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How To Measure The Level Of Viscous Liquids

viscous tar in a bucketLiquids with high viscosity present a unique challenge to level measurement sensors. While some sensors work just fine, others require a little extra maintenance, and some should simply be avoided.

What is Viscosity?

Before we get into a discussion about which level sensors work best with viscous liquids, let’s take a few moments to understand the definition of viscosity.

Viscosity for a fluid is the measure of its resistance to a gradual change in shape by shear stress or tensile stress. Another way to think of it is the thickness or stickiness of a fluid. For example, water is not a very viscous fluid whereas honey and molten glass are.

Liquid
(@ 25 °C unless specified)
Viscosity (cP)
Methanol 0.544
Water 0.894
Ethanol 1.074
Mercury 1.526
Blood (at 37°C) 3 - 4
Olive Oil 81
Corn Syrup 1,380.60
Ketchup 50,000 - 100,000
Peanut Butter ≈250,000
Pitch 230,000,000,000
The unit of measure for viscosity is cP or mPa·s (one millipascal-second). We've provided a chart of common fluids with their cP measure.

Pitch refers to a variety of super viscous materials like tar, asphalt, and bitumen. Though these may seem to be solids, they are actually liquids. To prove this, the world’s longest ongoing science experiment taking place at the University of Queensland demonstrates the flow of a piece of pitch over many years. The pitch was placed in a funnel and a cup was placed underneath the funnel to catch the drops of pitch as it flowed down. Since 1930 only 9 drops have fallen. The 8th drop fell in 2000 and the 9th fell just last year. You can even watch the 10th drop falling live on a webcam here.

Measuring the Level of Highly Viscous Liquids

Can you imagine trying to measure the level of pitch with a float level transmitter? We can’t either.

As you may have already guessed, some sensors are not suitable for measuring liquids that are highly viscous. Obvious examples are our magnetic float level transmitters. Highly viscous fluids will gum up the stem and most assuredly cause the float to get stuck. But don’t let this change your mind about magnetic float level transmitters in general. Even in applications where you might think the fluid is too viscous, our RPM probe, for example, still might work just fine. Our RPM is used to measure drilling mud, sewage tanks, and works just fine with a variety of oils. So we are confident it will work well with thicker fluids, just don’t use it with fluids like Honey, Corn Syrup, Peanut Butter, or anything that is really sticky.

However, the temperature of the fluid heavily influences viscosity. Take Honey for example, how often do you heat it up in your kitchen to allow it to flow better?

You could then consider using a magnetic float level transmitter to measure warm honey. The “runny” honey won’t cause the float to get stuck. But periodic washing will be required as crystallization on the stem would still occur.

A pressure transducer is another good example of a sensor that would not be ideal for viscous fluids. If the viscous fluid were to dry, it would block the transducers pressure port creating a nightmare of a maintenance project. Furthermore, if you attempt to try to clear out the pressure port you risk damaging the sensor.

Ultimately, the wisest choice would be to use a non-contact solution like an ultrasonic sensor. As long as the fluid, no matter how viscous, doesn’t give off heavy vapors and is relatively flat on the surface, an ultrasonic sensor will work splendidly.

If you have experience dealing with viscous fluids and have some expertise to add, please feel free to contact us. We look forward to hearing from you.


Ultrasonic sensors are extremely versatile! Non-contact measurement opens up a lot of possibilities. Read our guide to learn how ultrasonic programming can make it a perfect fit:

top image credit: Erich Ferdinand via flickr cc

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