The right gas/air mixture is essential for efficient combustion in an instantaneous water heater, so it is important for the blower in a gas/air modulation unit to always supply exactly the right amount of air. The unit of measurement used for describing this is the mass flow, which specifies how many grams of air flow through a defined cross section every second.
The mass flow conveyed by a fan can be controlled by the fan’s speed. For example, a speed of 8,000 rpm generates a mass flow of nine grams of air per second, which is needed for heating output of 20 kilowatts. However, these figures only apply for constant ambient conditions. If the pressure or temperature change, then the mass flow no longer depends directly on the speed and the desired output of the water heater can only approximately be reached. This means that if an exact output value is to be achieved independently of the ambient conditions, exact control of the mass flow is required.
How a mass flow sensor works
OP regulates the bridge voltage so that the overtemperature of the heating element with respect to the ambient temperature is constant.
QE is the heat added by electric power.
QF is the heat flux given off to the passing fluid by the thermal element.
UM is the voltage measured by the microcontroller and converted to a mass flow (calibration required).
This task is performed by a calibrated mass flow sensor, also called a thermal anemometer. How the measurement principle works: A heating element about three by five millimeters in size is electrically heated to a temperature that is always 15 degrees Celsius above the ambient temperature. This heating element is positioned in the gas/air modulation unit so that the air flows over it on its way to the burner.
The heating element gives off more heat to the surroundings than it would with no air flow. In principle, just as with hot food, which also cools off faster when one blows on it. In order for the heating element to maintain the temperature difference of 15 degrees Celsius, it needs to be heated more. That requires extra electrical energy, which is measured by a microcontroller. The measured voltage provides information about the mass flow. For example, at a voltage of four volts, 16 grams of air per second flow through the tube.
Such data can be used by the manufacturers of heating systems to set the mass flow exactly. Based on this information, the impeller speed in a pneumatic unit can be precisely adjusted. In an electronic unit, the measurement data can be used for regulation strategies. The signal can also be analyzed to find the causes of errors. For example, if no mass flow is measured even though the blower is working, that is an indication of an obstructed exhaust pipe.