Density measurement methods: 10 good reasons for a digital density meter

10 reasons to change from glass hydrometers and pycnometers to a digital density meter, the most comprehensive of all density measurement methods

Several years ago, I put myself in a very uncomfortable situation: I was preparing the market launch for a new digital density meter and asked a potential customer – who was using glass hydrometers – whether we could shoot the product video at his small distillery.

Disaster!

During preparation it happened: I broke the distiller’s glass hydrometer. What could I do? I offered to buy him a new one. Working at Anton Paar, I never thought I would ever purchase a glass hydrometer! I didn’t even know where I could order one. But in the end I gave him a brand new glass hydrometer in the same poor quality as the old one, and the distiller was very amused.

I’ll forgive you,” he said, “but only if you leave the prototype of your digital device here instead of the hydrometer!

Why did he make the change?

The distiller in question saw our work on the video and realized how a digital density meter could benefit him.  Listed below are ten reasons why it may be time for you to make the change and go for a digital density measurement method.

Reason 1: Time saved on measurement and cleaning

No matter which analog density measurement method we are thinking of: the time required for a measurement including cleaning is around 10 minutes or longer. 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.

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.

Reason 2: Independent of user, reduced risk of errors compared to other density measurement methods

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. It’s a matter of fact that different users will acquire different results. What’s more, the manual temperature correction via given tables is subject to reading errors.

Pycnometers are always adjusted together with the respective glass stopper, so 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.

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 even provide an automatic bubble detection to completely eliminate any user errors.

Reason 3: Low sample amount required

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 approx. 150 mL of sample. For a type-approved alcohol measurement with an accuracy of 0.1 %v/v you will require around 300 mL of sample 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 valuable 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.

A digital density meter usually requires around 2 mL of sample. If you use an automatic sample changer or you take the sample directly from the tank with a portable instrument, you also have to consider that several milliliters are caught within the sample hoses.

Reason 4: Full documentation & traceability

With analog density measurement methods (hydrometers and pycnometers), you still have to take manual notes and you have to take care not to make any mistakes when noting the values. The manual writing procedure also raises the issue of user influence that you could only overcome with the help of two-person integrity.

Due to the fact that modern digital density meters allow you 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 the best possible documentation and traceability of data. Data export and printing is commonly done via Bluetooth, USB or Ethernet. Many benchtop instruments already 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.

Reason 5: Easy handling of temperature influence

Unlike digital density measurement methods, a hydrometer measurement always requires a separate temperature measurement on the sample. Luckily, 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 even has to be controlled via a water bath for a defined period of time to ensure full temperature equilibrium.

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 your choice e.g. 20 °C. All you need is to calculate the temperature coefficient of your sample once. Benchtop density meters go one step further: They have a Peltier thermostat and automatically heat or cool your sample to the required temperature within a very wide range.

Reason 6: Wide measuring ranges and numerous units

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 (you will need 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 methods always replaces more than just one hydrometer, as there is no limitation regarding the measuring range. If you e.g. intend to measure alcohol, one density meter can cover 0 %v/v to 100 %v/v.

Reason 7: Handling of different measuring units

If you need to measure different samples according to different measuring units (e.g. alcohol, Brix, SG), you will require a hydrometer for each measuring unit.

With a pycnometer you will 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 occur.

Most digital density measurement methods have the most important concentration units and product-specific parameters preinstalled, so you just 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 long life

There’s a popular statement on the robustness of hydrometers saying: “There are only two types of hydrometers: Those that are already broken, and those that will break soon.”

It’s true! Due to their design they roll off the table far too easily! (I speak from experience…)

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.

Reason 9: Easy measurement of viscous samples

You can already imagine that using a floating measurement device with viscous samples will not be user-friendly, nor produce repeatable results. Heating up the viscous sample will help 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 definitely the point where you get to know the limits of analog density measurement methods.

Digital instruments provide the possibility to heat the sample up to a temperature where the viscosity decreases and facilitates the measurement. There are even 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 you 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: Simple cleaning

 

I hope I will never have to clean a pycnometer after an asphalt measurement! With both, hydrometers and pycnometers you will have to clean quite large surfaces with a suitable cleaning agent. That is something that will never be environmentally friendly or time saving!

When doing a measurement series of similar samples you can replace a sample directly with the next sample. To make sure you do not face any carryovers you just need to use enough sample volume. If this is not applicable or you require a high accuracy or need to clean the instrument after a measurement series, you use a syringe filled with suitable cleaning agent and remove the sample 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 even provide an air pump for drying the cell after cleaning.

What’s bugging you about your density measuring device?
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