Ohmic probe a inssues water table

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02 Apr 2022 19:15 #239096 by paulsao
Greetings.I just built a water table for my plasma machine. I use a thcad 10 and ohmic 3 for ohmic detection, but I am having some problems in parts where I have water, the torch takes a longer time to sense and when it goes up it no longer has the cutting distance (1.5mm) but a higher one.Could someone help me thanks.

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02 Apr 2022 20:13 #239102 by snowgoer540
Water is always a challenge for ohmic sensing.

Assuming you used the circuit from the diagram that is floating around the forum for THCAD ohmic sensing, then replacing the 24K resistor with a lower value will help make it less sensitive to water.  I don't run the THCAD ohmic sensing anymore, but I do run a 100 ohm 7W resistor.  It definitely helped make it less sensitive to water on the plates.  I think Phill runs 3x 100 ohm 10W resistors twisted in parallel to give 33.33 ohm of resistance.  

That said, I rarely have a "wet" plate while sensing.  I make sure to squeegee off any water before starting my cuts, and the air between cuts keeps the water away from where the next cut will take place.  I imagine every plasma cutter has different post-flow air settings, so if the air is off before the next cut, you might try a longer post-flow (if it's adjustable) to help a bit. 
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02 Apr 2022 22:29 #239112 by rodw

Greetings.I just built a water table for my plasma machine. I use a thcad 10 and ohmic 3 for ohmic detection, but I am having some problems in parts where I have water, the torch takes a longer time to sense and when it goes up it no longer has the cutting distance (1.5mm) but a higher one.Could someone help me thanks.

I don't think its related to using a THCAD for ohmic sensing.

I think what I am hearing you say is that water is adhering to the torch by surface tension on the way up. This keeps the sensing  circuit closed until the water breaks away causing a false (higher) reading for material height. 

Usually, this is only an issue for the first probe after which the air flow keeps the water away from the probe area. Before starting a job, hit the pulse torch button to trigger the post flow of air. This should clear the water before probing starts. That was the original intent of this feature and it was requested  by Islander261 who also contributed the alternative relay method for ohmic sensing used by Snowgoer. 
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04 Apr 2022 22:14 #239281 by paulsao
Yes Rod, that is the problem, I have an ohmic detection system (attached in the image) it works well the problem is that sometimes the water drops remain on the legs of the piece and give an error, I use that system because my torch is not machined for cnc, now I remember that I am using thcad 10 and an isolated source, could I connect it directly to the torch? I think I wouldn't have a problem.
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05 Apr 2022 08:38 #239309 by rodw
You can try it. The circuit is impervious to torch voltage so in theory it should work.
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05 Apr 2022 15:29 #239346 by paulsao
It doesn't work hahaha, actually I would be making a short circuit on purpose and the probe would always be activated.

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05 Apr 2022 20:45 #239386 by rodw
Well you could do what one member did and fashioned a probe with a wire ring that circled the torch and was fired down by a pneumatic probe when probing 
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12 Jun 2022 22:15 #245026 by Tsf62
Snowgoer,

I am new to Linux and would like to know the difference of you using the 100 ohm 7W resistor versus Phil using the 3x 100ohm 10W resistors in the relay ohmic sensing setup diagram Phil had shown in another post.

I have a Hypertherm 45xp and was going to use the THCAD 10 & THCAD 05 with it set to 20:1, but am thinking I might go with the relay setup instead. This is a new build of a 48x48 table I put together and am now just trying to get setup. I do have the machine torch with the ohmic sense shield.

Your expertise would be appreciated.

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12 Jun 2022 23:37 #245030 by phillc54
The only difference is the sensitivity.
I ended up with 3x100Ω during testing and was too lazy to remove them when finished. :)
I have left them as is to see if there are any detrimental effects.
On a table I think 1x100Ω would be quite adequate as the post flow of air from the previous cut will normally remove most of the water.

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13 Jun 2022 13:02 #245068 by snowgoer540
Yep, what Phill said...

I tested with 3 100Ω resistors that were not high enough wattage, and ended up with 1 100Ω resistor because I honestly just forgot what I had there when I ordered the resistor (from work).  My brain let me down :)

I haven't had any trouble with the one 100Ω resistor.  As Phill said the post flow keeps everything dry anyways.  Highly recommend the relay method.  It's been great.

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