Measuring Density of Viscous Samples: Four Challenges – Four Solutions

The oscillating U-tube technology is known to be a powerful tool for density measurement of liquids. But did you know that you can measure even viscous pastes and creams? In this article I will provide you with useful tips and tricks on four topics for performing a successful measurement.

Table of contents:

What are pasty samples?

Cosmetics

Shampoo, toothpaste, lipstick, facial cream, hand balm, and body lotion – even the density of sticky lip gloss and thick makeup has to be checked regularly.

Food

Ketchup, mustard, and mayonnaise – this is viscous deliciousness in tubes. Salad dressing, yoghurt drinks, and BBQ sauce can be measured too.

Others

Ointments in the pharmaceutical industry, vehicle paint and coatings in the chemical industry, and lubricants in the petroleum industry require density measurement as a means of quality control.

Four challenges that come along

Digital density measurement using the oscillating U-tube technology is my method of choice for quick, reliable results. However, I had to learn that viscous pastes are not as easy to measure as aqueous samples. Filling the measuring cell without bubbles seemed impossible to me and cleaning after having measured mascara troubled me every time – even getting the sample into the syringe without messing up my workplace took a lot of practice.

1. Taking a sample

Have you ever tried to draw up toothpaste into a syringe? It’s absolutely impossible. However, a properly filled syringe is essential for bringing the sample into the measuring cell without air bubbles.

2. Bubbles in the measuring cell

In order to get a correct measurement, it is necessary to fill the measuring cell homogeneously. Every air bubble influences the oscillation of the U-tube and hence changes the output value.

3. Viscosity

Try to imagine honey dripping from a spoon and compare it to water dripping from the tap. Honey is very thick and sticky; it takes very long for one drop to separate itself from the spoon to fall down. Water is fluid and moves very fast. This property is called viscosity and has an influence on the oscillation frequency of the U-tube.

4. Cleaning

24-hour makeup and waterproof lipstick are made to last – unfortunately they like to stick on measuring cells of density meters, too. Cleaning the instrument after measuring those samples can be very annoying and a good cleaning procedure is necessary.

Four solutions for easier measurements

By taking into account some tips and tricks, measuring pasty samples will be as easy for you as measuring any other substance. I found solutions for every step of the way – filling, measuring, and cleaning. Are you a user of Anton Paar instruments? Get the full Standard Operating Procedures here!

1. Taking a sample

Have you ever tried to draw up toothpaste into a syringe? It’s absolutely impossible. However, a properly filled syringe is essential for bringing the sample into the measuring cell without air bubbles.

2. Filling the measuring cell

There are several steps you can take to fill the measuring cell homogenously.

  • Fill the measurement cell carefully and make sure even the waste hose is filled completely. This increases the pressure slightly and helps you avoid bubbles.
  • If your sample is very viscous and it is hard for you to move the plunger, you can use a syringe clamp. This is a device you can fix onto your syringe and by turning the screw it is possible to fill your density meter even with viscous samples.
  • For bubble-free filling, fill the sample using a syringe clamp until it reaches the middle part of the U-tube. Close the outlet silicone hose with a plug; this will build up pressure in the cell and prevent air bubbles.

3. Useful features

  • Camera image: many digital density meters have a preinstalled camera, which records live images of your measuring cell before and during the measurement. You can see immediately if your sample is filled properly.
  • Air bubble detection: using advanced analysis, high precision instruments are able to tell you whenever the measurement cell is filled inhomogeneously.

4. Cleaning

Use two cleaning agents to clean your instrument properly. The first one should dissolve and remove your sample. If you are not sure which cleaning agent is suitable for your sample, take a glass plate, put a little bit of sample onto it and apply different cleaning agents. You will see immediately which one is best.

The second cleaning agent needs to remove the other cleaning agent and should be evaporated easily to ensure the measuring cell is quickly dried.

Conclusion

Now you are ready to measure viscous samples with your density meter!
If you apply the correct filling technique and follow our cleaning recommendation, nothing will stand in your way.

Do you have a very tricky sample to measure?
Write to us in the comments section and we will get back to you.

Are you interested in a density meter for pasty samples?

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