The challenges of accurately metering the flow of liquids at elevated temperatures are often based on the resultant changes in fluid properties rather than attributes of the measuring device used.  While the density of water changes by around 3% from 1 to 90°C, it could still be critical if your application requires an accurate mass of aqueous solution to be metered / dosed.  However, the viscosity of aqueous solutions changes between 1 and 90°C by a factor of 8, which can have a significant impact on fluid flow metering.

Aqueous solutions are one of the most common type of fluids that have to be metered and dosed in a wide range of industrial applications.    Above is a graph showing the changes in the viscosity and density of water at different temperatures.

As can be seen the density of water changes by around 3% from 1 to 90°C. While this is a relatively modest change, it could still be critical if your application requires an accurate mass of aqueous solution to be metered / dosed.  The change in viscosity over the same temperature range is however not modest, in fact viscosity of aqueous solutions change by a factor of 8 between 1 and 90°C.

For a Reynolds number (Rn) sensitive flowmeter this could be disastrous.  There would be nearly an 8:1 change in Rn and should the meter only have a 10:1 flow range, and be operating close to its extremes, it is possible that meter could drop out of the linear range.

The effect of these aqueous viscosity and density changes at different temperatures is not the same for all meter types. While most flowmeters require turbulent flow i.e. a Rn above ~2200, some are designed to perform in the laminar region below this value and an increase in temperature could take the meter into the turbulent zone.

Therefore – if you are looking to measure liquid flow at high temperature we recommend you follow our simple key considerations checklist:

  1. Check your liquids changing density / viscosity or physical properties at different temperatures.
  2. Check that your flowmeters operating envelope and principle of operation bearing in mind ‘1’ above.
  3. Consider if your flow line temperature has to be maintained for ease of fluid transport e.g. liquid chocolate or heavy oil. If YES, then does the flowmeter need to be insulated, heat traced or jacket heated,
  4. Does your process flowmeter need to be accurate at low temperatures?
  5. Is it acceptable for your flow measurement device to be ranged for the actual process running conditions and is flowmeter performance important during the “warm up period”?

If you want to discuss an optimised flowmeter solution for your elevated temperature application, please use this contact form:


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