Measurement principle
Thermal gas mass flow meters are designed based on the principle of thermal diffusion. These instruments use a constant temperature difference method to accurately measure gases. They offer advantages such as small size, high degree of digitization, easy installation, and accurate measurement.
The sensor section consists of two reference-grade platinum resistance temperature sensors. During operation, one sensor continuously measures the medium temperature T1 ; the other sensor self-heats to a temperature higher than the medium temperature T2 and is used to sense the fluid flow rate, called the velocity sensor. The temperature ΔT = T2 - T1 , where T2 > T1 . When fluid flows through, gas molecules collide with the sensor and carry away heat from T2 , causing the temperature of T2 to drop. To keep ΔT constant , the supply current to T2 must be increased . A faster gas flow rate results in more heat being carried away. There is a fixed functional relationship between gas flow rate and the increase in heat; this is the principle of constant temperature difference.
Among them ρg , fluid specific gravity (related to density)
V — Flow velocity
K — Balance coefficient
Q — Heating capacity (related to specific heat and structure)
ΔT — Temperature difference
Since the sensor temperature is always automatically and constantly about 30 °C higher than the medium (ambient) temperature, the thermal gas flow meter does not require temperature compensation in principle.
The thermal gas mass flow meter is suitable for media temperatures ranging from -40 to 220 ℃.
( 1 ) The fluid specific gravity and density are related.
Wherein ρg — the density of the medium under operating conditions ( kg/m3 )
ρn — Density of the medium under standard conditions ( 101.325 kPa , 20 ℃) ( kg/m3 )
P — Operating pressure ( kPa )
T — Operating temperature (°C)
from equations ( 1 ) and ( 2 ) that the functional relationships between flow velocity, operating pressure, gas density, and operating temperature have been determined.
Thermostatic differential thermal mass flow meter is unaffected by both temperature and pressure. It is a true direct mass flow meter, eliminating the need for users to correct for pressure and temperature.
Thermal gas mass flow meters have the following technical advantages:
● A true mass flow meter requires no temperature or pressure compensation for gas flow measurement, making it convenient and accurate. It can obtain the mass flow rate or standard volumetric flow rate of the gas.
● With a wide measurement range, it can measure gas flow rates from 100 Nm/s to 0.5 Nm/s , and can be used for gas leak detection.
● It has good shock resistance and a long service life. The sensor has no moving parts or pressure sensing components, so it is not affected by vibration on measurement accuracy.
● Installation and maintenance are simple. Installation and maintenance can be carried out without interrupting production, provided site conditions permit. (Special customization required)
● Digital design. Overall digital circuit measurement ensures accurate measurements and convenient maintenance.
● Using RS-485 communication, factory automation and integration can be achieved.
