Why does it not need pressure and temperature compensation?

Because the heat the gas carries away depends on its mass flow, not on its volume: when the pressure rises, the same volume holds more mass, and the meter sees it directly. So it gives mass flow — in kg/h, or as standard volume in Nm³/h — with no separate pressure or temperature transmitters. Only make sure that the reference conditions set in the meter are the ones your reports use.

Can one meter measure different gases?

Each meter is set up for a specific gas or a known mixture, because every gas carries heat away differently. Some meters let you choose among several gases or apply a correction for another one. With a mixture whose composition changes, such as biogas, the reading shifts with it — tell us the composition and how much it varies, and we will check what accuracy to expect.

FLOW MEASUREMENT TECHNOLOGIES

Thermal mass flowmeters: the mass flow of gases, measured directly

A thermal mass flowmeter has two small sensors in the gas flow: one measures the temperature of the gas, the other is heated above it. The flowing gas carries heat away from the heated sensor, and the more gas flows past, the more heat it removes; from the power needed to keep that sensor hot, the meter works out the mass flow directly — with no pressure or temperature compensation. It measures compressed air, nitrogen, natural gas and other gases of known composition, even at very low velocities, either inline or with a probe inserted into large pipes, ducts and stacks. Thermal mass flowmeters are not yet in our online catalogue; we supply them from Kurz Instruments and VP Instruments.

The manufacturers we work with

How a thermal mass flowmeter works

In the gas flow sit two small temperature sensors, usually platinum resistance sensors. One measures the temperature of the gas; the other is heated and kept at a fixed temperature difference above it. The gas flowing past carries heat away from the heated sensor, and the electronics supply exactly the power needed to keep that difference constant. Some meters work the other way round: they heat the sensor with constant power and measure how far its temperature drops.

The heat the gas carries away depends on the mass of gas that flows past the sensor every second — the mass flow — not on its volume. Compressed to twice the pressure, the same volume of gas holds twice the mass and removes more heat. So the meter measures mass flow directly, with no separate pressure and temperature measurements. It shows it in kg/h, or as standard volume — normal cubic metres per hour (Nm³/h), which is mass flow expressed as the volume the gas would take up at agreed reference conditions.

The relationship is not a straight line: the heat loss rises steeply at low flows and more gently at high ones, and even in still gas the heated sensor loses some heat. That is why a thermal meter is very sensitive at low velocities, and why it covers a wide range — typically 100:1 or more. Each meter carries its own curve, set up at the factory for a specific gas, because the heat a gas carries away also depends on its properties: air, nitrogen, natural gas and carbon dioxide each give a different curve.

There are two forms. In an inline meter, the sensors sit in a measuring section of their own, fitted into the line — the usual choice for small and medium pipe sizes, as in compressed air distribution. An insertion probe goes in through the wall of a large pipe, duct or stack and measures the velocity at its tip; from that, the known cross-section and the flow profile, the meter works out the total flow. In large ducts, where the flow is uneven across the section, several sensors — on one probe or on several — average it.

The gas flow
The reference sensor, which measures the temperature of the gas
The heated sensor, kept at a fixed difference above the gas temperature
The heat the gas carries away from the heated sensor (orange)
The heating power rises with the mass flow — steeply at low flows, more gently at high ones

Where a thermal mass flowmeter fits — and where it does not

A strong choice for

Compressed air: consumption per line, department or machine, and leaks — the flow that remains when production has stopped shows them. Compressed air is one of the most expensive utilities in a plant, and measuring it shows where it goes
Gases of known composition — nitrogen, oxygen, argon, carbon dioxide, natural gas and other process gases — with the meter set up for the actual gas
Mass flow, or standard volume, with no separate pressure and temperature measurements: what the process actually consumes, whatever the line pressure
Very low velocities and a wide range — typically 100:1 or more — so that both the small flows, leaks included, and the peaks are measured
Large pipes, ducts and stacks: an insertion probe costs about the same whatever the pipe size, and measures combustion air, flue gas and ventilation air — the limits on temperature and dust depend on the model
Little pressure loss and no moving parts: an insertion probe barely obstructs the flow, and nothing wears

Think twice when

The fluid is a liquid or steam: the method is for gases — for liquids, electromagnetic, Coriolis and other flowmeters are the choice, and for steam a vortex or differential pressure flowmeter
The composition of the gas varies widely or unpredictably: the heat a gas carries away depends on what it is made of, so the reading shifts unless the meter can be set for the actual mixture
The gas carries droplets, or its moisture condenses on the sensor: each drop evaporates on the heated sensor, takes extra heat away and shows up as a spike in the flow
Dust, oil mist or other deposits build up on the sensor: they insulate it, less heat is lost, and the reading drifts low — unless the sensor is cleaned
The flow pulsates strongly or can reverse, as right after a reciprocating compressor: many thermal meters cannot tell the direction of flow, and read reverse flow as forward flow
You need custody-transfer accuracy, for example for billing natural gas: there, ultrasonic or Coriolis meters with the required approvals are the usual choice

How it compares with the other technologies

Typical accuracy of each flow measurement technology, ± % of the reading, on a logarithmic scale. The further left, the more accurate. The technology of this article is highlighted.

Coriolis±0.1–0.2%
Electromagnetic±0.2–0.5%
Ultrasonic±0.5–2%
Differential pressure±0.5–2%
Variable area±1.6–4% *
Vortex±0.75–2%
Thermal mass (the technology of this article)±1–3%
Positive displacement±0.2–0.5%
Turbine±0.5–1%

Typical ranges for liquids (thermal mass: gases), ± % of the reading; for variable area, the accuracy class (*). The accuracy of a specific instrument depends on the model, the size and the installation — we confirm it for your application.

All technologies

Installing a thermal mass flowmeter

A thermal mass flowmeter measures the flow at its sensor — and an insertion probe at a single point in the pipe — so it needs a developed, undisturbed flow profile. Typically that means 10–20 D of straight pipe before the meter and 5 D after it (D = nominal diameter); after two bends in different planes, a valve or a reducer, the run before the meter has to be longer still, and a flow conditioner can shorten it. Pressure regulators and control valves belong after the meter. The tip of an insertion probe goes to the depth the manufacturer gives — usually the centre of the pipe — with the sensor turned to the flow, as the mark on the probe shows.

From the velocity at the sensor, the meter works out the total flow through the cross-section, so it needs the exact inner diameter of the pipe, or the dimensions of the duct. The flow goes with the square of the diameter: a 1% error in the diameter gives about 2% error in the flow. And the gas must reach the sensor dry and clean. Then:

A straight run before and afterTypically 10–20 D of straight pipe before the meter and 5 D after it. Pressure regulators and control valves go after the meter, beyond the straight run that follows it.

Large ducts and stacksThe probe goes in through a nozzle in the wall, to the depth the manufacturer gives. In a large duct or stack the flow is rarely even across the section, so several sensors — along one probe that crosses it, or on several probes — average the profile.

Compressed air: after the dryer and the filterInstall the meter where the air is dry and clean: after the dryer and the filter — usually also after the air receiver, which damps the pulsations — and not straight after the compressor, where the air is hot, wet and pulsating. Drain the condensate before the meter, and avoid low points where it collects.

Fitting a probe under pressureMany insertion probes go in through a ball valve and a compression fitting, so they can be fitted and withdrawn without shutting the line down. A safety restraint stops the line pressure from pushing the probe out while the fitting is loosened.

Put the meter where the gas is dry and clean, with the condensate drains before it working, and inspect and clean the sensor at intervals where the gas carries dust or oil.
Fit or remove an insertion probe with the line depressurised, or through a ball valve with the safety restraint in place — never loosen the fitting of a probe under pressure without it.
Set the gas, the inner diameter and the reference conditions in the meter: standard volume at 0 °C and at 20 °C differs by about 7%, so use the same reference as your bills and energy reports.
At start-up, check that the meter reads zero with the line isolated and the gas still.

Thermal mass flowmeters by industry

Power & EnergyCombustion air and natural gas to boilers and burners, for the air-to-fuel ratio and the efficiency; and flue gas flow in the stack, which, together with the gas analysers, gives the mass of each pollutant emitted.

Water & WastewaterAir from the blowers to the aeration tanks, measured per tank or per grid to control the aeration and the energy of the blowers; and biogas from the digesters to the engines, boilers or flare — where its changing composition and moisture have to be taken into account.

Food & BeverageCompressed air, nitrogen and carbon dioxide for packaging, carbonation and inerting, measured at each user — and leaks in the compressed air network, found by measuring when the lines have stopped.

ChemicalNitrogen for blanketing and purging, and process gases of known composition fed to reactors — measured as mass, whatever the pressure in the line.

Mining, Minerals & MetalsCompressed air networks in quarries and processing plants, combustion air and natural gas to kilns and furnaces, and the gas flow in large ducts and stacks.

Thermal mass flowmeters from Kurz and VP Instruments

Thermal mass flowmeters are not yet in our online catalogue. We supply them from two manufacturers of thermal mass flowmeters: Kurz Instruments, for combustion air, flue gas, biogas and process gases — including large ducts and stacks — and VP Instruments, for compressed air and technical gases, with energy-monitoring systems that show where a plant’s compressed air really goes. Tell us the application: the gas and its composition, the flow range, the pressure and temperature, and the pipe size or duct dimensions — and our engineers will suggest the right meter, with price and delivery time.

Kurz InstrumentsThermal mass flow meters for air and gases Learn more

VP InstrumentsCompressed air and gas flow monitoring Learn more

Thermal mass flowmeters: frequently asked questions

Our comparison table gives ±1 to ±3% of reading as a typical range, for gases. The accuracy depends above all on whether the gas is the one the meter was set up for, and — with insertion probes — on the flow profile and on how exactly the pipe diameter was entered. Some manufacturers state the accuracy as a percentage of reading plus a part of the full scale, so at the low end of the range the error, as a percentage, is larger. We confirm the expected accuracy for your application before you order.

An inline meter, with the sensors in a measuring section of its own, suits small and medium pipe sizes, as in compressed air distribution, and it is the easier way to the accuracy the manufacturer states. An insertion probe suits large pipes, ducts and stacks: it costs about the same whatever the size, and many can be fitted through a ball valve without shutting the line down. In large ducts with an uneven flow, several sensors average the profile.

Thermal mass flowmeters are not yet in our online catalogue. We supply them from Kurz Instruments — for combustion air, flue gas, biogas and process gases, including large ducts and stacks — and from VP Instruments, for compressed air and technical gases. Send us the gas, the flow range, the pressure, the temperature and the pipe size or duct dimensions, and we will propose the right meter, with price and delivery time.

Ask about thermal mass flow measurement

Tell us the gas and its composition, the flow range, the pressure and temperature, the pipe size or duct dimensions, and whether you need mass flow, standard volume or consumption per user — and our engineers will suggest the right meter, with price and delivery time.