Thought I would cut my teeth on an API phosphate test kit, bought new, expires Jan ‘25.Agreed.
I said the same to red2143 in a message.
"Here's the best solution I didn't think about in the thread.
Once you add reagent 1, cap and invert 10 to mix.... use THAT yellow as the c1 blank.
If the meter stays on for 10 minutes you have plenty of time to add reagent 2, shake 60 seconds, wait 5:00 minutes, then use that reacted sample as c2."
5 ml sample calibrated C1 after the initial dye. Hanna tube raised with plastic spacers in the checker, covered with a salifert plastic alkalinity tube, covered with black gaffa tape.
Add, 5 ml sample, add 6 drops # 1, shake 5 seconds.
Then into checker and set C1.
Then add 6 drops #2, shake 5 seconds.
Into the checker again, set the 3 minute timer.
Results using new salt water spiked with TSP;
Zero phos, API Reading 0.51 ppm
0.25, API Reading 0.76
0.50, API Reading 0.97
0.75, API Reading 1.20
1.00, API Reading 1.39
1.90 (Hanna), API Reading 2.08.
So, in the useful range of say 0.00 to 0.75, the API test kit is reading a linear 0.5 ppm higher than actuality, at least with this test kit. At 160 tests for 12 Great British Pounds, that’s a bargain