# Airwatt

Airwatt or air watt is a measurement unit of the effectiveness of vacuum cleaners which refers to airflow and the amount of power (watts) a vacuum cleaner produces and uses.[1][2] It can also be referred to as a measurement of the energy per unit time of the air flowing through an opening, which is related to the energy that electricity carries through the power cable (wattage).[3]

The airwatt is a useful measurement of vacuum cleaner motor efficiency, since the power carried by a fluid flow (in the case of a typical house vacuum the fluid is air) is equal to pressure times volumetric flow rate. The airwatt relates to actual airflow, while part of the electrical power (watts) consumed by a vacuum cleaner is dissipated into heat due to necessarily imperfect efficiency; two vacuum cleaners of the same airwattage have essentially the same suction, while devices of the same electrical wattage may have a difference in efficiency and thus have substantially different airwattage.[4]

## Definition

The formula used to compute airwattage differs between vacuum cleaner manufacturers. The standard airwatt formula is from ASTM International (see document ASTM F558 - 13)[5]

${\displaystyle P=0.117354\cdot F\cdot S}$

Where P is the power in airwatts, F is the rate of air flow in cubic feet per minute (denoted cu ft/min or CFM) and S is the suction capacity expressed as a pressure in units of inches of water. This makes one airwatt equal to 0.9983 watts, which rounds off to 1.000 watts.[6]

In terms of the orifice plate,

Air watts = ​18.5 × vacuum suction [inches of water] × air flow [cubic feet per minute]
Air flow [CFM] = 13.35 × D2 / vacuum suction

Where D is the diameter of the orifice holes.[7]

Using coherent SI units, power equals flow times pressure by definition. That is, where the power is expressed in watts (W), the flow is in cubic metres per second (m3/s) and the pressure is in pascals. Since one pascal (Pa) equals one newton per square metre (1 Pa = 1 N/m2), then:

${\displaystyle 1~{\frac {{\text{m}}^{3}}{\text{s}}}\cdot 1~{\frac {\text{N}}{{\text{m}}^{2}}}=1~{\frac {{\text{N}}\cdot {\text{m}}}{\text{s}}}=1~{\frac {\text{J}}{\text{s}}}=1~{\text{W}}}$

The power of the flow times the pressure will always be less than the power applied via the voltage and current (1 W = 1 V·A). The ratio of the power produced in the flow and pressure divided by the power from the voltage and current is the efficiency.

## Alternative measurement formula

${\displaystyle P={\frac {1}{8.5}}\cdot {\text{airflow}}[CFM]\cdot {\text{suction}}[{\text{inches of water}}]}$

CFM is always given statistically at its maximum which is at a 2-inch (51 mm) opening. Waterlift, on the other hand, is always given at its maximum – a 0-inch opening. When waterlift is at a 0-inch opening, then the flow rate is zero – no air is moving, thus the power is also 0 airwatts. So one then needs to analyse the curve created by both flow rate and waterlift as the opening changes from 0 to 2 inches (0 to 51 mm); somewhere along this line the power will attain its maximum.

If the flow rate were given in litres per second (L/s) instead of cubic metres per second (m3/s), then the pressure would be in kilopascals (kPa). Thus one watt equals one kilopascal times one litre per second: ${\displaystyle 1~{\text{W}}=1~{\frac {{\text{kPa}}\cdot {\text{L}}}{\text{s}}}}$

## Ratings recommendations

Hoover recommends 100 airwatts for upright vacuum cleaners and 220 airwatts for cylinder vacuum cleaners.[8]

## References

• ASTM Standard F558 Standard Test Method for Measuring Air Performance Characteristics of Vacuum Cleaners