Calibration of Pin-Point pH meter

[vincegoh]

Can anyone teach me how to calibrate pin-point pH meter?

I suppose I need to buy some buffer solutions. From where? And do I need to adjust anything on the meter itself so that it can be calibrated?

[Achilles Tang]

You can buy calibration fluid from Sealife or Petmart.

You need 3 cups.

First, wash the probe in tap water. (sometimes, if there is slime buildup on the tip, use a tooth brush to gently brush it off, if there is calcium deposits, use vinegar to soak it till its gone. Then wash thoroughly with tap water before starting the calibration exercise.

left cup is for the 4.1 fluid, centre is tap water (or better still, use distilled water), right cup is 7.1.

Take out your probe, make sure its attached well to the unit, make sure the 9v battery is fresh.

You will use see two knobs on the Pinpoint PH monitor body unti. One marked as '4,10' and one marked as '7'. These can be adjusted using a very small screwdriver (you should have a set which is for adjusting very small screws).

Dip in the tapwater cup, shake it dry.

Dip in the 4.1 solution. Use the screwdriver, adjust the '4, 10' knob till the digital readout is stablilized at 4.1.

Then take out the probe, wash it in the middle cup (tapwater), shake dry.

Dip in the 7.1 solution. Use the screwdriver, adjust the '7' knob till the digital readout is stablilized at 7.1.

Then take out the probe, wash it in the middle cup (tapwater), shake dry.

Go back to the 4.1 solution and calibrate again... and repeat the steps until dipping between all solutions will give the exact readouts without you needing to adjust the knobs anymore.

There! Your PH monitor is now ready for constant accurate monitoring till the next battery change!

Hope this helps,

AT

[vincegoh]

Thanks a lot. That is very detail, I think I will have no problem following.

Just curious why we calibrate it between 4.1 and 7.1? Since our readings is typical at about 8.4, why do we want to extrapolate? I means shouldn't it be calibrated between, let's say 7.1 and 10.1?

[Achilles Tang]

Interesting question. Hmmm... why?

[weileong]

The more expensive pH meters allows 3 points calibration at either 4,7,10 or 4,7,9. Some even offer customed calibration value.

The cheaper ones only 2 points and either 4,7 or 7,9 or 7,10.

Seems like we've no choice but to extrapolate. The mV/pH curve is in log form so you can already guess the accuracy especially if you do not calibrate on a frequent basis.

If you don't like that then by all means pay for the higher ends lab used type that offers 3 decimal place precision.

[vincegoh]

The more expensive pH meters allows 3 points calibration at either 4,7,10 or 4,7,9. Some even offer customed calibration value.

The cheaper ones only 2 points and either 4,7 or 7,9 or 7,10.

Seems like we've no choice but to extrapolate. The mV/pH curve is in log form so you can already guess the accuracy especially if you do not calibrate on a frequent basis.

If you don't like that then by all means pay for the higher ends lab used type that offers 3 decimal place precision.

Wah! How expensive u r refering to?

Pin Point already cost nearly S$300 liao.

Funny, since it is in log form, then all the more the calibration should be between 7.01 and 9.01, isn't it? I means the case is that we are restricted to 2 point calibration.

Pretty interesting, right?

[weileong]

I forgot cos just now rushing to go take my IPPT Aiya end up fail Cos keep losing my balance on the threadmill and the muscles used are different from running on the track.

Errm... expensive say like 4 digits?

Another reason why they normally have pH 4,7 for two point calibration and not 9 or 10 is because pH 9 or 10 solution will absorb CO2 from the atmospher air and will degrade quickly after that.

So it's no longer interesting after you know the reason right?

I also made a mistake, it's the concentration hydrogen ions vs mV generated in the pH electrode and not mV/pH curve.

The mV/pH curve is a straight line.

Usually pH 7 set the zero points (0 mV)

pH 4 or 9 sets the slope of the straight line (-120 mV for pH4 and 120mV for pH7). This vary a bit and depends on the made of the electrode

You could still obtain pH 9 solution and use it to verify the reading at pH 9. Plot two points with the other points at pH 7 then corelate the actual reading from the graph.

Another point of interest is that the pH does vary a little depending on the temp so if your pH meter do not have temperature compenstation then you'll need to ensure during calibration the buffer solution are at the same temp as the water in the tank.

It's quite troublesome if you're really going for that 0.xx accuracy in your pH reading which I think we do not need such precision for our reef tank.

[vincegoh]

i c

Since it is a straight line graph, then extrapolate is fine then. Especially the reason on the CO2 absorption, it make sense now.

Thanks

[vincegoh]

You can buy calibration fluid from Sealife or Petmart.

You need 3 cups.

AT

Wow... Sea life is selling the calibration fluid so X or it is market rate?

$6 per packet = 2 packets (one for 4.01 and one for 7.01) cost me $12...

Btw, AT, do we have to calibrate the meter everytime we change battery?

[weileong]

Why don't you go buy the hanna calibration solution.

500ml per bottle selling for around $25, I think can last you quite a long time. No worries of the need to prepare it or prepare wrongly.

[vincegoh]

Why don't you go buy the hanna calibration solution.

500ml per bottle selling for around $25, I think can last you quite a long time. No worries of the need to prepare it or prepare wrongly.

How it works? Do I need to buy 2 bottles to calibrate? Any different?

Anyway, i have bought the Milwa$%&* yesterday.