A fundamental calibration method for hydrocarbon density meters
Simple, Portable, Traceable
Pyknometry is the most reliable way of calibrating density meters. It is the only calibration method which overcomes possible measurement errors such as liquid viscosity, flow rate, temperature, mounting torque, pressure, position and vibration.
What is a Pyknometer?
A pyknometer is a stainless steel pressure vessel which is used to calibrate crude oil density meters. Pyknometry gives direct verification of the density meter’s reading, and so enables very accurate calibration of the instrument in-situ. Unlike secondary techniques, such as transfer densitometers, pyknometry is a fundamental calibration method which allows traceable calibration of on-line density meters at their on-line conditions.
How are Pyknometers used?
Pyknometry is a simple and foolproof calibration method.
Two empty and clean pyknometers are weighed and connected in series to the flowing oil directly downstream of the density meter to be calibrated.
The pyknometers are then filled with a representative sample of oil at the same temperature and pressure as the oil flowing through the density meter. The system measures the pressure immediately upstream and downstream of the pyknometers so that the average value gives the pyknometer pressure.
Once the pyknometers have stabilised at the oil temperature, the valves are closed. The pyknometers are then are removed from the pipeline, the outsides are cleaned, and they are reweighed.
The oil density at density meter conditions is then calculated from the mass of oil in the pyknometers, the volume of the pyknometers at the measured temperature and pressure, and the air density.
An independent density is calculated for each pyknometer. The two independent results can then be checked against each other.
Ordering Pressure Pyknometer Systems
HDF Pyknometers Ltd, the company in the H&D Fitzgerald group which had been responsible for pyknometer production, has been transferred to Oil & Gas Measurement Ltd based in Ely, Cambridgeshire.
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