velocity profile method

The velocity profiles program uses the log-linear technique to derive the mean flow and velocity profile. For practical purposes this method produces excellent results and has been verified by cross reference to both independent checks performed on Quadrina flowmeters, and to in-house testing. The calibration of each flowmeter is by the dynamic-gravimetric method, having a single point accuracy better than 0.05%. Each flowmeter is calibrated and issued with a computer generated Calibration Certificate prior to dispatch.

In the profiles program a 3, 5, 7 or 9 point traverse may be selected. On smaller pipes there are restrictions on the number of points allowed due to the size of the flowmeter head relative to the pipe diameter taking into account the spatial position of each of the traverse points. When the selected traverse is at variance with the number of points recommended then the laptop advises as such, allowing an alternative to be selected.

Profile data may be input manually in frequency (that is the frequency produced by the flowmeter which is directly and linearly proportional to the flow velocity). Or in m/s if the associated readout instrument is calibrated in such units. The readout or display instrument, in either of these cases, may be a local display built into the headbox of the flowmeter. Alternatively, the portable series of Jestim readout instruments type MEP-H or MEP-PR, that incorporate an LCD digital display, may be used.

For simplicity the ADCM instrument is preferred. It is under software control, interrogating the flowmeter when the profile program requires flow data. With each traverse point the ADCM unit takes several samples of the flowrate in quick succession. The sample period between each sample is about 0.05 seconds. This method gives an average value of flowrate based over the samples. It is a particularly useful feature when the flow fluctuations are quite large, to give a more steady, perhaps more meaningful flowrate result.

At each traverse point the computer gives the instruction 'insert flowmeter to the fall wall of the pipe and withdraw xxxx.x mm'. When the operator has set the flowmeter to the correct point it is only necassary to press the 'S' key on the computers keyboard to start the sampling. It is important to set the flowmeter as closely as possible to the position dictated by the computer.

Most importantly for the program is to know the exact internal diameter of the pipeline at the position of the flowmeter. Any error in this dimension will have a direct effect on the quality of the results obtained. The diameter can be quickly and easily measured by the Jestim Gauging Rod, a device that temporarily replaces the flowmeter at the beginning of the traverse test. Within a few minutes the internal diameter can be checked very accurately.

RSP - Last Modified 21-07-06 - Jestim Limited, All Rights Reserved