Thermal Mass Flow Meter

Thermal Mass Flow Meter Principle

A thermal gas mass flow meter is an instrument that measures gas flow rate using the principle of thermal diffusion. It requires no temperature or pressure compensation, making it the preferred product for measuring the flow rate of dry gases. It is widely used in fields such as coal gas, natural gas, compressed air, and semiconductor process gases, effectively monitoring gas flow rate in pipelines.

Working Principle

A thermal gas mass flowmeter is a meter that measures the mass flow of gas using the principle of heat conduction. The sensor of the thermal mass flowmeter consists of two reference thermal resistors (platinum RTD). One is the mass speed sensor T1, and the other is the temperature sensor T2 that measures the temperature change of the gas.

When these two RTDs are placed in the measured gas, the sensor T1 is heated to a constant temperature difference above the gas temperature, and the other sensor T2 is used to sense the temperature of the measured gas.

As the mass flow rate of the gas increases, the airflow takes away more heat, and the temperature of the sensor T1 decreases. To maintain a constant temperature difference between T1 and T2, the heating power of T1 must increase.

Check Wikipedia

According to King’s law of thermal effects, there is a certain mathematical relationship between heating power P, temperature difference △ T (T1-T2) and mass flow Q.

P / △ T = K1 + K2 f (Q) K3

K1, K2, and K3 are constants related to the physical properties of the gas.

Read more about: Thermal Mass Flow Meter Technology

Featured Applications

Thermal gas mass flow meters are widely used in fields and industries where high accuracy and stability in gas flow measurement are required.

Public Utilities

Biogas; coal gas; natural gas; liquefied petroleum gas (LPG); boiler preheating air.

Oil and Gas Industry

Energy exchange; well filler gas recovery; fuel gas metering; gas quality analysis; leak detection; natural gas measurement; flare gas monitoring.

Power Industry

Gas measurement during gas distribution in fuel systems; measurement of various gases in boilers and auxiliary systems; gas measurement in gas-fired furnaces; hydrogen measurement; measurement of primary and secondary air in power plant blast furnaces.

Chemical Industry

Flue gas circulation monitoring; gas flow measurement in sampling systems; gas flow measurement in induced draft fans; ammonia measurement in fertilizer plants; measurement of various gas flow rates in battery factories.

Metallurgical Industry

Gas measurement in steel plants; blast furnace gas measurement in ironmaking plants; coke oven gas measurement in coking plants; measurement and control of fuel gas (blast furnace gas, coking gas, natural gas, etc.) in steel rolling mills; control of hydrogen, oxygen, nitrogen, and other gases in heat treatment quenching furnaces, etc.

Pulp and Paper Industry

Gas measurement in wastewater treatment systems; flue gas flow monitoring; boiler secondary/tertiary air recovery; boiler fuel gas and air supply measurement.

Food and Pharmaceutical Industry

Fresh air introduction during processing operations; carbon dioxide treatment in breweries; hot air flow rate in bottle sterilizers; gas flow rate measurement during thermal oxidation; ventilation systems; boiler intake, exhaust gas, and process control.

Environmental Protection

Gas measurement during biogas utilization; chlorine gas measurement during chlorine treatment; gas measurement in aeration tanks during wastewater treatment; monitoring of SO2 and NOx emissions from chimney exhaust.

Other Industries

Compressed air measurement in factories; pulverized coal combustion process pulverized gas/air ratio control; hot gas flow control from vertical grinding mills in the cement industry.

Advantages in natural gas measurement

Thermal mass flowmeters offer several advantages that make them particularly suited for natural gas applications:

1. Direct mass flow measurement

Thermal mass flowmeters measure natural gas by mass rather than volume, which means the measurement is not affected by changes in pressure or temperature. This makes them especially suitable for natural gas applications, where gas density can vary significantly under different operating conditions.

2. High accuracy at low flow rates

One of the major advantages of thermal mass flowmeters is their ability to maintain high accuracy at low flow rates. With a wide turndown ratio that can reach 50:1 or even higher, they are ideal for applications such as leak detection, purge gas monitoring, and pilot gas measurement.

3. Minimal pressure drop

Thermal mass flowmeters are designed with no moving parts and typically use a small sensor probe inside the pipe. This design results in very little pressure loss, helping to reduce energy consumption and lower operating costs in natural gas systems.

4. Simple installation and low maintenance

Because there are no rotating components, thermal mass flowmeters experience less mechanical wear and require minimal maintenance over time. Many models also support hot-tap installation, allowing them to be installed or serviced without shutting down the gas line.

5. Fast response time

Thermal mass flowmeters respond quickly to changes in gas flow, making them suitable for real-time monitoring and control. This fast response is particularly useful in applications such as burner control systems and natural gas distribution networks.

6. Wide operating range

These flowmeters perform reliably across a wide range of pressures and temperatures commonly found in natural gas applications. Even when flow conditions fluctuate, the measurement remains stable and dependable.

7. Good repeatability and long-term stability

After calibration, thermal mass flowmeters deliver consistent and repeatable readings over long periods of operation. Compared with some differential pressure–based flowmeters, they tend to show less measurement drift and better long-term stability.

8. Compact and cost-effective solution

Thermal mass flowmeters are compact in design and are often a cost-effective choice for small to medium pipe sizes. In situations where ultrasonic or turbine flowmeters may be too expensive or difficult to install, thermal mass flowmeters provide a practical alternative.

What are the Disadvantages?

Thermal mass flow meters are very useful for measuring gas mass flow directly, but they still have several limitations and disadvantages that should be carefully considered before selection and installation.

Sensitive to gas composition: Changes in gas composition can cause measurement errors.

Only suitable for clean gases: Dust, particles, or condensation can coat the sensor.

Low accuracy at low flow rates: Measurements are less precise at very low flow.

Affected by temperature and pressure: Extreme temperatures or pressures can impact performance.

Installation sensitive: Requires straight pipe runs and stable flow conditions.

Not suitable for liquids: Cannot accurately measure liquid flow.