FAQ: “Where do HunterLab sphere instruments “top-out” as far as APHA measurements go? I have a request for 0-3000 APHA.”
HunterLab instruments can measure above APHA 500 but there is background information you should be aware of in this high yellow range.
When the APHA/Pt-Co/Hazen color scale is defined in reference documents such as ASTM D1209, the typical range is from 0 (distilled water) to a distinct yellow at 500.
Another name used for this same color scale is “Hazen Color”. When this name is used in a specification, the range is often above the typical 500 units associated with the APHA/Pt-Co, such as “1500 Hazen Color”. However, it is all the same yellow color scale for yellow liquids.
The instrumental correlation to APHA/Pt-Co/Hazen Color in HunterLab sphere instruments can measure above 500, including up to and beyond 3000. We would typically recommend using a 10 mm path length cell for this highly chromatic yellow sample.
It should be noted that when a liquid sample is a deeply saturated yellow color, there is more color information available than just the yellowness quantified by the APHA/Pt-Co/Hazen value. Relative to distilled water, the color will be much darker, much more yellow and tinted red or green. While a high APHA/Pt-Co/Hazen value does measure the intense yellowness above 500, a better and full descriptive metric to quantify all aspects of this high color would be CIE L*, a*, b*. It is recommended that CIE L*, a*, b* values be reported along with the APHA/Pt-Co/Hazen value for APHA measurements > 500.
The type of application where using a single number APHA/Pt-Co/Hazen Color rating > 500 is appropriate is when a process starts out with a liquid sample that is highly yellow, then gradually extracts the impurities that create the yellow color eventually leading to an end point where a low APHA/Pt-Co/Hazen value quantifies that the liquid sample is near “water white”.
While difficult to find, there are APHA/Pt-Co/Hazen liquid color standards that can be used to validate color measurement systems where the sample color APHA/Pt-Co/Hazen value is 500.