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uThing::iPH Calibration Procedure

Two-Point pH Calibration

pH calibration buffers pH calibration buffers

An ideal pH electrode outputs exactly 0 mV at pH 7.00 and has a slope of -59.16 mV/pH at 25°C. However, real probes always differ from this ideal response due to manufacturing tolerances, aging, and contamination. To ensure accuracy, a periodic two-point calibration is essential.

Ideal vs. Real Probe Response

A two-point calibration adjusts the meter's response at two distinct pH values, correcting for both offset and slope deviations.

  • Offset: This is the voltage reading at pH 7.00. An ideal probe reads 0 mV, but a real probe will have a small voltage offset.
  • Slope: This is the change in mV per pH unit. It's often expressed as a percentage of the ideal slope.

The graph below shows an example of a real probe calibrated at pH 7.00 and pH 9.01. The procedure calculates an offset of -0.3 pH and a slope of -61.36 mV/pH (104% of ideal).

Offset and Slope Calculation

The calibration algorithm calculates these offset and slope parameters, stores them in non-volatile memory, and uses them to provide corrected pH measurements. For best results, the two buffer solutions should bracket the expected pH range of your samples.

Calibration Steps

Step 1: Start Calibration Mode

Interactive Mode

To start the calibration, you need to send the character 'K' followed by a newline to the uThing::iPH's serial interface. * Using a serial terminal: Type K and press Enter. * Using a command line: $ printf 'K\n' > /dev/ttyACM0 (Linux/macOS). You can use a second terminal to monitor the output with cat /dev/ttyACM0.

The device will respond with a list of supported buffer sets:

**** Electrode calibration procedure (2 points -> linear) ******
 -- Options: (pH @ 25ºC) -
 ------------------------------------------------
'1': [1.68  4.01  7.00  10.01       ] Mettler US
'2': [2.00  4.01  7.00  9.21  11.00 ] Mettler Europe
'3': [2.000 4.00  7.00  9.00  12.00 ] Standard Merck buffer
'4': [1.680 4.008 6.865 9.184 12.454] DIN19266:2000
'5': [1.09  4.65  6.79  9.23  12.75 ] DIN19267
'6': [1.680 4.003 6.864 9.182 12.460] Chinese
'7': [2.00  4.01  7.00  10.00       ] Technical buffer
'8': [1.679 4.008 6.865 9.180       ] JIS Z 8802
--------------------------------------------------
-- Select Buffer Set [1-8] -

Step 2: Select Calibration Buffer Set

Select the set of standard buffers that matches the two solutions you will use. For example, to use the Mettler Europe set, enter 2 and press Enter.

Selected Set: 2

Step 3: First Calibration Point

  1. Rinse: Rinse the electrode tip with deionized water.
  2. Immerse: Place the electrode in the first buffer solution. It's good practice to stir gently for a few seconds and then let the probe sit still.

Buffer and Probe Temperature

For the most accurate calibration, ensure the buffer solution and the temperature probe are at the same temperature. Immerse both the pH probe and the external temperature probe (if used) into the buffer. This is especially important if the buffer was stored at a different temperature than the calibration environment.

The device will automatically detect the buffer based on the measured voltage.

Automatic Buffer Detection

The calibration algorithm monitors the measured voltage, compares it with the selected buffer set, and selects the buffer with the best fit.

 -- STEP 1: Rinse the probe with deionized water and insert in 1st buffer --
* Starting automatic buffer detection... [Enter 'C' to Cancel]

[\ ] +169.4 mV - Wait for value to stabilize and enter 'N' for next step
  1. Stabilize: Wait for the voltage reading to stabilize.

Settling Time

The time it takes for the reading to stabilize can vary. A longer settling time, especially if it increases over time, can indicate that the probe is approaching its end-of-life and may need replacement.

  1. Confirm: Once the reading is stable, enter N and press Enter to proceed.

* 1st calibration point - [Buffer detected: 4.01, measured: +169.3 mV (4.04 pH)]
Verify that the detected buffer matches the solution you used before continuing.

Step 4: Second Calibration Point

  1. Rinse: Remove the electrode from the first buffer and rinse it again with deionized water.
  2. Immerse: Place the electrode in the second buffer solution and wait for the reading to stabilize.
  3. Confirm: Enter N and press Enter to capture the second calibration point.
* 2nd calibration point - [Buffer detected: 7.00, measured: -2.86 mV (7.05 pH)]

Do Not Reuse Buffers

To avoid contaminating your stock buffer solutions, always discard the portion used for calibration. Never pour used buffer back into the original container.

Step 5: Confirm and Save Parameters

The device will display the calculated slope and offset.

*** Calibration Finished: SLOPE: 1.011 (101.1% percent), OFFSET: 0.30 ***

 [Enter 'W' to confirm and save to Flash (override previous calibration), 'C' to Cancel...]

Saving calibration data to Flash...

Enter W to save the new calibration parameters to the device's non-volatile memory.

Verifying Calibration

After saving, it's good practice to verify the calibration. Rinse the probe and place it back in one of the buffer solutions. The reading should be very close to the buffer's value.

Generally, a slope between 85% and 105% and an offset of ±30 mV (approx. ±0.5 pH) is considered acceptable. If your parameters fall outside this range, you may need to clean or replace your pH probe.