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MEDIUM FLOW UNCERTAINTIES |
Air Flowrate - Medium Rates
Technique
Bell prover techniques are currently used at Fathoms to calibrate fluid meter flowing air in the medium flowrate range - 1 SCFM to 100 SCFM. This system is also termed a dynamic one in that the fluid collection is initiated and terminated by the moving bell passing through a specially characterized vertical path length. The device consists of a cylindrical tank open at the top, with a central "dry-well" forming an annulus that is nearly filled with a sealing oil. Into this annulus is placed an inverted cylindrical tank - the bell - which is open at the bottom and having a dome shaped top. Its weight and support chain are balanced by counterwieghts so that it can be raised or lower at constant velocity with a pressure slighty larger than ambient so as to collect or discharge a measurable volume of gas. A smaller counterwieght is mounted on a cam so that it compensates for the buoyancy effects as the bell immersion changes. Rollers and guide rods provide lateral stability.
Operation
To make a calibration measurement, flow is produced and equilibriated through the flowmeter and it's adjacent piping with a bypass valve, so that the bell is just "floated" off its bottom support. To initiate the calibration, the bypass valve is closed so that all of the flow is diverted into the bell. Floowing a brief period in which accelerations, pressure transients, etc. subside, the bell attains a constant velocity with the steady state of conditions during which it passes through its stoke length and the corresponding time is measured. During each "run" or bell stroke, the temperature is measured, the collected air pressure is measured, and the timing is done using an electronic timer.
Summary of Uncertainties
Bell Provers. Typical values for the components of uncertainty on the determination of mass flow rate using the Fathoms bell provers are tabulated below. The flow rate uncertainty sum, in quadrature, of the forgoing yeilds a total system uncertainty is 0.14%.
| Bell Prover |
| Bell Strapping |
0.026 |
| Bell Wall Thickness |
0.007 |
| Bell Stroke Length |
0.001 |
| Thermal Expansion |
0.005 |
| Oil Film Adherence |
0.013 |
| Timing |
| Switching |
0.006 |
| Bell Motion |
0.040 |
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| Pressure |
| Barometric |
0.100 |
| "Gage" readings |
0.002 |
| Changes in piping |
0.002 |
| Effect on "constant" density |
0.010 |
| Effect on Density Change |
0.010 |
| Rise effect on oil level |
.066 |
| "Standard" Air Density |
0.020 |
| Temperature Effect on Density |
0.017 |
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