Start with the gas, not the price. Clean, dry gases go to thermal TA. Wet, dusty, aggressive or condensing gases go to vortex VA. Gas and liquid on one skid, or media to 550 °C, go to vane wheel FA. Liquids and test benches go to ultrasonic UA. Then confirm flow range, pipe size and temperature.
Höntzsch builds four measuring principles under one brand, and they are not interchangeable: two meters of the same nominal diameter can work on entirely different physics.
Why the medium decides the principle
Thermal TA measures by heat transfer: a heated element is cooled by the gas, and the resistance change becomes mass flow. Measuring mass directly removes the need for separate pressure and temperature compensation, and there are no moving parts. Höntzsch quotes a turndown of about 1:1000, so one instrument can read a leak-level flow and full compressor output. The limit is the medium — thermal is for clean, dry, pure gases. Condensate, oil aerosol or dust on the element shifts the reading, and that is the usual reason a thermal meter is blamed for drifting.
Vortex VA uses the Kármán vortex street. An obstruction sheds vortices at a frequency proportional to velocity, detected by an ultrasonic field in the sensor head. The reading is virtually unaffected by pressure, temperature, viscosity or gas composition, which is what is needed where the gas changes during the process. No moving parts and corrosion-resistant wetted materials, so condensate and particles do not upset it, and it works from 0.5 m/s while holding accuracy at high velocity. This is the family for wet, dusty and aggressive gases: biogas, sewage and landfill gas, flare and vent stacks, cement and waste-to-energy exhaust. Gases and gas mixtures only — never liquids.
Vane wheel FA is mechanical and the widest in scope. A vane wheel drives a pickup whose speed a proximity switch reads with no braking effect. That speed is virtually independent of the medium’s density, pressure and temperature, so one family covers gases and liquids: 0.15 to 150 m/s in air and gas, media to +550 °C, low pressure loss. Insertion probes start at a 16 mm probe diameter, measuring tubes (FA Di) at a 9.7 mm internal diameter, and a second proximity switch adds bidirectional reading and ± direction.
Ultrasonic UA covers liquids and test benches. UA Liquids measures flow in liquid-filled pipes for HVAC systems, pump verification, boiler tests, leak detection and filter sizing; the ExactSonic P measuring tube is the test-bench version, for engine intake air and turbocharger, valve or filter benches.
Then the range — and watch the units
Once the principle is fixed, the range decides the size. One detail causes more confusion than any other: the units. A vane wheel FA reports actual velocity in m/s; a thermal TA is calibrated in normalised velocity in Nm/s, referred to a defined reference state of temperature and pressure. Gas is billed on normalised volume or on mass, so converting either reading into Nm³/h or kg/h needs the internal pipe diameter and the gas density.
- Thermal TA — 0.08 to 200 Nm/s standard velocity, standard flow from 0.04 Nm³/h, TA10 models beyond 200 Nm/s at about one second response. Insertion from DN25; inline 8 to 53.1 mm inside diameter, 0.04 to 1196 Nm³/h.
- Vortex VA — from 0.5 m/s, accuracy held at high velocity. Insertion from DN80; tubes from a 24 mm inside diameter.
- Vane wheel FA — 0.15 to 150 m/s in air and gas, strongest at very low, bidirectional velocities.
Size to work in the upper part of the range at normal load: a meter specified at the top of its band has nothing left when the plant pushes past it.
Pipe size decides the mechanical design
Insertion probes suit larger pipes and ducts, and portable work. One probe covers a range of nominal diameters, an isolation valve lets it be fitted or removed without shutting the line down, and it can move between measuring points during a survey. Vane wheel probes start at a 16 mm probe diameter, thermal at DN25, vortex at DN80.
Inline measuring tubes suit smaller pipes where the flow path itself is calibrated: 8 to 53.1 mm inside diameter for thermal TA, from 9.7 mm for vane wheel FA Di, from 24 mm for vortex VA.
An insertion probe also reads at a point and assumes a developed profile, so give the straight pipe lengths available on the enquiry.
Temperature, pressure and approvals close it
Temperature often decides between families rather than refining the choice. Vane wheel FA reaches +550 °C and is the only family that handles gas and liquid with one instrument. Thermal TA covers −20 to +240 °C depending on model at pressures to 50 bar, so thermal cannot go on a hot exhaust stack however clean the gas. Vortex VA takes wet, particle-laden duties with explosion protection available.
Settle hazardous areas early, because approvals change the order rather than the price. ATEX and CSA versions exist across the families, vortex suits duties up to SIL 3, and in the latest thermal generation the same instrument can be used in category 3G and 3D — zones 2 and 22 — with M-Bus, without recalibration or a return to the factory. A standard unit with an external barrier added later is not equivalent.
Output and the measuring chain
What the sensor produces is the last decision, and the one most often left off the enquiry. Vane wheel FA and vortex probes output a velocity-tied signal and need a matching evaluation unit or transmitter to give flow rate and total, while thermal TA often has the transducer built in, with two 4–20 mA outputs, media temperature, quantity pulses, M-Bus, Modbus TCP, IO-Link or Wi-Fi.
A probe and its evaluation unit are one measuring chain and should be quoted together, and Ex versions must be ordered as Ex from the outset. Every Höntzsch meter is also calibrated in the manufacturer’s own laboratory before delivery, accredited to DIN EN ISO/IEC 17025 for the flow rate, flow velocity and volume flow of gases.
The four measuring principles at a glance
| Criterion | Vane wheel FA | Vortex VA | Thermal TA | Ultrasonic UA |
|---|---|---|---|---|
| Media | Gas and liquids | Gases and gas mixtures only | Clean, dry, pure gases | Liquids; test benches |
| Range in air/gas | 0.15 to 150 m/s | From 0.5 m/s | 0.08 to 200 Nm/s; from 0.04 Nm³/h | Sized to the application |
| Media temperature | Up to +550 °C | Wet and particle-laden duties | −20 to +240 °C, model dependent | Per application |
| Pipe sizes | Insertion from 16 mm; tube from 9.7 mm ID | Insertion from DN80; tube from 24 mm ID | Insertion from DN25; tube 8–53.1 mm ID | Tube or clamp-on |
| Best for | Gas and liquid, high temp | Wet, dusty, aggressive gas | Compressed-air and gas consumption | Liquid pipes, pump and boiler tests |
The selection errors we see most
Thermal TA on a wet or oily gas — the heated element fouls and the reading drifts. With condensate, oil carry-over or dust, use vortex VA.
Vortex VA on a liquid — VA measures gases and gas mixtures only; a liquid line belongs with the vane wheel or the ultrasonic range.
Comparing m/s with Nm/s — a thermal sheet in normalised units and a vane wheel sheet in actual velocity are not describing the same flow. Convert both to the unit the plant buys gas on.
A single insertion point in a disturbed flow — a probe after an elbow, reducer or damper without enough straight pipe reads the profile at that point, not the flow in the line.
A DN smaller than the chosen principle covers — vortex insertion starts at DN80 and vortex tubes at 24 mm; check the mechanical range before the technical one.
A probe ordered without its evaluation unit, or without Ex — both are order-time decisions, and both are expensive to correct.
Which one to choose
Choose thermal TA when the gas is clean, dry and pure — compressed air, nitrogen, carbon dioxide, argon, helium, natural gas, propane or hydrogen — and the job is consumption measurement, leak detection, compressor monitoring, aeration air, laminar flow in a clean room or isolator, or gas mass-flow proportional measurement on a test bench or combustion control loop. Use the DN25-and-up insertion probe on larger lines and the inline sensor on small ones.
Choose vortex VA when the gas is wet, dusty, aggressive or of varying composition — biogas, sewage gas, landfill gas, flare and vent gas, chemical and petrochemical process gas, cement and waste-to-energy exhaust — and where the duty must hold up to SIL 3 or sit in a hazardous area. Use the insertion probe from DN80, the tube from 24 mm, and the combination probe where velocity and temperature are both wanted from one tapping point.
Choose vane wheel FA when the medium is a gas up to 550 °C, when the line may see a liquid as well as a gas, or when velocities are very low or reversing and ± direction detection is needed. Take the FA Di tube for small pipes where accuracy is critical, and the insertion probe for large ducts, surveys and multiple measuring points.
Choose ultrasonic UA when the duty is a liquid — flow rate in liquid-filled pipes, HVAC systems, pump verification, boiler tests, leak detection or filter sizing — or when the measurement is on a test bench for engine intake air, turbochargers, valves or filters.
What to send for a quote
An enquiry that settles a Höntzsch meter in one pass is short: the gas and its composition, the flow range in the unit you work in, the pipe inside diameter and material, the media temperature and pressure, the hazardous area classification, the output required, and whether the measurement is fixed or portable. Add a photograph of the tapping point and, when replacing an instrument, the nameplate — cross-referencing an installed or obsolete model from its nameplate is routine work for us.
- Browse the Höntzsch brand page and the fuller brand directory for the manufacturers we cover.
- See the instruments and measurement range and the complete product catalogue for the categories we supply.
- Read more selection guides and technical notes.
- Or send the duty straight to the enquiry form.
Common vane-wheel and thermal meters are held in stock for fast shipment; less frequent types are sourced through the manufacturer and established distribution channels, with procurement documentation. Every unit we supply is original and new.