Greg McMillan Forum Moderator 36 Posts
Re: Can you explain linearity and accuracy of flowmeters?28 June 2011 at 1:29pm
Here are the general definitions and quirks in measuring flow using several methodology and the attending errors. [error=1-accuracy].
In general, accuracy is defined as the degree of exactness of measurements. That is the absolute quality of computed results (sensors and display units) accounting for temperature, pressure, errors in sizing devices, and pipelines with built-in imperfections (internal diameter). For process control errors, up to 2% in absolute terms is industry standard (typical orifice installation). However, for custody transfer flow instruments, accounting for all error causing deviations, the accuracy expectation is 0.001% of full scale- [error]. This is typical in national pipeline systems to measure flow of liquids in petroleum industry. Error can occur from morning to evening to night time measurements depending on seasons -- winter vs. summer. Hence, Prover systems are used per U.S. Bureau of Standards (Science & Technology) to certify flow measuring instruments in early morning hours.
Precision is the amount of details used to derive at that result [accuracy], like bored pipe, straight line run of pipes, and independent compensation of P&T to flow.
Linearity is defined as the closeness of a curve that approximates to a straight line throughout measurement range. Due to imperfections in pipe ID, coefficient of expansion of metals used in instruments, even the precise measurements have a built-in error in measurements of flow.
Flow in pipelines is computed by measuring velocity, volume transfer or weight transfer. All instruments employ one of the above techniques. In olden times PD meters or turbine meters with compensation were used for custody transfer. Nowadays mass flow meters are used to do the same since they are nor volume measurements.
In oval gear meters volume transfer is measured without accounting for temperature & pressure variations or changes in fluid characteristics [density or vapor content].If properly engineered, at lower rate of flow versus higher rate of flow volume transfer error will be noticeable- though in second decimal-hence neglected. Mag meters, vortex shedding meters, target meters etc this error below droop point is well defined, requiring selection of proper sizing to maintain accuracy for a given pipe size.
You can refer to US bureau of Standards on how errors are computed for custody transfer and gas pumps/scales for retail sales.
Editor's Note: Ram.G.Ramachandran of ERGOFIT USA answered this question.