Thermal mass flow

There are three main categories for thermal mass flow sensors:

1. Bypass Type Technology
2. Constant Temperature Anenometry as seen within our CS Instruments range.
3. “Chip” type sensors that have been micro-machined (CMOS) or grown in a deposition process much like a silicon wafer (MEMS). (For example, please see our Alicat OEM Mass Flow Controller).

Quick comparison

Why thermal mass? Thermal (calorimetric) mass devices infer mass flow from heat transfer, enabling direct gas mass measurement with no moving parts and wide turndown. That’s why they’re mainstays for utilities and process gases.

Choosing digital vs analogue:

• If you want serial/fieldbus integration, logging, recipe steps or alarms, choose digital.
• If your PLC standardises on 4–20 mA or 0–5 V only and you just need a clean signal, analogue is usually the fastest route.

How Does a Thermal Mass Flow Sensor Work?

A laminar flow element (LFE) is placed within the full bore flow path of the device with two main purposes. The first is to simulate laminar flow conditions such that the measuring principle operates at a Reynolds Number of less than 2000 and is therefore representative of the true flow. The second is to create a slight differential pressure such that a defined proportion of the total flow will bypass through the sensor – a capillary tube that connects upstream and downstream either side of the LFE.

Schematic of a Bypass Thermal Mass Flow Sensor:

Around the outside of the capillary tube there are three small windings. The first is a temperature sensor (T1), the second acts as a heater element and the third is a second temperature sensor (T2).

When no gas is flowing the conduction of heat will make T1 and T2 equal. When flow occurs, heat is stripped from the first temperature sensor but added to the second sensor. This creates a temperature difference, or delta-T, between the two sensors and it is this mechanism that forms the heart of the sensor.

The two temperature sensors actually form two legs of a Wheatstone Bridge with the change in their temperature changing their electrical resistance. The amount of heat transferred is influenced by the thermal properties of the gas as well as the amount of gas molecules passing through the sensor. It is therefore important for the gas type, or the gas mix ratio, to be known so that the calibration process can take the thermal properties into account. The remaining variation in heat transfer, or electrical resistance, is therefore only proportional to an increasing number of gas molecules i.e. the mass flow rate.

What are the Benefits of a Bypass Thermal Mass Flow Sensor?

Bypass thermal mass flow sensors are manufactured using 316L stainless steel and are therefore ideal for higher pressures or where the aggressive nature of the gas might corrode other materials.

The measurement technique is not adversely effected by vacuum and so is ideal for that industry.

Lastly, the inherent design of the devices, especially when including metal face‑seal process connections, are non-particulating and hence are perfect for the Semi-Conductor and allied Industries.

Where are Bypass Thermal Mass Flow Sensors Used?

• Chemical / Petrochemical Industry
• High pressure Hydrogen Economy
• Fibre Optic and LED Production
• Glass Industry
• Aggressive gases within Surface Treatment
• Semiconductor Industry
• The vacuum industry

What to specify (checklist you can copy/paste into an enquiry)

• Gas(es) to be measured/controlled and composition changes expected
• Max / min flow and units
• Inlet (P1) / outlet (P2) pressures, gas and ambient temperature
• Outputs & comms needed: 4–20 mA, 0–5 V, RS‑232/485/Modbus, etc.
• Process connection preferences (NPT/BSPP, compression, VCR/VCO)
• Area classification (Safe / ATEX), and any materials constraints (wetted parts)

These are the practical items we use to recommend the right MFM/MFC (and whether to add a power/readout box).

Application guidance

• Digital MFC - when you need closed‑loop control plus digital comms and data capture for QA/traceability. (Think automated profiles, calibration benches, R&D rigs.)
• Analogue MFC - when your PLC does all the logic and you only need setpoint in / flow out on 4–20 mA or 0–5 V.
• Digital MFM - when you need on‑device totals and serial logging (consumption/costing, energy studies).
• Analogue MFM - when you just need a stable retransmit into a logger/PLC with minimal configuration.
• Power‑supply/readout box - when there’s no plant PLC (or you need a portable setup) and you want a local readout/total with the correct supply voltage for the device. (Common on benches and FAT/SAT checks.)

Note on the sensing principle: Thermal mass devices route a portion of gas through a bypass with two temperature sensors; the upstream sensor cools and the downstream sensor warms in proportion to mass flow—this ΔT is the measurement signal.