Bag Method Calculator

Detailed Description

This calculator implements the bag method, a practical field technique for measuring compressed air leaks. A measurement bag (typically a trash bag with a known volume) is placed over the leak point and the time required for the bag to fill completely is recorded. The fill time and bag volume together yield the volumetric flow rate of the leak. This method is the second most accurate field technique for measuring compressed air consumption, after inline flow meters.

The bag method is commonly used in compressed air leak surveys to quantify individual leak points that cannot be instrumented with a flow meter. It requires only a pre-measured bag and a stopwatch, making it well suited for difficult-to-access leak locations. The annual consumption estimate is derived by scaling the instantaneous leak rate by system operating hours, enabling the economic impact of each leak to be estimated directly.

Procedure:

1. Pre-measure the volume of the bag (in cubic feet) by filling it with water or measuring its dimensions.
2. Hold the bag opening tightly over the leak source, allowing leaked air to fill the bag.
3. Record the fill time in seconds.
4. Enter the operating time to compute the annual impact.

The calculation methods are:

1. Flow Rate – computes the instantaneous leak flow rate in scfm from the bag volume and fill time.
2. Annual Consumption – scales the flow rate by annual operating hours and the number of measurement units, then converts to kiloscf.

Relevant formulas are documented below.

Flow Rate

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Instantaneous compressed air leak flow rate from bag volume and fill time.

The leak flow rate is determined by dividing the pre-measured bag volume by the time required to fill the bag, with the fill time converted from seconds to minutes. This gives the volumetric flow rate of the leak in standard cubic feet per minute (scfm), at approximately standard atmospheric conditions.

The formula assumes that the bag is held snugly over the leak and that all leaked air enters the bag (no bypass). Fill times under 5 seconds reduce accuracy because reaction-time measurement error becomes significant; fill times over 60 seconds are generally unnecessary since a smaller bag should be used for those cases.

\begin{equation}\label{eq:bag-method-flow-rate} Q_{leak} = \frac{V_{bag} \cdot 60}{t_{fill}} \end{equation}

Symbols

\(Q_{leak}\)	Compressed air leak flow rate \([\unit{ \scfm}]\)
\(V_{bag}\)	Pre-measured volume of the measurement bag \([\unit{ \cubicFoot}]\)
\(t_{fill}\)	Time for the bag to fill with leaked air \([\unit{ \second}]\)
\(60\)	Seconds per minute conversion \([\unit{ \second\per\minute}]\)

Annual Consumption

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Estimated annual air consumption from a compressed air leak measured by the bag method.

Annual air loss is computed by scaling the instantaneous leak flow rate by the annual system operating time, converting from standard cubic feet to kiloscf. The factor of 60 converts operating hours to minutes so the units are consistent with the flow rate in scfm. The factor of 1000 converts standard cubic feet to kiloscf, a more practical unit for annual volumes.

\begin{equation}\label{eq:bag-method-annual-consumption} C_{annual} = \frac{Q_{leak} \cdot t_{op} \cdot 60}{1000} \end{equation}

Symbols

\(C_{annual}\)	Estimated annual air loss from the leak \([\unit{ \kscf}]\)
\(Q_{leak}\)	Compressed air leak flow rate \([\unit{ \scfm}]\)
\(t_{op}\)	Annual system operating time \([\unit{ \hour}]\)
\(60\)	Minutes per hour conversion \([\unit{ \minute\per\hour}]\)
\(1000\)	Standard cubic feet per kiloscf conversion \([\unit{ \unitless}]\)

Note

The result is in kiloscf (kscf), where 1 kscf = 1000 standard cubic feet. This unit is conventional for annual compressed air volumes in industrial energy assessments.

Modules
	Bag Method Flow Rate Formula
	Instantaneous compressed air leak flow rate from bag volume and fill time.
	Bag Method Annual Consumption Formula
	Estimated annual air consumption from a compressed air leak measured by the bag method.

Files
file	bag_method.h
	Declarations for the bag method compressed air leak flow rate measurement.

Namespaces
namespace	bag_method
	Compressed air leak flow estimation using the bag fill-time method.

Classes
struct	bag_method::Input
	Input parameters for the bag method compressed air leak calculation. More...
struct	bag_method::Result
	Result of the bag method leak flow rate calculation. More...