Ohmic Sensor on Water Table

[MajorMetal]

I am having lots of issues with my THC using the ohmic sensor when cutting pieces with lots of fall outs. The plasma gas will cause the water to splash up on the torch and causes the ohmic sensor to quit working. I then have to pull the retaining cap and dry out the nozzle and electrode to get it cutting again. Is this normal? I am using the PM 45 with the 220719 cap and 45 amp consumables on a Dynatorch Super B table. Not sure if it is the Dynatorch or the PM45. Thanks

[BTA Plasma]

Ohmic should be only allowed to sense the plate top when it moves down or when the DC voltage is sensed. You should not be getting stray voltage when the machine is sitting there. It is possible your Ohmic sensor wires need to be swapped. The torch gives out DC voltage and if it is bleeding DC voltage (plasma issue) it will give you those errors.

[MajorMetal]

I don't think I am following you. The water splashing up onto the torch is causing the error with the ohmic sensing and hence it will stop the program and I have to dry out the cap, electrode and nozzle to resume cutting.

[BTA Plasma]

Ohmic is done by sending a DC current to the table and torch nozzle or "tip" using either positive or negative to the electronics and a positive or negative to the torch. You either have the positive or negative side hooked to the tip of your nozzle and the other polarity to the table. A plasma cutter gives off a negative charge from the torch and a positive from the work clamp. If the water is causing a circuit bridge from the consumables in the powermax 45 torch then the powermax may be bleeding negative charge. This would cause a problem if you have the positive side of the ohmic sensing hooked to the nozzle on your torch. Then you complete the circuit and the torch sensed Z home right away. It may be as simple as a polarity issue where you need to track down the other side in the controller and flip the 2 wires around. Why ask here though didnt Dyna give you an answer? If your getting stray voltage causing the ohmic to fail it could be the wrong polarity hooked to the plasma torch or the system never had isolation from stray current or the plasma cutter itself needs to be checked for stray voltage. If you are using a hand torch then you may repeatedly deal with this issue until you isolate the safety switch and the trigger switch inside the torch handle. They are setup to complete a DC circuit as well. If this is a machine torch then that doesnt apply.

[MajorMetal]

Thank you for the reply. I asked here because I do not have a clue why it is not working. I am not sure if its a machine or plasma issue. But since I can get it to work again when I dry the consumables, I assume it is related to the PM45.

[jimcolt]

Actually Ohmic is used for mutiple purposes on some machines.....such as most industrial height controls as well as with the Dynatorch and PlasmaCam systems.

-Used primarily to sense the material (initial height sense or IHS) before each cut to allow setting of the correct pierce height.

-Used durring cutting and traversing to sense plate collision.....which the height control then either stops the process or raises the torch quickly to avoid damage.

Since ohmic contact is essentially a process that senses the resistance difference when the shield touches the plate.....if the shield touches water from the water table it may sense the water...not the plate. This sensitivity depends on the design impedance of the ohmic sensing circuit from your machine manufacturer, as well as the conductivity of the water in your water table....usually very high as the water has suspended particles of metal in it!

Most companies suggest not using the ohmic contact sensing in situations with water on the surface of the plate. Industrial height controls have the ability to use torque or stall sensing when used on water table applications. I believe you can shut off the ohmic function either while cutting or for IHS in the software. I suggest contacting Leon at Dynatorch....I suspect he can offer you the best advice.

I use the ohmic on my home shop machine 100% of the time for IHS sensing as well as collision sensing. I have a downdraft table with no water and the process works flawlessly.

Jim Colt Hypertherm

Thank you for the reply. I asked here because I do not have a clue why it is not working. I am not sure if its a machine or plasma issue. But since I can get it to work again when I dry the consumables, I assume it is related to the PM45.

[MajorMetal]

Thank you Jim. Yes, I am able to shut off the ohmic sensing which is what I have to do anytime I have pieces that fall out and cause splashing. If I do not have splashing the ohmic works perfectly.