The APHA, Gardner and Lovibond® colour numbers
The method of visual comparison with physical standards has two inherent weaknesses – one is the variation in colour standards depending on edition or age, the other is the variables in the human observer. To rule out these weak points, there are instrumental measurement methods for determining the colour number. These are samples from the chemical and pharmaceutical industries, but colour standards are also used in the wastewater and plastics sectors.
Sample types can be: Lacquers – Oils – Solutions – Suspensions – Fats – Fatty acids – Resins – Syrups – Waxes
To this end, the ASTM, for example, has clearly defined the assignments of the visual standards to the colour values measured on the device. In principle, all three colour numbers are measured in cuvettes or ISO vials in total transmission (TTRAN), as the samples are transparent liquids. Semi-solid substances such as waxes or Vaseline are tested in a molten state.
The right device
The Vista colorimeter is ideal for measuring these colour numbers. This is a double-beam spectrophotometer with diffuse/0° measuring geometry, which can be used to measure liquid samples (and transparent solid samples if required) in transmission with simple handling. This device can be used to examine samples with all layer thicknesses from 10 mm to 50 mm. The selection of the colour number, together with the examined layer thickness, is referred to as the index. After the measurement, the colour values of the sample and various colour scales and indices, in particular the APHA, Gardner and Lovibond® indices, can be read off.
Selection of the cuvette
Depending on the number of colours, different indices are available for cuvettes, ISO vials, microcuvettes and ultra-microcuvettes with path lengths from 10 mm to 50 mm. This means that samples can be reliably measured in various simple or acid- or temperature-resistant plastic or glass cuvettes with the appropriate cuvette holders. The Vista offers the special option of calculating the output value for a coating thickness other than the measured one in order to obtain values in accordance with ASTM, DIN and ISO standards. This often eliminates the need to purchase expensive special cuvettes.
Practical example of the Lovibond® colour number measuring method
Read in our current HunterLab NEWS magazine how we measure colour numbers with the Vista transmission colorimeter, what to look out for in terms of sample handling and what alternatives are available. The magazine also contains other exciting articles on the subject of colour measurement of liquids and solids.
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Conclusion
Liquids from almost colorless to yellowish, yellow-green or reddish, classification and reproduction is simple and standard-compliant with the HunterLab Vista.
Would you like to know how we can determine the colour values of your sample? Get in touch with us, our colorimetry experts look forward to hearing from you.
