5

Mass Flow

Active

RTD

Heater

Reference

RTD

Mass

balancing

element

Transmitter

Thermatel switches rely on two miniature sensing

elements to detect heat transfer. Resistance Temperature

Detectors (RTDs) establish a thermal relationship that

establishes the basis for flow and level detection. One

RTD measures the temperature of the surrounding media

which serves as the instrument’s reference temperature.

The second RTD is heated to establish a temperature

differential above the reference temperature. In flow

sensing applications, the heated RTD cools with increas-

ing flow. In level sensing applications, the RTD cools in

the presence of media. In each case, cooling the heated

RTD decreases the differential temperature between the

two RTDs. This decrease is then converted into a relay

actuation which performs an alarm function.

Principle of Operation: Flow Meter

Thermal dispersion technology provides a mass flow

measurement of air or gas. This is accomplished by

precisely measuring the cooling effect as the mass (mole-

cular) flow passes the heated sensor. The sensor consists

of two elements: the reference which measures the tem-

perature of the gas and a second element which is heat-

ed at a variable power. This maintains the desired tem-

perature difference between the two sensors.

The illustration on the top of this page shows the

amount of power required to maintain a constant tem-

perature difference between the two sensors. Under low

mass flow conditions, there is minimal cooling and little

power is required. As the mass flow increases, more

power is required. This provides excellent low flow

sensitivity and high turndown capabilities.

Each instrument is factory calibrated and configured

for the specific application. If necessary, the TA2 can be

reconfigured in the field for different conditions.

Strengths



and Cautions



Direct mass flow measurement—does not require

pressure or temperature compensation which is

required by other technologies.



Thermatel offers excellent low flow sensitivity:

gas velocities from 25 ft/min (0.13 m/s) to over

50,000 ft/min (255 m/s).



Thermatel products are easy to install and

can be “hot tapped.”

An alternative flow meter technology should

be considered where condensed moisture is

continuously present.

Refer to Magnetrol bulletin 54-621 for further technical

information on Thermal Dispersion Mass Flow.