Why does my APHA/Pt-CO/Haze Color Standard reads half (or twice) what it should?

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FAQ: “We recently purchased an EasyMatch QC software and installed it on my computer. However my water standard is running within spec, but an 25 APHA Color Standard is reading at 11 APHA. Can you help?”

If you read back the cell + DI water as a PQ Performance Qualification step and get APHA = 0 closely, that is a good first step.

Reading a 25 APHA liquid color standard as a second PQ step and getting 11 suggests that you may have selected the wrong APHA metric for the Color Data display. EZMQC has APHA correlations for 10 mm, 20 mm and 50 mm cell path lengths and you have to select the correct metric to match your cell path length.

It sounds like you are using "APHA-20mm" but reading in a 10 mm path length cell, or using "APHA-50mm" but nearing in a 20 mm path length cell.

In Color Data View/Right Click/Configure/Indices, Select the correct APHA metric that matches the path length of the cell you are using; re-standardize in TTRAN, LAV (Large Area of View - 0.78 in/25 mm, UV Filter Nominal; then read back both distilled water (should read APHA = 0 closely) and the liquid APHA Color Standard (should be close to nominal value of 25).

The same issue would apply if your reading of your liquid APHA Color Standard was twice the assigned value. Make sure your APHA metric matches your cell path length.

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