Found the hole in my own idea so I thought I would post it just in case someone attempts it.
The problem is the "uncalibrated" meter is going to have different amounts of error in different resistance ranges. So measuring the divider resistors will result in an error percentage not less then the differential between two ranges used to measure the resistors. That's not to say a calibrated meter wouldn't work to create the references but at that point you may as well just measure the voltages. Ohhh well. Sorry for the wasted posts.
Jeff
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On 12/18/2012 8:14 PM, Jeff Machesky wrote:
Just some food for thought and criticism. Let's say all you can get is
a 2.5 or 5.0 volt precision reference. You could in theory use the
high resolution of your 5 1/2 or 6 1/2 + digit meter to trim resistors
to get precision divider ratios. Even if your meter was not in cal the
ratio would still be accurate. So for each calibration range you could
divide down your reference or source as needed to match your precision
reference. I think this method would get most of us well in the
ballpark. It would require a whole lot of patience however and I'm
sure would become less accurate at the higher voltages. Perhaps even
use a very low offset op-amp to buffer your readings. Once voltage is
accurate you could check resistance ranges with a small amount of
precision resistors and again trim the other half of a voltage divider
until your voltage equates to a value that matches your desired resistance.
So one uncalibrated meter plus one high precision voltage reference and
a bit of patience and a good calculator and you can get a fairly close
cal. Well in theory at least.
Any thoughts ?
Jeff
On 12/18/2012 8:54 AM, Chuck Harris wrote:
The 3478A is a nice meter, but bear in mind that it is 300V max.
It is accurate, stable, and works smoothly. It is easily controlled
over HPIB.
In its deficiency corner, is the 300V max,and a non back lit LCD that
tends to get lost in the shadows.
I have never needed to calibrate mine, but I am pretty sure that you
need to complete the calibration of all steps before you leave a
particular function, say DCV, or ACV, or Ohms....
The calibration procedure takes a measurement of a known standard
on a particular range, and creates an offset that it used when it
calculates the voltage to display. The 30mv standard only calibrates
the offset for the 30mv range. The 3V standard, the 3V range...
You shouldn't have to pay more than about $100 for a working calibrated
3478A. Tucker has them for sale periodically.
9 times out of 10, a voltage measured by a scope's trace is good enough.
Did I mention that the 3478A only measures up to 300V?
-Chuck Harris
Jeff Machesky wrote:
So it's about the time of year where I'm allowed to buy one piece of
test gear and I'm thinking I want a bench meter with higher
resolution. The best I've got so far is the Tek 2465BDM which has a
20,000 count 0.03% accuracy DMM. It's however a little out of cal and
I just don't want to spend the cash to get it back in perfect spec.
It's also a bit crazy to have a scope running just to use the DMM.
This brings me to the 3478A. While I could technically afford to buy a
new 120,000 count DMM I just can't justify the cost for the hobby type
work that I do. Plus I love working with the older gear and saving
money.
Couple quick questions about the 3478A:
Can one range be calibrated at a time? This is not highly clear in the
service manual. Can I calibrate just the 3V range for example?
Are they reliable, or will I have to fix up most of what I would buy
on say ebay?
The problem I have with the meter on my Tek is that you have to
run the
entire cal for each measurement type, If you start with DC you have to
go all the way up the range from millivolts up to several hundred
volts. As we all know it's easy to get a precision calibrated low
voltage references such as a DMM check board, it's another thing
to get
500 volts or higher reference at the spec of the meter.
So is this really a meter to buy if your willing to send it off to be
calibrated or have a calibration transfer standard meter that's above
the specs of the 3478A? In other words if all you have is just a few
standards are you wasting your time?
Has anyone had any luck using a precision reference, some precision
resistors and an op-amp to generate other reference voltages with any
degree of accuracy? Also has anyone found a good source of say 500
volts 60 Hz AC? The right audio amp and a sig gen can get you some of
the other odd cal standards. High voltage DC is just so much easier to
obtain then a nice high voltage sine wave.
Sorry for the random questions, and thanks for any replies. The
54111D is still running great. My thanks again for everyone who helped
me fix the ROM's.
Jeff
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