accuracy of weld rig digital amp gage?

Date 02-05-2004 18:53

Every machine and every gage will be different thats why you need to have them calibrated at regular intervals for ISO requirments.

Most gages have a +/_ factor standard to live up to but from there to the stinger & back there can be a lot of variables.

I don't know of any rule of thumb and I really don't think it would be very significant. Volts and Amps are usualy stated as a range and can very with the length or the arc.

I maybe misunder standing your question. I am sure some like Brande will have more input and possibly exactly what your looking for.

By jwright650 (*****)

Date 02-05-2004 19:04

Bill,
My welders(machines and personnel) here vary to the point I couldn't say that I have a rule of thumb. Out of the 40+ machines only two of them are dead on and the rest vary +/_ of the meters on the machines. As you probably know, the welder, ESO, and lengths of cabling (ground and work) make for an interesting bunch of variables that effect the meter vs actual at the gun.
Maybe someone else has a better handle on a rule of thumb to use.
John Wright

By Bill A

Date 02-05-2004 19:17

Thanks for the response. I should have broken my question into two parts:
1) typical difference between machine gages and separate instruments used to measure the electrical parameters at output terminals of the machine. (If the machine gage says 170 amps will a handheld gage say something within a couple amps of 170 or will it say something further off, like maybe 160 or 180?)
2) typical difference between measurements made at the machine and measurements at the electrode+grounding clamp. (Example, the GMAW machine said 21.5volts. I measured 19.9 between the grounding clamp and outlet of wire feed box.

By jwright650 (*****)

Date 02-05-2004 19:33

Bill,
What you are describing is not uncommon. You'll also notice a difference between a cold machine and one that has been warmed up and running for a while. The cables and all will heat up changing those figures you are trying to nail down. It is not uncommon to have the dial/meter read 180A and it is actually producing 200A or more. Sometimes the numbers go the other way and the machine is not as hot as the dial/meter says. Voltage will stray around as well, from a couple tenths to several volts. We send out our hand held gages to have them verified by a lab with instruments traceable back to NIST and then check the machines outputs with those gages to verify that we are still within our welding procedures. Keep your gage's certs on file.
John Wright

By RonG

Date 02-05-2004 19:57

1) If you are using Tong clamp instrument I would allow +/_ 10 amps for sure. Are you at the stinger or up close to the machine? I really don't think these instruments are that accurate or sensitive and the current flow is will vary allways. Even in a fixed SAW machine the numbers go up and down with the arc length.

2) Are you measuring potential or welding voltage. If you do not have an arc going you are not measuring working volts.

By CHGuilford (****)

Date 02-06-2004 15:36

Bill,
We use mostly FCAW and SAW; almost no SMAW.
Typically our ammeters on the power supplies agree within +/- 10 amps with a calibrated clamp meter for both FCAW and SAW.
As you probably know, the amperage can be measured anywhere in the circuit; it doesn't have to be near the arc.

Our FCAW wire feeder digital readouts typically read 1 to 2 volts higher than what the calibrated meter indicates. I have not checked the power supply voltage readings when doing those calibrations.

Our SAW tractor volt meters and the power supply meters read the same as the voltmeter as the calibrated meter reads, at least as far as I can determine. The SAW meter is analog and the cal. meter is digital.

When using SAW we use double 4/0 cables for the ground and electrode cables and keep the lengths to 50' max as much as possible. At ~800 amps we get a 3 volt drop between the power supply voltmeter and the tractor meter. If the voltage drop climbs to more than that, I know that I will find a poor connection somewhere that will soon burn out if not fixed.
Our power supply and tractor ammeters read the same.

You will have some voltage drop between the power supply and the electrode while welding. Your 21.5/ 19.9 is really pretty normal.
Picture a long garden hose for watering your lawn. The water pressure (similar to voltage) will be higher at the faucet than at the nozzle. Friction, fitting constrictions, hose length and diameter, etc.(similar to resistance) all act to reduce the pressure available at the nozzle. The flow or gallons per minute (similar to amperage) is the same anywhere in that part of the piping system. Assuming no change in available system pressure, you can push only the amount of water that the hose will allow. Hooking up to a bigger pipe won't increase flow, so you could hook up the water meter anywhere in that system and get the same reading.

I don't know if all that helped with what you're looking for.
Chet Guilford

By kam

Date 02-05-2004 21:25

I had a similar post a while back.

http://www.aws.org/cgi-bin/mwf/topic_show.pl?id=4342#18601

if this doesnt get you there try doing a search for calibration of power supplies. Somewhat related and might be interesting to you.

regards

kam

By Mike Brace (**)

Date 02-06-2004 13:10

Bill: The Pipe Pro will be accurate to +/- 10 amps and +/- 3 volts. Now remember if you take reading at the arc this reading will generally be lower than at the studs due to cable loss. It will also make a difference if the cable is laid out straight or in a coil. For inspection/quality the readings should be taken with in 10' of the arc. Hope this helps.

By Bill A

Date 02-06-2004 16:00

thanks to you and everyone else that responded with the useful input. To me the bottom line is that if we are trying to verify compliance with welding procedure parameters that include a specific range of heat inputs, then we really need to monitor using separate instrumentation close to the arc, instead of taking the lazy way out and looking at the gages on the machine.

By PhilThomas (**)

Date 02-08-2004 03:44

Actually, to ensure conformance vs the WPS qualified parameters, you should strive to measure the same way as the amps/volts were measured in preparing the PQR. If the PQR was performed with the machine meters, then you should be measuring with the machine meters in manufacturing.

To do otherwise will result in different heat input and potentially knock your mechanical properties out of the acceptable range.