Air Watts Calculator

Have you ever wondered how to calculate air watts? Using the Air Watt Calculator to make Air Watt Calculations provides a number that is based on CFM and Inches of water lift to compare shop vacuums.    It is easy to convert air watts to CFM with one of these air watt calculators. 


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    Air Watts

    Manufactures have been using Air Watts to rate output power of a vacuum.  Restricting the inlet to 2 inches and then measuring the airflow, then closing the inlet completely to measure the suction, should provide a standard basis for comparison.  By measuring at the same point at the inlet of the vacuum two measurements can be taken.  Suction, when no air flows through the internal suction fan,  and Airflow, when the inlet is wide open.   Internal resistance of the system affects both suction and airflow therefore measurements at the inlet gives comparable results.   Thus one system can be reliably compared to another.   A very high Air Watts for Appliances (maximum of 12 Amps) is around 1440 Air Watts. But here is the catch.  You cannot depend on the Air Watts from different manufactures being reliable.   However, “Generally”, manufactures are relatively accurate on CFM and Suction.

    Note: If you use one manufacturer’s “Airwatts” for comparison to another manufacturer’s “Airwatts” ,only use Airwatts calculated in accordance with ASTM F2105.   This is a precise standard.   I have found that the data sheet for some quality vacuum motors do have this calculation for all flow rates from zero to peak flow and they can supply a data sheet.   You won’t find this data available for typical (“appliance rated”- under 12amp) vacuums.



    The formula for Air Watts is the  Air Flow Rate (CFM) * Vacuum (Inches of Water)/8.5.  The default numbers in the calculator above CFM –  203 and Suction = 45 inches of water lift are for the Ridgid WD19560 shop vacuum. The Air Watts Calculation = 1075.  This is a contractor grade vacuum running at no more than 12 amps and with a 16 gallon canister.  Cat 6 Tools has been using the Ridgid WD19560 shown below  to test the Holey-Moley Vacuum Assisted Post Hole Diggers for years..

    Rigid WD19560 Shop Vacuum
    Rigid WD19650 Shop Vacuum


    The Cat 6 Hurricane Vacuum shown below has a peak flow rate of 140 CFM and full suction of 145 inches of water.  The Air Watts Calculation = 2306, a factor of 1.894 times higher than the Ridgid.  Using these numbers for comparison you can easily conclude that the Cat 6 Hurricane has twice the performance of the Ridgid. When physically tested with dirt, sand, and gravel, this holds true.


    Air Flow

    For shop vacuums Airflow is typically rated in Cubic Feet Per Minute (CFM).  This is the maximum amount of air moving at no load (no lift). Since Air Watts is directly proportional to air flow it also couples to the suction of the motor fan that provides suction  Air Flow is measurements are done with a restriction of a 2″ opening simulating resistance to air flow.

    Because the driver (motor) is direct drive to the turbine blade or fan blade, both suction and airflow are affect each other.  Typically, when one parameter is rises, it is at the expense of the other.  So you will see some vacuums with higher suction and lower airflow or vice versa. If you wonder how to calculate air watts, this calculator makes it easy.

    Air flow through a system, like the wind in a dust storm is what keeps the dust particles in suspension. The higher the flow rate the greater the capacity of the airflow to carry dirt. This is a key performance aspect of a shop vacuum when moving larger particles such as sand and gravel.

    Airflow is also inversely proportional to the total resistance to air flow within the complete system including hoses, deflectors and filters.  Since shop vacuums do not have piping networks , and since the suction and flow measurements are at the inlet, the only system parameter that may change is the filter and this would be the filter that normally comes with the shop vacuum.  The resistance within the system is normally constant.  If the filter is new, airflow will have little resistance within the filtering system.

    Therefore, when comparing new units, use the air watt calculator and CFM for comparison.


    Suction – Lift or Sealed Pressure

    Suction measurements can be in inches of Mercury, or inches of water.   For most shop vacuums suction (Lift or Sealed Pressure SP) measurements are in inches of water.  This is how far up a tube the suction will lift a column of water. At this maximum point, air flow (CFM) is zero. It is important to know that at Maximum Flow lift is zero, and at Maximum Suction Lift air flow (CFM) is zero.

    For reference maximum lift would equal atmospheric pressure that is approximately equal to 33 feet, or 396 inches of water.  Most shop vacuums have single stage fans (one set of fan blades).  The more sets of blades the greater the suction is obtainable at the expense of flow rate.  Therefore it is important to compare the suction along with the air flow when comparing vacuums. See how to calculate air watts using the air watt calculator which provides a good number for vacuum comparison.


    Here is a link that provides a lot of Shop Vacuum Comparison Data on Wet/Dry Vacuums by Christopher Komuves

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