How to Buy a Flow Meter
Flow meters come in different styles, technologies, abilities, etc. No one meter that can do everything – but they all have their purpose. Before you can buy with confidence, learn what questions must be answered!
How Does Viscosity Affect the Different Flow Meter Technologies?
So now that we know what viscosity is and what it does, how does it affect your choice of flow technologies? Viscosity effects different meters in different ways. Here are some examples…
Inferential Flow Meters
Inferential meters measure flow by measuring another physical characteristic and “inferring” the volumetric rate. This may be the speed of a turbine or the speed of sound through the liquid. Inferential meters such as turbine meters do not accurately measure fluids with changing viscosity without a correction factor. Therefore in the most part they are recommended for thin fluids with a low viscosity range between 0.5 and 10 cP such as water and diesel oil.
Magnetic (Mag) Meters
Mag flow meters are not affected by viscosity however they rely on conductivity of the fluid and most viscous fluids are non-conductive as they are long chain hydrocarbons. Most conductive fluids are water based and contain dissolved ions.
Positive Displacement (PD) Meters
The accuracy of Positive Displacement meters, such as Oval gear meters, are not greatly affected by changes in viscosity which is one of their main advantages. A PD meter can be calibrated at 3 centipoise and be within ±0.5% accuracy in a range of 1 centipoise to 1,000,000 centipoise.
Unless pressure is a concern, you should not need to consider the effect of fluid viscosity if it remains with a range of 3 to 500cP. For oval gear meters equipped with High Viscosity Rotors, that range increases to 3 to 2,000cP.
For liquids with viscosities of less that 3cP, there are several considerations that need to be factored into your final choice of flow meter(s).
- Stated minimum flows need to be adjusted to 10% of the stated maximum flows. For example, the OM006 (1/4 inch) meter has a published range of 0.5 to 27 GPH. If used for measurement of fluids with a viscosity of 3cP or less, adjust the range to 2.7 to 27 GPH.
- Although we can’t offer specifics, accuracy can vary slightly when measuring any liquid that has a viscosity ranging from 1.0 to 3cP that has no lubricating qualities.
- The most difficult fluids to measure with an Oval Gear meter are non-lubricating liquids with viscosities below 1.0cP.
- For fluids with viscosities greater than 2,000 cP, end users should give serious consideration to the use of High Viscosity (HV) rotors. Doing allows for additional flow ranges under similar conditions.
If your fluid is viscous with little or no particles, start thinking Positive Displacement (Oval Gear Meter). Particulary if that fluid has the opportunity to change viscosity based on process conditions.
Your last choice for a Viscous fluid would be an inferential meter such as a turbine. In fact, you should probably avoid Turbines and other inferential type meters altogether if your fluid is anything but a clean, low viscosity (1-10cP) liquid like water, fuel, etc. Interested in learning more? Contact Cross to discuss your particular application with an instrumentation specialist: