Any electronic gurus in the club?

[philagothos]

I got a "great deal" on a pH meter of dubious Asian descent a couple months ago and am having a hard time getting it to calibrate to the high range pH. It uses two potentiometers for adjusting the readings. One is intended to bring it to 7.0, the other I think is supposed to calibrate it to 11.0, but I only have 7.0 & 10.0 solutions, so I was trying to calibrate it to 10.0, but the closest I could get it was somewhere in the 11 range. I'm contemplating trying to replace one or both of the potentiometers to make calibrating easier/more effective. I can only find one identifier on each pot and they both have the same markings, but their readings on my cheap multimeter were very different. Also, they only turn 270 degrees (don't know if that is standard or not) and are VERY sensitive to the slightest touch.

So, my questions are:
1) Do you think replacing these pots will make it possible to actually get the thing to calibrate to pH 10.0 solution?
2) Is a multi-turn pot available/economical and would it make it any more accurate/less sensitive to tiny nudges?
3) How do I determine what pots I have and what ones I need?
4) Where can I get the new pots?
5) Wanna help me solder this thing so I don't destroy/melt it?

Thanks,
-Kevin H.

[tcottle]

Kevn

Is the meter new or used? If it is used the probe maybe bad. A pH meter is designed to give a linear response to a measurement that is logarithmic. Because the probe can drift and vary from probe to probe the manufacturers provide two calibrations: zero and gain (slope). The 7.0 pot is the zero and should be set first. The 11.0 pot is the gain and should be set second (duh!) Once set you should check the 7 again to make sure that is it is still correct and readjust if necessary. You should be able to calibrate using a 10.0 solution. Since you can't I suspect the probe is defective

That said I would not recommend replacing the pots

Todd

[philagothos]

Thanks Todd! I was beginning to wonder if I was going to get a response on this one.

As far as I know the unit is new-ish. It was a demo unit sent to PA by the manufacturer. They didn't like it and decided not to market it, so they sold their only unit at their yard sale a couple months back.

I guess I could get a new probe and try that first. If it doesn't work I'll at least have a good probe for when I buy a Pinpoint monitor down the road. Maybe I can even find someone about to replace their old probe and buy it for a couple bucks.

Thanks again,
Kevin

[tcottle]

When you bought the probe was the bulb dry? I am pretty sure that they are done for if the bulb is not kept wet

[philagothos]

It had a protective cap over the end, and there was moisture in there. However, it was only a couple drops of moisture, not filled with liquid. This is my first pH meter and thus my first probe, so I don't know just how wet they are supposed to be to keep them in good shape.

[tcottle]

Gotta love wiki!

http://en.wikipedia.org/wiki/PH_meter

[tcottle]

<a href=showthread.php?s=&postid=8683065#post8683065 target=_blank>Originally posted</a> by tcottle
A pH meter is designed to give a linear response to a measurement that is logarithmic.

Doh! that should have read "response to a measurement that is expodential"

What happened to ChemE? - he should have slapped me on that one!

[philagothos]

Thanks for the Wikipedia link. It sounds like my probe is probably shot. I'll try a new probe before I take the leap to replace any pots.

Thanks again!