Cone DP Meter Calibration Issues
Figure 1: Cone meter drawing with wall cut away to show cone assembly.
The cone meter is a simple and robust generic differential pressure (DP) meter. It has been shown to be remarkably resistant to the effects of both asymmetric and swirling flow. The cone meter is often the meter of choice when there is limited straight pipe length available for a flow meter. Therefore, cone DP meters are becoming increasingly popular gas flow meters.
Figure 1 shows a drawing of a cone meter design with a wall cut away to show the cone assembly within the meter body. Flow direction is left to right.
The cone DP meter operates according to the same physical principles as other DP meter types. The differential pressure measured across the cone is proportional to the flow rate. The cone DP meter is a member of the generic differential pressure (DP) meter family and uses the generic DP meter flow rate equation. For a given cone meter geometry this equation uses the discharge coefficient to link known fluid properties and the read DP to the flow rate.
The discharge coefficient of a DP meter is at the center of the discussion of the meter’s accuracy. Just how a DP meter’s discharge coefficient is found and how accurate it is are very important issues when it comes to choosing and using a DP meter.
Like most DP meter types a cone meter must have its discharge coefficient found by calibration. This means testing the cone meter’s performance over the application’s full Reynolds number range. For a given pipe size and fluid properties the Reynolds number is a non-dimensional expression of the flow rate.
Like all DP meter designs, the cone meter can have its discharge coefficient expressed as a constant value. For potentially more precise flow rate predictions the discharge coefficient value can be linked to the Reynolds number. If the discharge coefficient is expressed as a constant value then the cone meter’s flow rate prediction is found directly from the generic DP meter flow rate equation. If the discharge coefficient is expressed as a function of the Reynolds number then the cone meter’s flow rate prediction must involve an iteration of the generic DP meter flow rate equation. With modern flow computers this is a very simple, almost instantaneous and trivial increase in calculation complexity.
A cone DP meter calibrated across the full Reynolds number range of the meter’s application can have the discharge coefficient found to a low uncertainty. From experience it is known that a cone meter’s discharge coefficient is approximately 0.8. However, for any given cone meter its actual discharge coefficient can be found to be somewhere between 0.74 and 0.86, i.e. 0.8 ±8%. That is not to say an individual cone meter’s discharge coefficient varies by 8% - it does not. Any given cone meter will have a discharge coefficient that lies within that range and can be set as a constant value or fitted to the Reynolds number to typically ±0.5 %.
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