Manual to Digital - METTLER TOLEDO

Manual to Digital

Overview

Exploring Density and Refractive Index Measurement

Your comprehensive source regarding density and refractometry. Explore each tab to learn about common manual and digital methods for characterizing substances and ensuring quality products every time.



Common Terminology


Density (ρ) =
mass of sample/volume of sample
=
m/v


Specific Gravity (SG) =
density of liquid/density of water
=
ρ/ρ0
, typically measured at 20 °C where SG =
ρ/0.9982 g/cm3


Degree Brix = one degree brix is 1 gram of sugar in 100 grams of solution


Refractive Index (RI) = a dimensionless value defined by the ratio of the speed of a light beam in a vacuum to its speed in the substance;
n1/n2
=
sin(𝛽)/sin(𝛼)

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Refractive Index measurement
A) Excitation: magnetic
B) Detection: optical

Industry Regulations and Standards
Compliance for All of Your Measurements

Complying with industry regulations means quality products and safety for your employees and your customers. METTLER TOLEDO instruments provide accurate and reliable measurements while ensuring compliance with international and regional regulations. Learn more about common regulations and which ones our instruments comply with in your industry.


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Lab Expertise Library
Expert Knowledge at Your Fingertips

Want to learn more about density and refractive index measurement? Increase your density and refractometry knowledge by exploring our resources including tips and hints guides, white papers, webinars, and more.


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Manual Density

Manual density methods are quick, simple, and require relatively inexpensive instrumentation. These methods are good for determining rough density values of inexpensive samples. The hydrometer and pycnometer methods are excellent for academic settings where the main objective is teaching the measurement principle.



Hydrometer

A hydrometer, also known as an aerometer, is a glass body that determines the density by being dipped it into a liquid sample. The hydrometer consists of a cylindrical stem and a bulb weighted with mercury or lead shot to make it float upwards.  

After a hydrometer is dipped into the sample, the glass body will float at the level where the mass of the hydrometer is equal to the buoyancy effect. The higher the density of the sample, the less the hydrometer will sink in the liquid sample. The density of the sample is measured by noting the intersection of the calibrated scale on the stem with the surface of the sample.

Main Applications

  • Quick quality control check of an approximate density reading, mainly for process control
  • Suitable for repeated measurements of the same type of sample (wine, beer) due to the limited measuring range
  • Not suitable for expensive samples (large volume required)

Advantages

  • Simple method
  • Quick measurement for ambient temperature readings
  • Relatively inexpensive instrument

Disadvantages

  • Breakable glassware
  • Long and difficult thermostating process
  • High cost of total ownership
  • Very small measuring range (typically 20 units, requires several hydrometers to cover a wider range)
  • Specialized instruments with direct readings in %Alcohol, %Sugar (°Brix) or other density related values available
  • Operator dependent readings, therefore limited accuracy and repeatability
  • Large sample volume required
  • Large amount of sample waste
  • Difficult to clean and dry
  • Intense operator training required

Pycnometer

A pycnometer is a flask of a defined volume used to measure the density of a sample. To measure the density of a liquid, the flask is first weighed without sample (M1). Then it is filled with the sample and weighed again (M2). The difference between M1 and M2 is equal to the mass of the liquid sample. When the mass of the sample is divided by the volume of the flask the density is calculated manually.

Main Applications

  • Educational: what is density and how to measure density
  • Production control: where more precision is required (compared to hydrometer)
  • Analytical labs: where GLP is not required
  • Not suitable for expensive samples (large volume required)

Advantages

  • Simple method
  • Relatively inexpensive instrument

Disadvantages

  • Breakable glassware
  • Long and difficult thermostating process
  • Density must be calculated following measurements (some balances include this calculation)
  • Operator dependent technique, therefore limited accuracy and reliability
  • Large sample volume required
  • Large amount of sample waste
  • Intense operator training required

Density Kit

A density kit is an accessory to be used together with a balance to determine the density of a sample. METTLER TOLEDO's density kit can be used to determine the density of solid, liquid, porous, and viscous substances. The kit contains a support plate, brackets, glass beakers, thermometer, holders for floating and non-floating solids, and a bottle of wetting agent. By combining this kit with a balance, density can be determined using methods utilized by hydrometers and pycnometers. An internal program on the balance is utilized to calculate the density automatically.

Main Applications

  • Density of solids
  • Density of porous materials
  • Density of viscous material (using a gamma sphere)

Advantages

  • No manual reading
  • Capable of GLP printout
  • Relatively inexpensive – accessory to balance

Disadvantages

  • Long and difficult thermostating
  • Large sample volume required

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Digital Density

Digital density meters, also known as densitometers, use oscillation tube technology to accurately measure the density of a sample in a short time. The sample is injected into a U-shaped glass tube and put into oscillation, which stabilizes at a specific frequency. This frequency changes when the tube is filled with various sample types: the higher the mass of the sample, the lower the frequency. This frequency is measured and converted into density automatically on the instrument. Calibration of digital density meters is carried out using distilled water. Digital density meters have a high level of accuracy when compared to manual methods and ensure reliable density measurements. Densitometers are great solutions for expensive liquid samples and quality control of final products.



Measuring tube with defined volumetric capacity
A) Excitation: magnetic
B) Detection: optical

Handheld Density Meters

A handheld density meter is a portable device capable of measuring the density of a liquid. METTLER TOLEDO's portable density meters measure density and specific gravity (SG), as well as alcohol, °Brix, API, °Baumé, °Plato, sulfuric acid (%w/w), or user defined units.

Main Applications

  • Incoming goods inspection
  • Concentration measurement
  • Replacement of hydrometers and pycnometers
  • Battery acid tests
  • Measures °Brix, °Baumé or °Plato in Food and Beverages
  • API compliant measurements of petroleum products (ASTM D7777)
  • Control of electroplating and photo solutions

Advantages

  • Density measurement without needing to transport samples to the lab – increased efficiency
  • Temperature compensation using a temperature correction coefficient
  • Controlled sampling for samples of varying viscosity
  • Only a small sample volume is required for measurement, which is excellent for expensive samples

Disadvantages

  • No thermostating capabilities
  • Not as accurate as benchtop density meters

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Benchtop Density Meters

A benchtop density meter is a modern, compact instrument suitable for measuring density in a wide variety of industries. Most benchtop density meters have a built-in Peltier thermostat to control the temperature very precisely without using a water bath. This temperature of the sample is incorporated in the determination of density on the instrument. To ensure greater measurement accuracy, a benchtop density meter is recommended rather than a handheld device. METTLER TOLEDO's benchtop density meters are a flexible and expandable solution for any density application.

Main Applications

  • Quality control of final products
  • Incoming goods inspection
  • Concentration measurement
  • Purity checks
  • Expensive and limited sample sizes (flavors and fragrances)
  • Alcohol measurement in wine and beer

Advantages

  • Small sample volumes (less than 2 mL)
  • Fast measurement (less than 1 minute)
  • Peltier thermostating
  • High accuracy (up to 0.00002 g/cm3
  • Measurement protocol (GLP)
  • Automation (compatible with sample changer, computer, and multi-parameter solutions)
  • High throughput, short ROI
  • Simple to use and short training needed
  • Operator independent results
  • Durable

Disadvantages

  • Higher upfront cost compared to manual methods, but short ROI

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Manual Refractometry

Manual refractometry methods utilize Snell's Law to determine the index of refraction. A vital component of any refractometer is the prism used to refract the light. These prisms are typically ruby, sapphire, or topaz. The wavelength of the light source affects the refractive index, therefore, it is necessary to filter the light to a specific wavelength, commonly the D-line of Sodium (NaD = 589.3 nm). The index of refraction is dependent upon the temperature of a liquid sample, however thermostating is very difficult using manual methods.

Manual refractometry methods are often used for determining approximate values of samples and are excellent for learning the basic measurement principle.



Example of refraction; drinking straw in water

ABBE Refractometry

An ABBE refractometer is a benchtop device used to measure the refractive index of a sample. ABBE refractometers are named after Ernst Abbe, a German scientist who worked for the Zeiss Company and developed the first laboratory refractometer. Most ABBE refractometers use daylight as the light source for measurement, causing relatively inaccurate results, but some advanced models use a light source with a defined wavelength to increase accuracy.

A few drops of the liquid sample are placed on the measurement prism of the device. After adjusting for compensation, the refractive index is determined by looking through an eyepiece and utilizing the internal scale of the instrument to read where the sample optical line and the built-in scale intersect. Thermostating with ABBE refractometers is challenging because it requires a connection to an external water bath, which can be difficult to maintain accurate temperature control and refractive index measurements. Therefore, most ABBE refractometers are used without a thermostat.

Main Applications

  • Production control
  • Raw material control
  • Academia – teaching measurement principle

Advantages

  • Relatively inexpensive instrument

Disadvantages

  • Long and difficult thermostating (requires external water bath)
  • Operator dependent readings, therefore limited accuracy
  • No measurement protocol (not suitable for GLP/GMP)
  • Tedious calibration

Portable Optical Refractometer

Portable optical refractometers are manual handheld devices used for refractive index measurement. A few drops of the liquid sample are placed on the prism surface of the device. In order to determine the refractive index, the portable optical refractometer is lifted to eye level similar to a telescope. The refractive index is the intersection between the sample optical line and the built-in scale. Thermostating is not feasible with portable optical refractometers, and therefore some models feature a temperature compensation scale. In order to accommodate various applications and their specific concentration measurements (°Brix, salinity, %alcohol, battery acid, etc.), users must have different refractometers with dedicated concentration scales

Main Applications

  • Quick control of an approximate refractive index value, especially for °Brix measurements
  • Sugar control in wine production

Advantages

  • Simple method
  • Inexpensive instrument
  • Special instruments with direct readings in °Brix or salinity

Disadvantages

  • Small measuring range (requires several refractometers to cover a wide range)
  • No thermostating
  • Operator dependent readings, therefore limited accuracy
  • No measurement protocol
  • Potential safety concerns for hazardous materials

Digital Refractometry

Digital refractometers determine the refractive index of a sample automatically without needing to lift the device and read the intersection of a built-in scale as is required for manual refractometers. As a result, digital refractometers are utilized to determine the refractive index of a larger variety of samples, including volatile chemicals. Similar to manual refractometers the sample is placed on a measurement cell containing a prism, which is one of the most crucial elements of the instrument. For most digital refractometers, the angle of total reflection of the sodium D-line (NaD = 589.3 nm) is used as the measurement method. Digital refractometers use an optical sensor to convert the refracted light into a refractive index reading and other desired units, such as %alcohol, °Brix, %w/w, %v/v, specific gravity, freezing point, HFCS, and other user-defined concentration units.

Digital refractometers are simple to use while maintaining high levels of accuracy and reproducibility when compared to manual methods. Refractometers are great solutions for identification and quality control of various types of samples.



Plano convex lens focus and aberration

Handheld Digital Refractometry

A handheld digital refractometer is a portable device that determines the refractive index of a liquid sample automatically. A few drops of the liquid are placed on the measurement cell. For handheld devices, the refractive index is measured digitally and automatically. An advantage to digital handheld refractometers is the ability to read the measurement value without needing to lift the device to the eye.

METTLER TOLEDO's handheld digital refractometers are both convenient and versatile. They are an ideal solution for taking measurements in the field and have the capability to be used as a portable benchtop refractometer. The Refracto 30PX uses a glass prism held by a stainless steel measurement well and the Refracto 30GS uses a sapphire prism held by a gold-plated brass ring allowing it to have the broadest measurement range in comparison to other portable instruments.

Main Applications

  • Quick control of a refractive index value, especially for °Brix measurements
  • Sugar control in wine production

Advantages

  • Simple method
  • Relatively inexpensive instrument
  • Fast measurements
  • Some feature temperature compensation

Disadvantages

  • No thermostating
  • Limited accuracy in comparison to bench top

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Benchtop Digital Refractometry

Benchtop refractometers quickly and accurately determine the refractive index, °Brix, HFCS, and other user-defined concentrations of a wide variety of samples. A high resolution optical sensor measures the total reflection of a light beam emitted by the sample after passing through a sodium interference filter at 589.3nm and a prism.

METTLER TOLEDO Excellence Refractometers provide the most efficient and accurate results for even the most rugged applications. The devices utilize a sapphire prism to allow for the broadest measurement range, a stainless steel SUS316 measurement well for simple cleaning, and a perflouroelastomer seal for applications that involve aggressive chemicals. These refractometers also contain built-in Peltier temperature controls which provide both reliable temperature stability and rapid heating/cooling rates for temperature dependent measurements.

Main Applications

  • Quality control of final products
  • Food and beverage industry – sauces and drinks/juices
  • Pharmaceuticals including liquid drugs

Advantages

  • Automation – faster and more efficient measurements
  • High level of accuracy
  • Limited exposure to sample leads to safer working conditions
  • Wide sample variety including liquids, pastes, and solids
  • Operator independent
  • Durable

Disadvantages

  • Higher upfront cost compared to manual methods, but short ROI

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Multiparameter

METTLER TOLEDO offers an innovative solution for measuring multiple physical properties with just One Click™. The same sample is passed through each instrument using Tygon tubing for seamless automation and laboratory integration. 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 easy integration of other measuring instruments.

Main Applications

  • Flavors and Fragrances
  • Anything you'd require both density and refractive index as well as pH, color, and titration

Advantages

  • Simultaneous measurement of all parameters saves time (70% labor time) and prevents changes in the sample
  • All results are displayed on the touch panel and can be transferred to a PC, LIMS or ERP system
  • Can be used with a sample changer and barcode reader for further automation (increases throughput by 50%)

Disadvantages

  • Relatively high upfront cost compared to manual methods, but short ROI

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