I am answering many of your questions. Bob, > But the capacitor is the first place to look. > Any kind of film capacitor is going to be an order of magnitude > better. You are right, I changed the caps to metal polyester. > Also, .47 or 2.2 uF are both too much for 7000Hz. > It pushes the resistor values way too low, making > the current through the 555 significant enough to > shift things around. I had to use 0.47 uF as coupling. 0.1 uF would give me too low a=20 frequency and I would not get the required resolution. Will wait for the=20 results tomorrow morning. lgtngstk, > Are you using the same saltwater? I was thinking it > could possibly be evaporation in addition to the things > already mentioned. It is the same salt water. The probe is immersed over night. The salt concentration is not too high. It is about 2500 TDS.=20 (calibrated through a TDS meter) Jack, > Capacitors too; varying according to construction > - The data sheet is your friend. You are right about this :-). Robert, > OP said his probes were in salt water. Salt water corrodes/oxydizes. > His electrode impedance (R and or C) is changing > because of corrosion. If he pulls the electrodes out of the water, > washes them clean, then measures again, > hours later, he will likely see less change, proving the postulate. > Can he try changing out the water for distilled during the overnight > period? Does the effect continue over time or flat line after a day > or two or 3 without activation of the circuit? > Does running the circuit reverse the effect? You have to eliminate > chemistry effects and he hasn't said > what kind of electrodes he's using so I'm betting corrosion effects. As mentioned above, the water is not too salty. It has a TDS of about 2500. The electrodes are to be in water always for the final product. The electrodes are made of steel I suppose (from HM Digital). Peter, > Using very sensitive scales weigh the cup to find out Good idea, but very little evaporation would not change the frequency=20 reading to a large extent. Sean, The answer to your question is here http://emesystems.com/OL2mhos.htm Denny, I have answered above. Tomorrow morning, I will let you all know the outcome of my further=20 experiments. Thank you all. Cheers Ravi Denny Esterline wrote: > On Wed, Sep 10, 2014 at 6:12 AM, Sean Breheny wrote: > >> I think that the OP needs to post a schematic or at least describe the >> circuit better. Where are the probes connected to the circuit? >> >> > My thoughts exactly. Also, what are the "probes"? > With the right (wrong?) metals in salt solution overnight, I could easily > see the surface corrosion having an effect. > > -Denny > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .