Ok, if I'm unstanding this correctly, the measure of uncertainty for the hanna dkh checker is +/- 5% of the reading +/-0.3 dkh.
That means for the 8.8 reading on 7/29, it could be 9.54-8.06 and the 7.6 reading on 7/30 it could be 8.28-6.92.
So, the 2 readings, even though they are 1.2 dkh different, are within the measure of uncertainty.
Furthermore, now the hanna checker reads 8.1, which can be between 7.4-8.8 and the red sea reads 9.1. I can't find red seas measure of uncertainty listed, but I'm sure with its range, the two testing methods would be within the measure of uncertainty of each other.
On the
HI772 dKH Alkalinity Checker the accuracy statement =
Accuracy @ 25°C/77°F ±0.3 dKH ±5% of reading
So if your Checker reads 10.0 dKH the measure of uncertainty would be ±0.5 dKH or a range from 9.5-10.5 dKH.
If 5% of the total reading is great than 0.3 dkH then the use the 5% as your accuracy. If 5% of your total reading is equal to or less than 0.3 dKH then use 0.3 dKH as your accuracy statement.
8.1 dKH, (accuracy statement= 5% of 8.1 dKH =0.4 dKH) measure of uncertainty range of 7.7 dKH -8.5 dKH
7.6 dKH (accuracy statement= 5% of 7.6 dKH= 0.38 dKH (round up to 0.4 dKH)= measure of uncertainty range of 7.2 dKH -8.0 dKH
We do not use accuracy statements for any
chemical test kits, nor do most analytical chemistry brands. This is because an accuracy statement is usually reserved for instrumentation with digital displays of some sort. Human interpretation of color is subjective and thus you cannot measurably quantify what that measure of uncertainty will be due to differences in how people observe colors.
Companies who manufacture testing equipment only, like Hanna, use this same principle. Most titration based alkalinity tests, like LaMotte, will use a range/resolution/sensitivity but that is not the same thing as accuracy/measure of uncertainty.
Our
HI84531-01 Mini Titrator for Measuring Titratable Alkalinity in Water and Wastewater has an
Accuracy (@25ºC/77ºF) Low Range: ±1 mg/L or 3% of reading, whichever is greater; High Range: ±10 mg/L or 3% of reading, whichever is greater. Even though this is a titration, it has an accuracy statement because it doesn't rely on the human eye to determine an end point, but rather a pH electrode and automatic dosing system. Same is true for things like the kH guardian, pH electrodes, ORP electrodes, conductivity/TDS meters, refractometers but not chemical test kits which use color changes. It would be disingenuous to put an accuracy statement on something you cannot quantify.
If you are using a chemical test kit which uses a color change there are somethings you must do in order to adequately determine your end point.
1. Color must be observed under a full spectrum light (different light wavelengths can cause colors to appear differently)
2. Color must be observed under the same intensity light each time
3. Color should be observed against a white background
4. Make sure syringe is properly filled and you are not over/under tirtrating the sample
5. Some Chemical Test Kits for alkalinity require to observe any color change or a change to a specific color, make sure you know whether the kits wants you to observe a color shift in general or a shift to a specific color (i.e yellow to blue).
Our HI3811 Alkalinity Chemical Test Kit requires a color change from green or blue to yellow, following an acid titration using phenolphthalein and bromophenol blue.
