Measuring TIG voltage, who cares?

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If you were doing precision automated TIG welding of say aircraft or space shuttle quality requirements, why would you care about measuring the voltage during welding?  Where's the best place to measure voltage, across the power source (+) (-) lugs, or right up close to the arc between the collet body and the workpiece, what's the difference?
Reply:Originally Posted by pulser.....Where's the best place to measure voltage, across the power source (+) (-) lugs, or right up close to the arc between the collet body and the workpiece, what's the difference?.....
Reply:What are you interested in knowing; the output voltage of the machine or the voltage across the arc?  The reason I ask is that the power cable going between the machine and the work drops considerable voltage due to electrical resistance. If you want to know the voltage at the machine, measure that, if you want to know the voltage at the arc, measure that (electrode/collet/etc. to work piece).
Reply:Who cares?Not me.. just as long as whats welded dont have a chance in hell to come apart i'm happy.. ...zap!I am not completely insane..Some parts are missing Professional Driver on a closed course....Do not attempt.Just because I'm a  dumbass don't mean that you can be too.So DON'T try any of this **** l do at home.
Reply:Originally Posted by razerWhat are you interested in knowing; the output voltage of the machine or the voltage across the arc?  The reason I ask is that the power cable going between the machine and the work drops considerable voltage due to electrical resistance. If you want to know the voltage at the machine, measure that, if you want to know the voltage at the arc, measure that (electrode/collet/etc. to work piece).
Reply:Originally Posted by Sober_PollockIf the power cables between the machine and the work drop a "Considerable" amount of voltage.....They're probably too small.
Reply:This was a baited question.Razer is precisely on the correct track.  The voltage measured at the power supply is higher than the voltage at the arc, due to resistance in the power and ground cables and various connnections.Zap has a valid point, most people don't care what the voltage is.  But as presmised in the question, this hypothetical situation is a critical automated welding application and you need to know that each weld is going to be identical (as much as possible).  So, say you measure the voltage at the machine on day one, and its 16 VDC.  As time goes on, about a month later, now measures 20 VDC.  What happened?  The resistance in the circuit increased, you have lose or degraded connections, or corrosion in your water cooled power cable.  This is a good thing to know before things degrade any further.Now if you had only been measuring voltage at the arc, you would never know that your connections were lose.  The voltage at the arc is a function of arc gap, current leve, and gas composition, it is not affected by other voltage drops within the circuit.  And since the power supply is constant current, and the arc is the same with or without added circuit resistance, you really don't see any difference in the weld anyway.  So Zap is right, no reason to care about measuing voltage.more later
Reply:What if your using an automatic voltage control (AVC) to maintain a constant arc length based on the voltage measurement?  You would want to measure voltage at the arc, not at the power supply.  If the AVC looked at power supply voltage, the arc length would change any time the circuit resistance changed due to a loose connection or whatever.
Reply:You care about voltage a) as mentioned for maintaining arc length, and b) for monitering arc power, where the total energy put into the weld is critical, among other reasons.
Reply:Arc voltage can be very impotant anyone tried burning An arcos lincoln NU 5 with a low voltage machine?