hi all,
m designing a ph meter using ca3140 for the first stage where we in m directly connecting the probe of the ph electrode to pin 3 of the opamp and thru pin 2 feedback is attached... and also tried attaching 100k & 10M resistance in between pin 3 and electrode... but since the impedance of the probe is high m not getting proper results...what should i do? please help me out
You may already know this, but I was told for glass pH probes that leakage current is an issue in the circuit itself and the pcb's generally must be made. Look into "guard conductors" for information on how leakage current is controlled. Here is a link to a op-amp datasheet that covers some of this information and also has a pH probe circuit as one of its typical applications.
**broken link removed**
I'm not sure this is your problem, but I hope it helps.
3140s run about 10^10th at the input. A more modern op amp with a higher input resistance would be better. Ypu might like an OPA127 - 10^13th and only 5 pa leakage.
Someday I would like to build networked pH meters for my ponds. It does not have to be bang on just good enough to tell when things are changing. When ponds go bad they move from 7 or 8 to pH 6 or less.
What sort of probe are you using?
Does anyone know where to get or how to build inexpensive ones?
So far all the ones I have seen on the net are out of my price range.
Someday I would like to build networked pH meters for my ponds. It does not have to be bang on just good enough to tell when things are changing. When ponds go bad they move from 7 or 8 to pH 6 or less.
What sort of probe are you using?
Does anyone know where to get or how to build inexpensive ones?
So far all the ones I have seen on the net are out of my price range.
I am building a low-cost pH / ORP (oxidation-reduction potential = redox) system. Multiple sensors will be connected, each sensor having its own potential seperated amplifier using a DC-DC convertor to drive the circuit. The analogue value will then be translated into a digital value by a low cost PIC processor (PIC12F675). On command of a host (PC or any intelligent controller) a master controller will read out all the values from the sensor-circuits and transport them to the host. Reading out the sensor-circuit will occur with opto-couplers to assure galvanic seperation of all the circuits. This way I will be able to read multiple sensors used in my reef tank without interfering each other.
I am still looking for a pH probe but think of using a HANNA probe: HI-1332/B.....Hanna also have ORP probes which are very usable.....Also Milwaukee has some usable pH probes (e.g. MA911)
The PIC controller in the circuit will also be used for software calibration purposes. This gives me the most design flexibility....
Overall, the circuit is low cost as the main components just cost a few euros (1 euro = 1.30 US$ at this time). The PCB will be the most expensive part though.....
Keep us posted regarding probes esp if you can find ones at a reasonable price. I would need several and at about $40 a pop it would add up. I seen some less expenisve ones using a cloth junction for $18 but I have doubts about that.
"Real men write machine code by hand and load it in with toggle switches"- For a better experience, sit on a tall stool and punch the code into the register with the indicator/switches, and manually toggle it into core memory.
"Real men write machine code by hand and load it in with toggle switches"- For a better experience, sit on a tall stool and punch the code into the register with the indicator/switches, and manually toggle it into core memory.