Good Density Measurement™

Since 1967 Anton Paar GmbH has specialized in providing highly accurate and reliable density meters for research and industry.

This brochure sums up our experience and insights into measurement practice gained in over forty years.

Follow these guidelines and you will be well on your way to accurate and reproducible density results.

Good Density Measurement™ requires care and attention in five basic areas: The water check, adjustment, sample preparation, sample filling, and cleaning.

If you carry out density checks at regular intervals you can ensure the high and stable accuracy of your density and concentration measurements.

• Fill ultra-pure (e.g. bi-distilled or deionized), freshly degassed water into the measuring cell.
• Start a measurement.
• Compare the determined density with the referencevalue: ρ_Water = 0.998203 g/cm³ | for T = 20 °C

The water check fails if the measured density value is not within a required tolerance range. The tolerance range depends on the application and is more stringent in the pharmaceutical industry than in the soft drink industry, for instance.

Example

A common tolerance limit for soft drinks is ± 1 x 10^-4 g/cm³. This means the water check is positive if the determined density is between 0.9981 g/cm³ and 0.9983 g/cm³.

If the water check fails, try the following:

Use fresh ultra-pure water.

• Repeat the water check.

If the water check still fails:

• Clean the measuring cell thoroughly.
• Repeat the water check using fresh ultra-pure water.

If the water check still fails:

• Perform an air/water adjustment.

An adjustment changes the instrument constants. For the consistency and comparability of results an adjustment should only be performed if the water check fails and using fresh water and cleaning the measuring cell does not help. Poor cleaning is the cause of the majority of measuring errors – an adjustment should be seen as a last resort.

• Common adjustment media are dry air and ultrapure (e.g. bi-distilled), freshly degassed water.
• Follow the automatic adjustment procedure on your instrument.
• Make a record in your adjustment logbook.

Reasons why the side effects of cleaning need to be compensated by an adjustment:

There are different methods for the degassing of liquid samples. The best method for your application depends on the kind of sample, the kind of gas, and the amount of gas that is dissolved in the sample. Be aware of the fact that you may slightly change the composition of many samples during sample preparation due to evaporation of volatile components.

• Observe all safety regulations regarding the handling of the samples, cleaning, rinsing and waste liquids (e.g. use of safety glasses, gloves, respiratory protection, etc.).
• Check the chemical resistance of all materials which come into contact with the sample before starting the measurement.

• Heat up your sample to get lower viscosities.
• For highly viscous samples use a heating attachment to prevent sample freezing in the sample inlet and outlet.
• If you use a sample filling system, check whether the viscosity corresponds to the given specifications.

• Close the sample vials with caps.
• Gently swirl the vials to bring condensed droplets back into the bulk liquid.
• For highly volatile samples use a sample filling unit which supports sample filling under pressure.

Using sample filling units is the only way of eliminating filling errors due to the operator. As sample filling units repeat measurements in the same way every time, this is the best way to get repeatable results. Even critical samples, like highly viscous samples or samples with volatile components, are filled smoothly. Some sample filling units additionally support automatic cleaning.

Workflow

• Fill your samples in the corresponding sample vials and prepare the magazine.
• If your sample unit supports automatic cleaning, make sure that sufficient cleaning liquid is available.
• Empty the waste container before starting a series of measurements.
• Check the instrument settings.
• Prepare the sample list for your instrument.
• Start the measurement.

Using a syringe is the traditional way of filling a sample into a density meter. It requires some training to achieve repeatable results and to avoid bubbles in the measuring cell.

Workflow

• Press the plunger smoothly and slowly without stopping.
• Check whether the measuring cell is filled without bubbles.
• Check the instrument settings.
• Start the measurement.

For paste-like materials, always use a syringe. If the samples have a very high viscosity, you can fill them into the syringe by pulling the plunger completely out of the syringe, filling it from the back using a spoon and then inserting the plunger again.

• you perform adjustments,
• you measure a sample that is not miscible with the previous sample (e.g. water after a petrochemical sample),
• you want to use a minimum sample amount for your measurement,
• you measure a sample that could chemically react with the previous sample.

The best way to clean your instrument is by using sample filling units which support automatic cleaning. In this case make sure that the applied cleaning liquids suit your samples.

Workflow

•  Clean the measuring cell using two cleaning liquids:
  • Cleaning liquid 1: Cleaning liquid 1 dissolves and removes sample residues in the measuring cell. It needs to be a good solvent for all sample components.
  • Cleaning liquid 2: Cleaning liquid 2 removes cleaning liquid 1 and is easily evaporated by a stream of dry air in order to accelerate drying of the cell. Cleaning liquid 2 needs to be a good solvent for cleaning liquid 1.
• Dry the measuring cell by activating the internal air pump.
• Check whether the cleaning and drying was successful by measuring the density of air (= air check).
• Compare the determined density with the reference value: ρ_Air = 0.001199 g/cm³ | for T = 20 °C, p = 1013 mbar

* Using enzymatic lab cleaner is recommended for special cleaning. After using the lab cleaner you need to rinse the measuring cell with water.