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How to select a suitable thermowell

Calibration of a flow meter

Our flow meters are generally designed and constructed according to ISO 5167, which describes in detail the geometry, construction, requirements, and calculations of various types of flow meters.

What is calibration of a flow meter?

It is an empirical method of finding the discharge coefficient1. The calibration institute sets up our meter in their pipeline, where they have a way of knowing the exact flow, usually by the way of one or more calibrated coriolis meters. The standard only has a few requirements regarding calibration. The main one is that the meter must be calibrated to the full range of operational Reynolds numbers2.

Why should a flow meter be calibrated?

There are a few reasons to calibrate flowmeters.

  1. To verify the accuracy of the meters. This is often a client requirement, especially in the offshore industry.
  2. To improve on the accuracy of the meter. Depending on the type of meter, the accuracy can vary from 0,5 to 5%, according to ISO 5167. Clients often have strict requirements on the accuracy of the flow meter, ranging anywhere from 5% to 1%. When calibrated, depending on the institute and exact conditions of the calibration, this can be improved all the way down to 0,2%.
  3. To confirm function and accuracy of meters which are beyond the scope of the standard. The standard has set limitations in terms of size, beta, and Reynolds number. These are often exceeded, especially for a lot of the larger or high-pressure meters which are produced.

What requirements are needed of a Calibration Facility?

All the calibration facilities used by EMCO Controls are certified. This, as a bare minimum, means they are accredited to ISO17025, which is the requirements for the competence of testing and calibration laboratories. This is certified by both DANAK and ILAC MRA. They often have numerous additional certifications as well.

Why gas calibration?

Generally, meters are calibrated on the phase medium as the operational use, even though this is not a requirement. This is usually for practical reasons; a meter meant for gas will usually require extremely high flow of water to reach the same Reynolds numbers. As most of the meters, EMCO products are meant for hydrocarbon gas applications, the logical solution is a gas calibration. This choice is only helped by the fact that we have one of the most capable calibration institutes in the world, FORCE Technology, just a short drive away, and with the opening of their MEGA Loop in May 2023, they will double both their flow and size capacity.  

1 What is a discharge coefficient?

It is a corrective factor, which tells how close to the theoretical that the meter performs. For example, a venturi has a discharge coefficient of 0,995, where as a cone meter has one of 0,82. This is due to the difference in geometries and the placement of the pressure tappings.

2 What is Reynolds number?

Reynolds number is essentially an indicator of the behaviour of the flow. In very rudimentary terms, it describes if the flow is laminar or turbulent. However, a requirement of the standard is that all flow is turbulent, in other words, a Reynolds number of over 5,000 to 200,000, depending on the type of flow meter. Reynolds number is calculated as:

where Re is the Reynolds number, ρ is the fluid density, V is the velocity in the pipeline, D is the pipe inner diameter, and μ is the dynamic viscosity of the fluid.