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  field verifications

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Author Topic:   field verifications
Bud
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posted 13 April 2001 01:02 PM           Edit/Delete Message   Reply w/Quote
I work for a company that wants to send a technician to our customers' sites to perform electronic service and upgrades. This person travels with various electronic equipment that we have calibrated by an approved vendor once per year. I have a question about that equipment that is bound to be handled and mishandled through its many travels. Is it possible to perform some quick and easy verification of the test equipment in the field without doing a full-blown calibration/verification procedure? The equipment includes a multimeter to check continuity, a waveform generator, a hipot tester, and a portable oscilloscope. Does anybody has some ideas on quick and simple ways to verify in the field that the test equipment has not been damaged in transit? We want to catch problems with our own equipment before we use it on someone else's equipment. I'm hoping there are some electronic gurus out there that can assist me. Thank you in advance.

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Jerry Eldred
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posted 13 April 2001 01:17 PM     Click Here to See the Profile for Jerry Eldred   Click Here to Email Jerry Eldred     Edit/Delete Message   Reply w/Quote
I'll take a stab at it.

The multimeter- If it's a handheld portable meter such as a Fluke 77 or 87 or similar, those are designed for field use any way. Something as simple as touching the test leads together for continuity check ought to be sufficient. If it's continuity testing, and you are using a less portable meter, the same method would apply, and look for a repeatable stable zero indication (minus lead resistance).

The wave form generator can be checked by putting a waveform on the oscilloscope. Use wave shape, amplitude and frequency similar to values you will be testing. Do this for a couple of different settings at the extremes of where your technician intends to use the generator.

The hipot tester, with nothing connected to the output can be run up it's voltage range, and watch for a predictable front panel meter display. Maybe before the first trip, just after the units have been freshly calibrated, make up a brief document as to what you run up to, what the dial setting is turned to (depending on what type of hipot you have), and verify when you turn the dial up to that position you get a full scale indication on the hipot. If its less than about a 50KV hipot this is fine. For some of the higher output voltage models (100KV for example), environmental conditions can make a difference on how high you can run it. Make sure to have adequate safety instructions in the document so noone gets unwanted electrons coursing through their body.

For the portable oscilloscope, many of them have a cal output that can be used for a self-check. Use the probe normally used in the field work, and check for proper waveform period, amplitude and risetime.

Please feel free to ask further if you need better elaboration on this.

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Bud
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posted 16 April 2001 11:27 AM           Edit/Delete Message   Reply w/Quote
Thank you, Jerry, for your prompt reply. That is exactly the kind of information that I was looking for. Is there anything we can do if the portable oscilloscope does not have a cal output?

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Jerry Eldred
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posted 16 April 2001 11:49 AM     Click Here to See the Profile for Jerry Eldred   Click Here to Email Jerry Eldred     Edit/Delete Message   Reply w/Quote
That makes things even a little simpler. You can do what is sometimes called a "back-to-back" check. Set your generator for an appropriate output amplitud/waveform/frequency, and use it in lieu of the calibrator output.

Make sure to remember that this is NOT a calibration. It is a good confidence checker. Doing the back-to-back checks between the oscilloscope and the generator will give you some reasonable confidence level that they are both working properly. Odds are that if you have an erroneous decrease in the output level of the generator, you shouldn't have the same level of erroneous change in the oscilloscope input (we hope). If you get an incorrect display on the oscilloscope you can know with pretty good certainty that one or the other of the two are bad.

Conversely, if you get a "good" reading, you will have a pretty high degree of confidence that they are good.

One additional caution is to remember absolutely that these units are ONLY as good as their specs. If your vertical and horizontal specs on your scope are +/-2%, it will only read that well. If your generator amplitude is only +/-5%, it will ONLY read th at well. If you are checking them "back-to-back", you can ONLY expect readings within those limits. Your generator can legitimately change by 5% and still be good. Your scope could also drift by another 2% and still be good. You could see a change (in the example) of as much as 7% or more, and your instruments will still be "good." As you or your tech is checking them in the field, make sure to take into account the allowable specs of both instruments. If one was reading at the low end of spec and drifted all the way to the high end of its spec, it is still legitimate. Factor those quantities into your method for doing the checks in the field. Make your method such that you are only looking for equipment failures rather than legitimate instrument variation. As a calibration person, I have had to explain to many people these details. They used +/-2% meters and couldn't understand a 1% difference between two meters they used to check the same value. It is important to remember those details, and not deceive yourself into believing that test equipment is certified to remain "exact".

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Bud
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posted 17 April 2001 12:27 PM           Edit/Delete Message   Reply w/Quote
Thank you again, Jerry, for your valuable insight. I feel a lot better about our equipment traveling now that I have this information. This is without a doubt the best metrology forum I have seen and I hope it will be around for many years to come.

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