In this article, we will learn calorimetric flow meter measurement and its working principle. In the market calorimetric flow switches are also available. We will also learn about calorimetric flow switches.
Calorimetric Flow Meter:
Calorimetric flow meter work on the principle of heat transfer by the flow of fluid. This is also known as a thermal mass flow meter and hot wire anemometer. Typically calorimetric flow meters are situated along the direction of the flow.
Heating of an element is placed in the flow. As the sensor is positioned to measure the temperature of the device, a second measuring sensor reads the temperature of the flow downstream from the heater.
PT100 RTD is used as a heating element. As you can see in the below attached image of the calorimetric flow measurement setup.
Two temperature sensor used in the flow stream.
Here as T2 and T1, RTD is used to measure temperature.
The rate of flow is determined by the difference in the two temperatures with a constrain power input.
This difference in temperature is a linear function of the mass flow and the heat capacity. The flow meter can then be calibrated to indicate directly in mass flow units.
Applications of thermal flow meters are limited to use with fluids that have known heat capacities.
Usually, they are clean gases or a clean mixture of pure gases of known composition. Liquid applications are less common; because liquids generally contain more impurities than gases.
Most of the thermal flow meter designs have a temperature range between 100 and 150 degree Celsius.
Conductive surfaces of a thermal flow meter can become contaminated and should be routinely cleaned to maintain the performance levels. These devices are generally not applied to abrasive fluid services.
Calorimetric Flow Switch:
Calorimetric flow switches, which are often referred to as thermal flow monitors, use the physical laws of heat transfer in the flows. A distinction is basically made between two technical solutions: continuous and regulated heating.
Continuous heating:
A flow switch which is based on the calorimetric measuring principle consists of a measuring probe with two temperature sensors integrated into it.
One of the sensors is heated continuously with the aid of an integrated heating element (wire-wound) with a constant heating power and measures the temperature at the heating element.
The second sensor determines the temperature of the medium in the pipe. Consequently, a temperature difference occurs between the two sensors, which is registered by the electronics.
The higher the flow velocity of the medium in the pipeline, the smaller this temperature difference is. The basis for this is the cooling effect of flowing media.
The molecules in the medium, which are flowing past the probe tip, collect “packages of heat” and transport them away.
The more molecules flow past, the greater the cooling effect. The number of molecules passing by increases continuously with the increasing flow velocity.
Regulated heating:
The measuring probe is basically identical in design: There are two temperature sensors in the medium, one of which can be heated.
In this technical solution, the heating power is regulated so that the temperature difference between the two temperature sensors is kept constant throughout.
Consequently, as the flow velocity increases, the heating power must be increased in order to keep the size of the temperature difference constant.
The applied heating power is thus a direct measure of the flow velocity in the medium.
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