Hypersensing with THCAD - better way to do ohmic sensing

phillc54 wrote: Yes, accurate and consistent probing is what we should be looking for, who cares if you save some milliseconds per probe. I would think that comparators would also be with a look. Do you know what John uses, he seems to have reasonably consistent results.

John uses a relay circuit using opto22's. He's being doing it for along time and has a process down pat but I know triggering pre-flow is part of that.

tommylight wrote: Well i think i was silent long enough about this...
Find a 1KOhm resistor at 1W ( or 2x 2.2K @0.5W, or 4x 4.7K @0.25W, in parallel ), and attach it to the THCAD5 input, that should make all your water problems go away. It is to sensitive and that is very good, but not when water is involved. That will make it require more current passing through the system to trigger ( or change measured values in this case )
470Ohm/2W should do even better, just a bit over 5 mA going through it.

I don't think you can do that as it will change the scaling so full scale won't be 24 volts anymore. PCW did wonder if 24v was enough. Its still all a journey of discovery. The stuff we now know and have documented is pretty amazing. Stuff thats not publicly available anywhere.

Yes, you can do that and it will not move the scaling, 24V will still be 24V, just the current to get there will be higher, hence lowering the sensitivity to water.
Humour me, give it a try.
I have 4 THCAD's wandering the globe somewhere collecting viruses, still did not get them, so i can not try that yet.
I did say, parallel to the input, not in series, did i ? Guess not.

Tommy, is this what you're saying?

Yes, but if you have that 390K resistor, connect it before the resistor.

OK you can lower the sensing resistance to lower the sensitivity of the probe
_BUT_ beware that the resistor must withstand full plasma voltage and the 390k
resistor must remain in place since it sets the full scale THCAD range (= 24V)

A resistor to raise the sensing current would go from +24V to the probe
Note that 1K is probably way too low because it could dissipate ~100W
at 300V, 10K 10W might be a more reasonable compromise (roughly 50X
higher sense current)

Hmmmm that is new to me, thank you PCW.
I was under the impressions that the probing is switched off before firing the torch.
Adding a relay will solve that issue.
Now i have to wonder, if the probing THCAD is on all the time, the use of another THCAD seems redundant.
Will think about it a bit, while waiting for my THCAD's to arrive.