Comparision of different measuring techniques for density and refractometry - METTLER TOLEDO

Comparision of different measuring techniques for density and refractometry

What are the advantages and disadvantages of the different measuring techniques for density and refractometry?
What are the major applications?
Please find the answers below:

 

Density


Hydrometer
The hydrometer (aerometer) is a glass body which is dipped into the sample. After a short equilibration time it will swim at a certain level (when the mass of the hydrometer is equal to the buoyancy effect). The higher the density of the sample, the less the aerometers will sink. The level of equlibration reads the density on the calibrated scale.

Advantages:
Disadvantages:Main applications:
Simple methodBreakable glasswareQuick control of a "rough" density value, mainly for process control

Quick measurement (*)Long and difficult thermostatingSuitable for the measurement of the same kind of sample (wine, beer) due to the limited measuring range
Inexpensive instrumentVery small measuring range (requires several hydrometers to cover a wider range, typically 20 units)

Not suitable for expensive samples (large volume required).
Special instruments with direct readings in Alcohol%, Sugar% (BRIX) or other density related values available

Operator dependent readings, therefore limited accuracy 
(*) if no thermostating requiredNo measurement protocol (not suitable for GLP)

 
 Large sample volume required

 
 Difficult to clean and dry 


Pycnometer
A pycnometer is a glass beaker of defined volume. It is weighed without sample (M1), then filled with the sample and weighed again (M2). The difference between M1 and M2 (=Mass of the sample) divided by the volume of the beaker is the density of the sample.

Advantages:
Disadvantages:Main applications:
Simple methodBreakable glasswareEducational: what is density, how to measure density

Relatively inexpensive instrumentLong and difficult thermostatingProduction control: where more precision is required
Inexpensive instrumentDensity must be calculated (some balances include this calculation)

Analytical labs: where GLP is not required
Special instruments with direct readings in Alcohol%, Sugar% (BRIX) or other density related values available

Operator dependent readings, therefore limited accuracyNot suitable for expensive samples (large volume required).
(*) if no thermostating requiredNo measurement protocol (not suitable for GLP)

 
 Large sample volume required

 


Density kit
The density kit is used together with a balance. A glass body of defined volume is weighed in air (M1), dipped into the sample and weighed again in the sample (M2). The difference between M1 and M2 (buoyancy) divided by the volume of the glass body is the density of the sample. A special holder can also be used to measure density of solids, using a reference liquid (water, ethanol or user defined).

Advantages:
Disadvantages:Main applications:
No manual readingLong and difficult thermostating

Density of solids
GLP printoutLarge sample volume required

Density of porous materials
Just an accessory to a balance, therefore relatively inexpensive Density of viscous material (using a gamma sphere)


Digital density meter
A hollow glass tube vibrates at a certain frequency. This frequency changes when the tube is filled with the sample: the higher the mass of the sample, the lower the frequency. This frequency is measured and converted into density. Calibration is carried out with air and distilled water. A built-in Peltier thermostat controls the temperature very precisely, without using a water bath.

Advantages:
Disadvantages:Main applications:
Build-in user management with optional finger print reader Quality control of final products
Build-in product management with automatic quality control  
Build-in SOP  
Automation (sample changer, computer)  
Extension with other instruments for multiparameter measurement  

Refractometry


Portable optical refractometer
A few drops of the liquid to be measured are put on the prism. The refractive index can be read directly from the built-in scale, looking into the refractometer. Some models feature a temperature compensation scale. There are refractometers for specific concentration measurement with dedicated concentration scales (BRIX, salinity).

Advantages:
Disadvantages:Main applications:
Simple methodSmall measuring range(requires several refractometers to cover a wide range)

Quick control of a "rough" density value, especially for BRIX measurements
 No thermostating

 
Inexpensive instrumentOperator dependent readings, therefore limited accuracy

Sugar control in wine production
Special instruments with direct readings in BRIX° or salinity

No measurement protocol 


ABBE optical refractometer
A few drops of the liquid to be measured are put on the prism. The refractive index can be read directly from the built-in scale, looking into the refractometer. Connection to an external water bath allows thermostating required for accurate refractive index measurements. Some models use natural light, the most advanced a light source with defined wave length.

Advantages:
Disadvantages:Main applications:
Relatively inexpensive instrumentLong and difficult thermostating (requires external water bath)

Production control
 Operator dependent readings, therefore limited accuracy

Raw material control
 No measurement protocol (not suitable for GLP/GMP)

 
 Tedious calibration 


Digital refractometer
The latest in refractive index measurement. A high resolution optical sensor measures the total reflection of a light beam emitted by a special LED light source after hitting the sample. This total reflection is converted into refractive index, BRIX, HFCS or user defined concentrations. A built-in Peltier thermostat controls the temperature (no water bath required).

Advantages:
Disadvantages:Main applications:
Build-in user management with optional finger print reader Quality control of final products

Build-in product management with automatic quality control 

Build-in SOP
  
Extension with other instruments for multiparameter measurement
  

Multiparameter


Combination of density and refractometry
Our digital density meters and refractometers can be combined and extended with other instruments for simultaneous measurement of density, refractive index, pH, conductivity and color. The open architecture allows to easily add any other measuring instrument.


Advantages:
Disadvantages:Main applications:
Simultaneous measurement of all parameters saves time and prevents changes in the sampleExpensiveFlavors and Fragrances
All results are displayed on the touch panel and can be transferred to a PC, LIMS or ERP 

Can be used with a sample changer and barcode reader for further automation