# How much does air weigh?

### How mass flow sensors measure the exact amount of air.

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).

## Measure­ment prin­ciple

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 mass flow sensor in a gas/air modu­la­tion unit

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.