Viscosity is a measurement of the “thickness” of a liquid and the resistance of that fluid to flow. It measures the friction between individual fluid particles and also between the fluid particles and the surfaces they move across.
For example, high viscosity fluids such as oil, honey, etc. are thick making them hard to pour and requiring additional power to pump. Low viscosity fluids like water or alcohol are thin and easily pumped.
What Affects Viscosity?
Temperature affects the viscosity of most fluids. Petroleum products in particular. As the temperature of the fluids increases, the viscosity decreases and the fluid thins out as a result. Lowering the temperature can have the opposite effect on a fluid.
The presence of water will also have a thinning effect on most fluids. Some fluids will become more viscous as the shear rate increases, while others will become less viscous. For example, when agitated, quicksand will become less viscous. Liquids that contain insoluble droplets of another liquid are referred to as emulsions. Emulsions decrease viscosity which results in increased flow and greater shear.
What is Shear?
Shear stress is a parallel-acting force, where two surfaces (in this instance, two layers inside a fluid) slide against each other. This is in contrast to Compression, where there is a perpendicular-acting force; Tension, where there is a stretching force; or Torsion, which is a twisting force. Again, Shear is a sliding force.
A good example of Shear would be if you were to place your hands together, palms flat against one another and then rub them together. You would quickly note a small amount of resistance based on the smoothness of your skin. Do the same with a piece of course sandpaper in each hand and the resistance (or shear stress) will be much greater.
The same thing happens within a given fluid. In the earlier example, if you were to coat your hands with a thin fluid like water and then rub them together; The shear stress would be minimal. But if you were to coat your hands with a viscous fluid like honey and rub them together, you would quickly notice a resistance similar to the coarse sandpaper example above.
In the hand rubbing examples, the amount of resistance is quantified and referred to as “Coefficient of Friction”. In the fluid/plate example, it’s called Viscosity…
With the very tight tolerances built into an oval gear meter, the maximum amount of shear occurs at the points where the rotors rotate on each other and the outer walls of the metering chamber. The end result is twofold. Slippage can occur when thin fluids are measured at low flows, while high viscosity fluids can place large forces on the rotors at high speeds, leading to damage to the meter.
In both examples, the shear rate is the rate at which the shear deformation occurs.
Units of Viscosity
Viscosity is typically referred to in two different units of measurement.
- CentiPoise (cP) – A centipoise is one one-hundredth of a poise, and one millipascal-second (mPa:s) in SI units
- Centistokes (cSt) - 1 Stokes = 1 cm2/s
Q) How do I convert from centipoise (cP) to centistokes (cSt)?
A) If you are working with hydrocarbons like fuel or lubricating oil, the conversion is simple. To get centistokes from centipoise, divide by the density (cP/cSt). Since most hydrocarbons will have a density of about 0.85 to 0.9, the centistokes value will be about 10% to 15% higher than the centipoises value.
To go in the other direction, multiply centistokes by the density to get centipoises.
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.