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The Essential Guide for Measuring Density

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Density Measurement Guide - How to Achieve the Best Results in Day-to-Day Density Measurement
Density Measurement Guide - How to Achieve the Best Results in Day-to-Day Density Measurement

This guide explains what precautions should be taken to prevent errors when measuring the density, specific gravity or concentration of liquids. Following its recommendations will allow you to optimize density measurements, particularly those made with digital density meters. The following topics are covered:

  • Tests and adjustments: Which is better, regular tests or regular adjustments? How often should a density meter be tested or adjusted, and with which substance(s)? Which tolerance should be applied?
  • Samples: Which types of samples can be measured with a digital density meter? How should difficult samples be measured? What are possible effects on measurements?
  • Sampling: What is the difference between sampling with a syringe and with an automatic filling system. How can air bubbles be avoided?
  • Cleaning: What are the best methods of cleaning the cell? Which solvents should be used for each sample type? What is the effect of improper cleaning on measurement accuracy?
  • Verification and documentation of results: How to ensure that a measurement has not been affected by air bubbles or residual solvent; how to convert results into other units or concentrations; how to automatically verify if a result is within specifications for a given product (quality control).
Download the Density Measurement Guide and get Tips and Hints to avoid errors when measuring the density of liquids

1. Test and adjustments

The commonly held opinion that frequent adjustment of an instrument guarantees accurate results is not true. Any adjustment results in changes being made to the internal instrument settings. If the adjustment is not properly performed, all the measurements performed afterwards will be invalid.

Instead of frequent adjustment, it is better to regularly verify the measuring accuracy of the system with a sample of accurately known density (e.g. distilled water or a standard) which is called test, calibration or check. The density measured is then compared to the known nominal value of the sample.

 

Discover more information in the Density Measurement Guide

 

One Click test with standards – Video

See how to run a test with a standard to check that your density meter or refractometer is still working properly.

Test (Calibration)

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Adjustment

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2. Samples

Viscous samples

Be careful when measuring viscous samples; make sure that they do not contain air bubbles when you introduce them into the measuring cell. It often helps to warm the sample in a closed vessel and to allow it to stand for a few minutes before the measurement. The viscosity of the sample decreases on warming and air bubbles can more easily escape.

The shear forces that arise when viscous samples are measured can lead to inaccurate results that are typically artificially high. All METTLER TOLEDO DM Density Meters are able to automatically correct measurement errors owing to sample viscosity. If your density meter has such a viscosity correction function, you should always switch it on if you want to obtain the best possible accuracy for samples with a viscosity greater than 25 mPa·s.

METTLER TOLEDO SC1 and SC30 automation units are able to handle samples with a viscosity of up to 30’000 mPa·s (approximately the equivalent of liquid honey). For samples which have a higher viscosity or which are solid at room temperature (e.g. paraffin) the heated versions of the automation unit, SC1H and SC30H, can be used.

Discover more information in the Density Measurement Guide

Aggressive samples

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Volatile samples

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Samples with dissolved gases

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Non-homogeneous samples/suspensions

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3. Sampling

With a syringe

Manual sample handling with a syringe is always operator-dependent, which means that it is error-prone, difficult to reproduce, and time-consuming. Make sure that your operator is properly trained and injects the sample in a reproducible manner. 

Use an excess of sample to fill the cell! Existing contamination in the cell may result in invalid measurements:  

Make sure that the sample fills at least 10 cm of tubing beyond the cell so that contamination can be pushed out and only new sample is left in the cell. 

Fill the measuring cell slowly and with laminar flow (5–10 cm per second) to ensure complete wetting of the cell walls (no trapped bubbles along the walls). Make sure that no air is trapped within the syringe. The plunger must be pressed slowly and continuously without stopping.

Discover more information in the Density Measurement Guide

Automatic filling

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Avoid bubbles

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Automated density determination – Video

Switch from syringe to the SC1 automation unit to improve your density measurements, increase accuracy and avoid operator-dependent results.

4. Cleaning

Deposits of previously measured products are not always visible. For example, if products containing oil or fat are measured, the measuring cell may be coated with a very thin film of oil. To prevent this, the cell should be cleaned with suitable rinsing solutions (as described later) and dried – preferably after each measurement.
If all samples measured are of similar type and able to dissolve the residues in the measuring cell (e.g. during measurement of a variety of different juices), it is also possible to do a large over-sampling with the new sample to ensure complete removal of the previous (“analytical rinse”).

Restrictions:

  • Use at least a sampling pump (e.g. METTLER TOLEDO FillPal™); effective over-sampling is difficult to achieve with a syringe.
  • Immerse the sampling tube of the pump in the sample, then remove it so that air is sucked into the tube (~2–3 cm air in the tube) and immerse it again in the sample. Repeat this procedure approximately 5 times before the cell is filled for the measurement. This ensures that the old sample is properly flushed out of the cell.
  • Verify the procedure to make sure that the repeatability and error limits required are maintained. Measure the most critical sample first (for instance, the one with the highest sugar content), followed by deionized water, and repeat this step a couple of times.
  • If for instance sugar-containing products are measured, make sure that the cell remains filled with either sample or water between measurements to prevent drying out of the sample and crystallized sugar on the cell walls.
  • Completely clean and dry (as described below) the measuring cell at least once after each working day.

Discover more information in the Density Measurement Guide

Rinse

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Dry

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Fully automatic cleaning

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Automated density determination – Video

Switch from syringe to the SC1 automation unit to improve your density measurements, increase accuracy and avoid operator-dependent results.

5. Verification and documentation of results

Automatic conversion of results

Often a result must be converted using a table. Looking values up in a table, or interpolating from one, is error-prone and time-consuming. Automatic conversion using built-in tables (e.g. alcohol, Brix, temperature compensation according to API) prevents reading or calculation errors and saves time. A digital density meter of the latest generation allows the use of built-in conversion tables to show results directly in the desired units. METTLER TOLEDO DM Density Meters have the following built-in result units / concentration tables:

  • Specific gravity, light and heavy Baumé, T waddell
  • Sugar: Plato, Brix (Emmerich, NBS 113), HFCS 42/55, Invert Sugar, KMW, Oechsle, Babo
  • Alcohol: OIML, AOAC, Proof Degree, HM C&E, Gay Lussac
  • Petro: API degrees and gravity tables for crude oils, refined products and lubricants
  • Up to 30 user-defined concentration tables (can be entered as tables or formulas)

Discover more information in the Density Measurement Guide

Error detection

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Result limits

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Proper Documentation

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