Density Measurement Guide

• Test and Adjustments: What is best, regular tests or regular adjustments? How often should the density meter be tested or adjusted, with which substance? Which tolerance should be applied?
  

• Samples: Which types of sample can be measured with a digital density meter? Which are the difficult samples and how to measure them? What can be the effect on the measurement?
  

• Sampling: Difference between sampling with a syringe and with automatic filling system. How to avoid air bubbles?
  

• Cleaning: How should the cell be cleaned, depending on the sample? Which solvents should be used? What is the effect of improper cleaning on the measurement accuracy?
  

• Result verification and documentation: How to make sure that the measurement has not been affected by air bubbles or remaining solvent? How to convert the result in other units or concentrations? How to automatically verify if the result is within product specification (Quality Control)?

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

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

Viscous samples

Be careful when measuring viscous samples and make sure that the samples 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 in the measuring cell can lead to inaccurate results. The value of the density displayed by the meter is then usually too high. All METTLER TOLEDO DM Density Meters are able to automatically correct the sample viscosity measurement error. If your density meter has such viscosity correction, 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 (which equals approximately liquid honey). For samples which have a higher viscosity or which are even solid at room temperature (e.g. paraffine) the heated versions SC1H and SC30H can be used.

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.  

Do NOT fill the cell just barely! Possible contamination would remain in the cell and yield a false measurement:  

Make sure that the sample comes out of the cell at least an extra 10 cm, so that contamination can be pushed out and only new sample is left in the cell.  

Fill the measuring cell at a slow speed and with a 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 entrapped in the syringe. The plunger has to be pressed slowly and continuously without stopping.