Real reason for a drip cup for probes?

[mojo]

I'm setting up a new aquarium controller (Prolifux II Ex) with a few probes - temp, pH, ORP, and conductivity. I'm really intersted in getting the best measurement I can (at least from the conductivity proble, since I use it to continuously monitor my salinity level). I know that some people recommend a drip cup, so that the probes are in a separate container, and the water drips in and then out of the cup. My question is - why? I can think of two possibilities:

- Consistency: no bubbles, no rouge waves, no currents, very predictable environment, etc.
- Stray electrical currents: These probes make their measurement by measuring the flow of ions, and any stray voltage will cause them to have an incorrect reference. But the only way to truly get rid of the stray voltage is to cut water from the probes completely - even a steady stream wouldn't be acceptable.

Can anyone else provide any insight? Are these assumptions correct? Are there other reasons?

 

[donnyholland]

I know that as far as your ph for your calcium reactor goes... We had to move our ph probe because the one in the reactor got severely funky after only a week. Not sure what caused it but now its in the effluent drip cup so its easly calibrated and cleaned.

Not sure about the other probes though haven't heard that.

/donny

 

[roland jacques]

I always assumed it was your reason #2 Stray electrical currents.

 

[cameron]

Same here on #2, but not sure how a probe REALLY works just a general notion. I know probes are to be spaced a certain distance apart supposedly for this reason.

 

[mojo]

Cameron;122415 wrote: Same here on #2, but not sure how a probe REALLY works just a general notion. I know probes are to be spaced a certain distance apart supposedly for this reason.

Here's some reading up, if you're so interested. (I was building my own controller for a while, but never had the time to complete it):

http://www.nico2000.net/Book/Guide6.html">http://www.nico2000.net/Book/Guide6.html</a>
[IMG]http://www.thermo.com/eThermo/CMA/PDFs/Articles/articlesFile_13455.pdf">http://www.thermo.com/eThermo/CMA/PDFs/Articles/articlesFile_13455.pdf</a>
[IMG]http://www.topac.com/ISE.html">http://www.topac.com/ISE.html</a>
[IMG]http://www.eidusa.com/ph_ISE_Electrodes.htm">http://www.eidusa.com/ph_ISE_Electrodes.htm</a>

And a little bit on dissolved oxygen, which works based on a different principal:

[IMG]http://users.bigpond.net.au/eagle33/elect/do2-cct.htm">http://users.bigpond.net.au/eagle33/elect/do2-cct.htm</a>

I don't have anything handy for conductivity probes. Basically, it's just measuring the resistance between the leads, but in order to keep electrolysis from taking place, it's typically done with an AC current, and the potential read at the z-cross point of the sine wave...