A word about temperatur compensation with pH probes.
Some basics:
Usually it is said that pH probes should be calibrated with buffer solutions at 25°C.
This is not really correct, it is just the usual compromise so the buffer solutions are made for use at 25°C.
Beware: Buffer solutions vary their pH value with temperature too!
For really exact pH gauging the solution should have the same temperature as the water you want to check.
Remeber you have to use buffer solutions made for this temperature - makes things somewhat complex.
To make things even more complex:
If you want to meter values above ph7 you have to calibrate with ph7 buffer and pH 9 or pH 10 buffer.
If you want to meter values below ph7 you have to calibrate with ph7 buffer and pH 4 buffer.
The output curve of the probe differs between above / below pH7 slightly.
The temperature compensation depends on 2 factors.
- The deviation of the temperature (for example calibrated at 25°C, gauging at 15°C)
- Deviation of the pH value from the neutral point (pH 7).
On pH7 you don't have any temperature deviation. If water has pH 7, it has pH7 at 10°C and it has pH 7 at 50°C.
For a pool or a fishtank the temperature deviation usually is negligible.
Let me give an example.
Requierement: You calibrated your pH probe at 25°C.
Your fishtank or your pool shows a pH of 6 at 25°.
What pH would be shown without temperature compensation if you raise or lower the temperature by 20°C?
At 5°C it would show pH 5.93. (Difference -0.07 pH)
At 45°C it would show pH 6.07. (Difference +0.07 pH)
Lets say at 25°C you see pH 5.
At 5°C it would show pH 4.86. (Difference -0.14 pH)
At 45°C it would show pH .14. (Difference +0.14 pH)
As you can see temperature compensation is not the most important thing for most home users.
Anyways, you may simply use a DS18B20 waterpoof sensor and calculate the temperature compensation
with a rule.
A table with some compensation values is shown in the attached picture.
left column shows deviation from calibration temperature.
(Sadly picture is in German language, but the table should be clear enough).
- temp-pH.jpg (489.75 KiB) Viewed 18231 times
Regards
Shardan
