Calibration Priciple Low Flow
Flow calibrations are usually performed using a system that includes, a source of flow, the intrument being calibrated, connecting conduits, and a scheme for determining the fluid flowrate. When the calibration is based upon the bulk flowrate, i.e., time rates of fluid or mass, the scheme for determining the fluid flowrate, in a highly accurate manner, is generally a timed collection technique. For each of these schemes, the error budget should be known and maintained so the performance levels are as quoted.
Technique
Mercury sealed piston displacement techniques are currently used at Fathoms to calibrate fluid meters flowing air at low rates. The piston is housed in a cylinder, which is a precision-bore glass tube. The plastic (PVC) piston is groved horozontilly retaining the mercury which forms the seal between piston and cylinder.
Operation
To make a flowmeter calibration measurement, flow is produced and equilibriated through the meter and it's adjacent piping. The calibration is performed by diverting this flow into the prover so that the piston rises, attains a constant velocity, and then passes through the collection interval where the timing is done. The pressures and temperatures of the flow through the meter and the flow in the prover are measured during each "run",i.e, the timed collection interval.
Summary of Uncertainties
Piston Provers. Typical values for the components of uncertainty on the determination of mass flow rate using the Fathoms piston provers are tabulated below. The flow rate uncertainty that can be obtained from a root-sum-square combination of these uncertainties is: 0.015%.
| Collection Volume |
| Cylinder area |
0.023 |
| Piston Stroke length |
0.001 |
| Piston Rocking |
0.012 |
| Thermal Expansion |
0.003 |
| Timing |
| Device |
0.007 |
| Actuation |
03014 |
|
|
| Pressure |
| Barometric |
0.100 |
| "Gage" readings (on density) |
0.002 |
| Changes in piping |
0.001 |
| "Standard" Air Density |
0.020 |
| Temperature Effect on Density |
0.017 |
|
|