🧪 Pomona pH Calibration Checker

Enter the voltage your pH probe gives in pH 7 and pH 4 buffer. You get the calibration numbers for your logger, a check that they are believable, and (optionally) how the probe compares with its first calibration.

🔒 Runs entirely in your browser. Nothing is uploaded.

1. Buffer readings

Volts in pH 7 buffer

Volts in pH 4 buffer

Volts in pH 10 buffer (optional)

Use the settled reading (the raw column of the SD log, or the serial output). Rinse the probe between buffers and let each reading settle for a minute.

Stricter check for your board (optional)

Expected sensitivity, mV per pH

Bare probe about 59; PH-4502C-style board about 180. Take it from a first good calibration of your own board.

Slope direction

Catches swapped buffers.

Compare with the probe's first calibration (optional)

First calibration: volts at pH 7

First calibration: volts at pH 4

A healthy probe keeps about the same sensitivity. Below 90 % of its first calibration it is weakening; below 80 % it is worn.

2. Result

Enter the two buffer readings above.

How it decides

Sensitivity (mV per pH) must be 20-600. Identical, swapped or wrong buffers fall outside that.
The pH 7 voltage must be 0.05-3.4 V, the range an ADS1115 on 3.3 V can read.
With your board's expected sensitivity and slope direction: swapped buffers and a worn probe (below 75 % of expected) are flagged.
With a pH 10 buffer: the line through pH 7 and pH 4 must predict pH 10 within 0.3.

These are the same rules the Pomona cress logger firmware and Pomona Core use. This page matches the Python rules on 10,000 test cases. The check can tell a calibration is implausible; it cannot prove a plausible one is right: keep using fresh buffers. Advisory only.