Can performing density measurement be compared to driving a Formula 1 racing car?
It’s certainly true that if you are not a trained and professional racer (i.e. you do not follow sample handling rules in density measurement) or the road is not perfectly even (i.e. poor sample preparation or contaminations) you will probably not become a Formula 1 world champion even if you drive the best car available. As far as density measurement is concerned, if you don’t follow strict sample handling guidelines and your sample is poorly prepared or contaminated, you won’t achieve the best density results.
Density: one parameter, many industries
Digital density meters are used all over the world in the quality control of incoming raw material or finished products, for product characterization, and to monitor production processes. This already suggests the tremendous variety of different liquid and gaseous samples. The only thing all these samples in different industries have in common is the need for quick and precise density or concentration results.
What do you need to consider to achieve flawless density measurement in real-life scenarios? Awareness of all the affected elements and the right consideration of sample, instrument, and process along with basic understanding of metrology will lead you to great density results.
What do you need to pay attention to concerning the sample?
Consider the sample´s nature
The characteristics and composition of samples to be measured are at least as diverse as the industries in which they are produced. The quality parameter density is sometimes needed for samples which are (almost) solid or for samples which are highly volatile at room temperature. Unwanted gases such as CO2 dissolved in the sample or unevenly distributed particles also require special attention. An important requirement to obtaining reliable and stable results with any type of sample is a homogeneous composition. This requires excellent sample preparation…
What does excellent sample preparation entail?
Removing dissolved gases prior to measurement is possible by stirring, shaking, sonification or boiling. Before using any of these procedures you need to consider the impact on the sample as well. For example: while sonification can be used to degas sea water, boiling would not be appropriate since this would increase the concentration of salt as water evaporates.
If the sample contains any kind of particles, filtration is a suitable method of sample preparation.
For higher sample throughput and easier handling, the viscosity of samples is decreased by heating them up prior to measurement. Again, special attention is required to avoid changing the sample’s composition by accidental evaporation. Samples with highly volatile components (e.g. jet fuel) are measured either under pressure or at low temperatures to make sure the composition is not changed by accident.
What about sample filling?
Depending on the amount of samples to be measured regularly, you can use either manual or automated filling procedures. Samples such as LPG or aerosols are even filled semi-automatically directly out of a pressurized container into the density meter, which keeps the gaseous sample liquid and therefore stable during measurement.
The importance of the right cleaning
The neutral status of a digital density meter requires an empty and dry U-tube. To verify this condition, you need to carry out an air check (i.e. density measurement of air with subsequent comparison to a known reference air density value). If the result of this check is “passed”, the density meter is ready for storage, shipment, or temporary downtime.
To develop and define cleaning procedures which are suitable for your particular samples it is best to use a clean and empty glass plate. Pour a little sample onto this glass plate and flush it off with different solvents to see which solvent works best. The solvent which dissolves the sample without you needing to wipe it off is the one you should use for cleaning the sample out of the density meter’s U-tube (as long as it is chemically compatible). A second volatile solvent is then used to remove the first solvent and dry the U-tube afterwards.
What do you need to consider about the instrument?
Checks, i.e. calibrations, are a means to verify the validity of an instrument’s measuring results. A sample with known characteristics is measured and compared to a trusted reference value. A calibration (in contrast to an adjustment, see below) does not change anything on the measuring setup. Suitable substances for calibrations are air and water since they are stable, cheap, available everywhere, and easy to provide in a pure state.
In contrast to calibrations, adjustments do modify the measuring setup which, in the case of a density meter, means modifying the adjustment parameters. Adjustment parameters are used to precisely calculate the density based on the U-tube’s oscillation characteristics when measuring samples for which the density is not yet known. For the determination of these adjustment parameters you need at least two samples with known densities.
Environment & location
Since density is highly dependent on temperature, it is obvious that highly accurate density measurement requires highly accurate temperature regulation. A digital density meter should therefore not be placed in direct sunlight, near a heater, in a drafty place, or close to an air-conditioning vent or unit.
Also, the lab bench should be stable as any vibrations might interfere with the oscillation of the U-tube density sensor and lead to incorrect results.
For long-term stability, it is also recommended to leave the instrument switched on even if it is not used, for example overnight. This is due to the fact that glass is not a solid but a non-crystalline amorphous solid whose characteristics benefit from stable temperature conditions.
If used at very low temperatures or high levels of humidity, the temperature-regulated measuring cell’s environment produces condensed water which might even freeze at temperatures below 0 °C. This has to be accounted for by means of a drying cartridge helping to provide dry air for the U-tube’s environment.
Metrology: what you need to know about the science behind the measurements
Often people talk about precision when they actually mean accuracy – or repeatability when they actually mean reproducibility. Luckily, these terms have recognized definitions. The most important are given below.
The accuracy expresses qualitatively how close the measurement result comes to the true value of a measurand.
The quantitative measure of accuracy is expressed by the term “uncertainty of measurement”.
The precision expresses qualitatively how close the measurement results are to each other under given measurement conditions. Precision can be stated under repeatability or reproducibility conditions.
The uncertainty of a measurement specifies an interval within which the true value of the measurand is expected.
The “Uncertainty of measurement” includes instrumental measurement uncertainty (arising from the measuring instrument), uncertainty of the calibration standards, and uncertainty due to the measurement process (sample preparation, sample filling, etc.).
The repeatability is defined as the closeness of the agreement between the results of successive measurements of the same measurand carried out under the same measurement conditions. These ideal conditions lead to a minimum dispersion of the measurement results.
The reproducibility is defined as the closeness of the agreement between the results of measurements of the same measurand carried out under different measurement conditions.
These conditions lead to a maximum dispersion of the measurement results.
5 steps to perfect density results
As you can see, there are a number of factors to consider when carrying out density measurements. However, if you follow the 5 steps below you will have your measuring procedure under control.
Are you interested in a free density guide?
Anton Paar launched the first-ever digital density meter in 1967. Since then company has grown enormously and the density meters have developed and become the modular and versatile instruments you find in thousands of laboratories today.
The Good Density Measurement™ website created by Anton Paar provides essential guidelines and helpful tips for accurate results achieved by digital density meters, whatever your sample.
Your free printed copy of the Good Density Measurement™ guide is just a click away.
Would you like more information on Anton Paar’s density meters which are still going strong (in a new instrument generation) after 50 years of innovation?