I got the AtlasScientific pH kit from Sparkfun. I am confused regarding calibration. In Sparkfun’s video: https://www.sparkfun.com/news/1305 at (3:35 mins in),
Joel notes:
The pH stamp allows calibration of the pH sensor. You can either do this by telling it what the solution you are testing (4,7,10) or tell the temperature of the solution you are testing. Thereby giving a more correct reading without calibrating to a specific level.
I’m confused. Don’t you need to both adjust the temperature (see why in this post: http://www.all-about-ph.com/ph-versus-temperature.htm and calibrate in each pH solution?
In other words, Joel uses the term “calibrate” which his prototype does not do. His prototype adjusts for temperature - which should be automatically done throughout the readings and does not require the calibration solutions.
What is the correct way to calibrate?
thanks very much.
I work a lot in a metrology shop, and I do a lot of pH calibrations. Not all pH probes are temperature corrected. If you calibrate with only the pH value, then it is valid, at that calibration temperature only. If you want (or need) pH calibration with temperature adjustment, you could use a correction formula in the code, and a thermistor to measure the solution temperature. Some pH probes have RTDs built in. This one doesn’t appear to.
You can use a lookup table like this one: http://www.young-train.net/multimedia/t … age_19.htm
and interpolate to correct the temperature.
The approximate change is… 0.003 pH/(d°C/dpH)
d°C is how far away, in degrees C from 20, the probe is.
dpH is how far away from 7 pH is.
The standard temperature of pH calibrations is 20°C. That’s the starting point…so the approximate change at a ph of 10, for a solution of 40 °C is
correction = (0.003)*(40-20)/(10-7) = 0.003 * 20 / 3 = 0.02.
So maybe you don’t even need temperature correction? It depends how far away from neutral pH of 7 you are, and the temperature.
Thank you very much for the additional info. You clearly understand pH!
The question I have however is not applying the temperature correction. I can do this quite handily. In fact, this is what I am saying Joel is doing in the video BUT he says he is calibrating the pH. Note: Joel say “you can EITHER…” I do not see that. Rather, I believe you have to do both. I am not an expert so I am checking since I assume Sparkfun videos are checked for accuracy (this might not be true?) or someone would have noted it by now. I am checking since past assumptions have not been correct - perhaps that is the case here.
Thus - I still am perplexed. Shouldn’t a person of the pH stamp: 1) just check the temperature and update the pH stamp with the info (easy to do and what Joel does as his calibration test 2) go through the calibration steps as noted in Atlas Scientific’s document: http://dlnmh9ip6v2uc.cloudfront.net/dat … it_4.0.pdf
Again - many thanks for the additional info.