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I think there are much bigger issues than ohmic sensing from what you describe, namely bad wiring/shielding/grounding in general, so my strong advice is:
-do not fire the torch before grounding things properly
-make sure the power socket and the machine are grounded and the shop has proper ground rods
-make sure the PC is grounded
-make sure the plasma source is grounded
-disconnect anything ohmic related from Mesa
-disconnect the THCAD from Mesa
-connect THCAD only to plasma source
-wire and test the new Mesa, make sure everything works properly, wire the Torch ON to plasma source
-power on the plasma source and cnc machine and start QtPlasmaC or PlasmaC or whatever you are using
--enable LinuxCNC, jog around a bit, then use the Torch PULSE on screen set to 1 sec (default) ONLY ONCE !!!!!
-did anything move/burn/make strange noise/screen blank/PC turn off or lock/Mesa reset or lost connection/THCAD smoking ?
If any of those is yes, turn OFF everything and go back to wiring/shielding/grounding (goggle Hypetherm grounding they have a nice general guide)
If all above are no, try the pulse again and again, waiting 5 seconds between, make the pulse time 3 seconds and test several times again
If all that goes OK, try to do some manual cutting (use F9 to probe and fire the torch then jog manually), do several short cuts
If all good, power down and wire the THCAD to Mesa, do the same procedure from the line with two - signs above
ONLY if all tests go without issues proceed to wiring and testing Ohmic sensing. Again test from line with two -

Do you have a blinking yellow light on the THCAD?

If you set  the THCAD frequency divider to 1X you can probably
measure a change in the frequency as a voltage change on the F+
or F- relative to ground. Not sure of the polarity but full scale
should be about 2.5V at full scale and maybe 0.3V at 0V

I would need to study a bit about the probe wiring.
 

Since replacing the 7i97t, I haven't had any of the ohmic wired up while the plasma is running. I have only tried to get it working with the plasma off, so the 7i97t encoders should be fine.
I was more referring to the thcad output.
Can measure voltage between f+ and f- with the ohmic circuit open and again with it closed to test the thcad?

For the capacitor you mentioned, would I wire the torch tip to capacitor and the other end of the capacitor to star ground. Then run a separate wire from torch tip to thcad? Or would I wire both the star ground and the thcad to the capacitor?

I am trying to get ohmic sensing to work using a mesa 7i97t, thcad-2(jumpers on low range and 1/32), and a 24v power supply(meanwell hdr-15-24).

I have tried rodw's ohmic comp and ohmic comp3, as well as try many variation in the hal file mentioned in the original ohmic post/thread on this forum.
While I have no doubt there are issues with my hal file, I can't seem to get any response from the encoder in halshow, so I'm starting to wonder if I'm even getting a signal.
I tried using a multimeter to test the thcad outputs, but I don't know what I should be looking for. Is there a good way to test this?

I have it wired up similar to the diagram in the manual. I am using a 370k(or maybe 390k, can't remember) resistor between 24v+ and the thcad.
Since all the controls are on the gantry(this is a retrofit), I connected the 0v on the power supply to the buss bar on the gantry which in turn is wired to the star ground on the frame. I also connected the thcad shield terminal to the same buss bar.

The plasma is a ht hsd130 which uses a high frequency start. I previously damaged a 7i97t(which has been replaced) starting the plasma. I wonder if I could have damaged the thcad, as well. 
How can I protect the mesa boards from the high frequency start? Would a relay between the torch tip and thcad be enough? I assume it would need to be a special relay.

Attached is a hal file I have been using.

 

I'm not good at electronics. i will try implement a hypersensing circuit from plasma-cnc-primer docs with yours schematic. similarly, without surgery in a long cable.

for ohmic sense i only have thcad-10. i have thcad-300 but i used it for THC in this plasma cutter with 1:1 devider.

I found almost all the components (FY0022 + FY0023 connector,24VDC >20A relay ). but i can't find Isolated dc-dc converter with on/off signal.

Yeah the thread is long and full of failed attempts and iterations.
That's my development cycle, sadly.
The last post summarizes it. And this post show the performance forum.linuxcnc.org/show-your-stuff/56269...m-60?start=20#336805 It keeps performing like that even on water and rusty pieces.

Because the torch is the way it is, the ground leads in the torch, used during the arc start, needs to be cut and a small voltage applied which one measure to detect contact with the material. The rest is implementation and packaging in a nice format.
If you are willing to do surgery on the long cable to the torch you can cut it open and access the ground leads in there.
Instead, I made a short extension cable where all modifications were done.

I also used two separate voltage readers but I don't think that is necessary.
One uses the built-in voltage port, one measures directly between material ground and torch tip.
I will cut some parts later today so I can compare the voltage output of the two.
Of course, Mesa's THCAD cards can be used, the full voltage version.

Hi all

i bought a cheap plasma  Herocut Cut55i and i have machine plasma torch IPTM-80 like this
 
 

i try to do ohmic Hypersensing with a MESA THCAD-10 on my cnc table

Can you explain how physicali connect ohmic sensor?

Can i do ohmic ring something like that to the copper shield
 

or it is impossible on this type of torch and all that remains is floating switch?

thank you


 

Yes i am, i have 3 or 4 of those in daily use, 1 with Proma THC150 and others with Mesa THCAD.
I also have a china one with same electronics and pilot arc, the HF type, i intentionally wired the THCAD10 to the main terminals with 1MOhm resistors on each side, still in daily use for nearly a year, no issues.
The thing is, with correct wiring and grounding and shielding, it works fine, but any wiring error will result in burnt electronics the first time the torch fires, hence i refrain from advising such things.

The THC voltage is raw cutting voltage. a THCAD can use this with the right scaling resistors. I would expect there are some resistors between these terminals and the cutting terminals to avoid lethal current! Check with a multimeter.

here the thcad manual is your friend. 200 V full scale will be best but 300v is also Ok

Another cheap option for reading thermistors is LM331, a voltage-to-frequency converter
www.ti.com/lit/ds/symlink/lm331.pdf?ts=1760588182451
Or an industrial solution by Mesa, the THCAD

Hello
I'm having a weird Qtplasmac 2.9.238.315 bug where my torch goes down when it should be going up - but only during THC corrections.
Normal Z axis movement is OK as it should, with homing, probing, etc.

THC is being used from voltage (THCAD-2) and arc OK signal is present. 

I tried reversing the Voltage Scale (from 0.009 to -0.009) and no changes. - it still is correcting in the reverse way. how weird is it!

Where could i start?

Thanks everyone

Here is a suggestion:

forum.linuxcnc.org/plasmac/55286-servo-d...ofit?start=10#331245


( pncconf has more built-in QTPlasmaC/THCAD support  so may be an easier path)

PCW wrote:

If using a THCAD for ohmic sensing has issues with water. the most likely issue
is that the sense  resistor is too high a value.

I agree and I am pretty sure i mentioned somewhere a long time ago that a lower value resistor should be used for sensing. Something that will result in 20-50mA current should do just fine, maybe more as the water used will get lower resistance the more cuts are done and as it evaporates.

Thanks everyone for the feedback. I’ll keep you posted as things progress.

If using a THCAD for ohmic sensing has issues with water. the most likely issue
is that the sense  resistor is too high a value.