fixed monitoring for continuous protection, portable units for entry surveys and maintenance.