Thermal Mass Flow meter

How do Thermal Mass Flow Sensors work?

This is a basic explanation on some of the most common types of Thermal Mass sensors. There are of course, other variations which may add a twist to the concept and the measurement. These devices are popular for measuring air and gas flow, but some manufacturers have designed the sensors to also work with liquids.

Thermal probes are versatile instruments that can be conveniently installed in existing pipework at the site location, often using screwed or flanged connections. These connections allow for easy integration into pipes and ducts to accurately measure the temperature differential (ΔT) of a flowing medium, which is essential in various industrial applications.

Engineers and Technicians recognize this as a measurement of a differential. In this case it is the difference between the actual temperature being measured (by a PT 100) and the constant temperature of the heated sensor.

This ΔT measurement is fundamental for accurate flow and energy calculations, as it provides real-time data on how much the medium has heated or cooled, based on the difference from the reference temperature. Such information is invaluable in monitoring thermal energy flow in HVAC systems, heat exchangers, and industrial processes, where maintaining specific temperature ranges is critical to system performance and efficiency.

To understand how an insert probe works, it’s helpful to visualize its internal components. Inside the probe, there are two distinct sensors: one that is heated and another that remains unheated. Although these sensors are often encapsulated together, giving the appearance of a single rod, it’s crucial to recognize that there are always two separate sensors. The heated sensor is designed to maintain a constant temperature, serving as a reference point, while the unheated sensor actively measures the actual temperature of the medium within the pipe or conduit.

This two-sensor design enables the probe to calculate the temperature differential (ΔT) between the heated and unheated sensors, providing valuable insights into flow conditions. The heated sensor’s stability allows it to determine how much heat is lost to the flowing medium, which helps gauge flow speed and turbulence. By combining the data from both sensors, operators can achieve precise assessments of flow dynamics, essential for maintaining efficiency and control in applications such as HVAC systems, industrial flow control, and environmental monitoring.

When the flow increases, the heated sensor will begin to cool. In order for the heated sensor to maintain it’s steady temperature, more energy will be needed to heat it up. The amount of energy required for the heated sensor to maintain it’s constant temperature is directly proportional to the flow velocity e.g. feet per second or meters per second.

In a pipe or a conduit, flow velocity can be used to calculate the volumetric flow if the diameter of the pipe/conduit is known. An over simplified calculation (without corrections for Reynolds numbers) can be understood by spending time with a flow velocity to volume calculation e.g.