Density can be measured analogically using glass hydrometers or pycnometers. This article compares the classical methods to measurements employing digital density meters.
Analog and digital density measurement comparison
Time for measurement and cleaning
Analog density measurement
The time required for a measurement including cleaning is around 10 minutes or longer in analog density measurement. Hydrometers require the sample to be filled into a measuring cylinder, whereas it has to stabilize before you can read the result from the scale and follow up with the necessary steps. For pycnometers the exact filling of the flask and subsequent temperature regulation via a water bath is the most time-consuming part of work. Cleaning might also take a lot of time as the sample has to be removed from quite large surfaces with a suitable cleaning agent.
Digital density measurement
For density measurement on a digital device, the sample is usually taken out of the storage container with the help of a syringe, which is then directly attached to the filling adapter of the instrument. Portable instruments even suck the sample into the measuring cell directly from the storage container via a built-in pump. The temperature is either controlled by the device or the influence compensated by software. Cleaning is done by filling several milliliters of suitable solvent – and benchtop instruments even feature an air pump for drying the measuring cell after cleaning. A total of 3 minutes per measurement including cleaning are realistic.
Risk of errors
Analog density measurement
For hydrometers, there is no common standard operating procedure defined, e.g. there is no specific rule about whether the result has to be read while looking from the top, from the bottom, or straight onto the scale. Different users acquire different results. Additionally, the manual temperature correction via given tables is subject to reading errors.
Pycnometers are always adjusted together with the respective glass stopper, therefore mixing them up would lead to incorrect measurements as well. The density is calculated via a formula, whereby most operators use a PC to calculate the density.
Digital density measurement
As the temperature is either controlled by the device or its influence compensated via software, there is no danger of making a mistake. The only precondition for a successful measurement is to ensure proper sample degassing before measurement, which is also required for the analog density measurement methods. Most benchtop instruments also provide an automatic bubble detection to completely eliminate any user errors.
Sample amount required
Analog density measurement
The amount of sample required for analog density measurement methods like a hydrometer measurement depends on the accuracy required. Cheap, inaccurate hydrometers with a wide measuring range require approximately 150 mL of sample. For a type-approved alcohol measurement with an accuracy of 0.1 %v/v around 300 mL of sample are required as the glass measuring apparatus is also bigger and needs to float within the sample. The question of sample amount is especially relevant for samples that must not be poured back into the sample container after measurement due to danger of microbiological contamination (e.g. wort / must) as a lot of sample has to be disposed of.
Pycnometers are available in different sizes whereby sample volumes are usually defined in the respective operating standards. Volumes around 25 mL to 30 mL are very common.
Digital density measurement
A digital density meter usually requires around 2 mL of sample. If an automatic sample changer is used or the sample is taken directly from the tank with a portable instrument, it has to be considered that several milliliters are caught within the sample hoses.
Documentation & traceability
Analog density measurement
With analog density measurement methods (hydrometers and pycnometers), manual notes have to be taken for documentation. The manual writing procedure also raises the issue of user influence that could only be overcome with the help of two-person integrity.
Digital density measurement
Due to the fact that modern digital density meters allow users to enter sample IDs, the measured results are not only stored on the device but even allocated to the name of the sample. This ensures documentation and traceability of data. Data export and printing is commonly done via Bluetooth, USB or Ethernet. Many benchtop instruments offer the possibility of a user management, electronic signature, and a logging function for all changes made on the instrument. This fulfills traceability requirements e.g. for the pharmaceutical and food industry.
Handling of temperature influence
Analog density measurement
Unlike digital density measurement methods, a hydrometer measurement always requires a separate temperature measurement on the sample. Some hydrometers have a thermometer already integrated. The measured result and temperature are used to manually search for the corrected value in a given table. This step is subject to reading errors.
For a measurement with a pycnometer the sample temperature has to be controlled via a water bath for a defined period of time to ensure full temperature equilibrium.
Digital density measurement
Portable devices provide the possibility of an automatic compensation of the temperature influence on the result, meaning that the sample is measured at the given temperature, e.g. 27 °C but the final result is shown for a reference temperature of the user’s choice e.g. 20 °C. What is required is that the user calculates the temperature coefficient of your sample once. Benchtop density meters go one step further: They have a Peltier thermostat and automatically heat or cool the sample to the required temperature within a very wide range.
Measuring ranges
Analog density measurement
Due to its limited size, the more accurate a hydrometer is, the smaller the measuring range it covers will be, as there is only limited space on the scale. This results in the requirement for more than just one hydrometer for each application (e.g. a set of 20 hydrometers to cover the whole relevant alcohol range from 0 %v/v to 100 %v/v if the accuracy required is 0.1 %v/v).
Digital density measurement
Digital density measurement methods always replaces more than just one hydrometer, as there is no limitation regarding the measuring range. If users e.g. intend to measure alcohol, one density meter can cover 0 %v/v to 100 %v/v.
Handling of different measuring units
Analog density measurement
When measuring different samples according to different measuring units (e.g. alcohol, Brix, SG), a hydrometer for each measuring unit is required.With a pycnometer users always calculate density in a first step and then have to consult a density / concentration table to see what the concentration is. This is when reading errors can potentially occur.
Digital density measurement
Most digital density measurement methods have the most important concentration units and product-specific parameters preinstalled, so users need to switch to another unit and can measure different sample types according to different measuring units. For all measuring units installed, a digital instrument will always provide the full relevant measuring range.
Reason 8: Robustness and life-expectancy
Analog density measurement
Due to their design, glass hydrometers are very prone to rolling off tables and breaking.
Digital density measurement
Many digital density measurement methods have measuring cells made out of glass. However, either they are designed as a benchtop instrument which will have a stable and fixed position on the bench, or they can be equipped with appropriate protection measures – like protective housings or carrying straps – to ensure they withstand bumps and knocks.
Measurement of viscous samples
Analog density measurement
Using a floating measurement device with viscous samples is user-friendly, nor produces repeatable results. Heating up the viscous sample helps ensure easier handling but makes it necessary to compensate the measured result manually via the correction table.
Filling a pycnometer flask – including the capillary in the stopper – with a viscous fluid is a challenge. This is the point where the limits of analog density measurement methods are reached.
Digital density measurement
Digital instruments provide the possibility to heat the sample up to a temperature where the viscosity decreases and facilitates the measurement. There are also heated sample changers available which include heated hoses, making them suitable for samples like crude oil or heavy fuel oil. For samples like bitumen or asphalt special density meters with the capability to measure the sample at up to 200 °C under pressure are used that are pre-heated in the oven before measuring them. Also with portable devices users still have to heat the sample manually and make sure to be quick so that no sample freezes within the measuring cell. All modern digital density meters provide a viscosity correction of the measured result, meaning that the influence of the viscosity on the result is detected and corrected by the software.
Reason 10: Cleaning
Analog density measurement
With both, hydrometers and pycnometers users will have to clean quite large surfaces with a suitable cleaning agent. This is not environmetall friendly and costs a lot of time.
Digital density measurement
When doing a measurement series of similar samples users can replace a sample directly with the next sample. To make sure that there will not be any carryovers users just need to use enough sample volume. If this is not applicable or a high accuracy and / or the need to clean the instrument after a measurement series are required, a syringe filled with a suitable cleaning agent is used and the sample is removed from the cell by moving the plunger forth and back. No mechanical action is used to get rid of sample residues in the cell. Benchtop instruments also provide an air pump for drying the cell after cleaning.