Alex,
I would not worry about the full scale gain calibration for the Ohms ranges unless for some reason you need it to be dead on at this time.? The 3468A has a? ONE YEAR spec of 0.016% of reading plus 2 counts.? This works out as follows for expected maximum reading error:
? ? ? ? ? ? ? ? ? Full Scale(FS) reading:? ? ? ?error = 166 ppm? ? ? ? ? ? ? ? ? ? ppm = parts per million
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 2/3 FS reading:? ? ? ?error = 170 ppm?
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 1/3 FS reading:? ? ? ?error = 180 ppm
? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 1/6 FS reading:? ? ? ?error = 200 ppm
? ? ? ? ? ? ? ? ? ? ? ? ? ? 1/10 FS reading:? ? ? ?error = 226 ppm
Your readings are out of spec but not that unreasonable for a calibration that may have occurred many years ago.??
The 20K resistor measured 19.9661K in 4W mode (actual value is 19.97140K).? ? ? ? ? ? ??
The 98.975K resistor measured 98.978K in 4W mode (actual value is 99.00352K).?
?Error of 20K in ppm:? ? ? (19.9661 - 19.9714) / 19.9714 X 10^6? =? - 265 ppm?
?Error?of 98.975K in ppm:? ? ? (98.978 - 99.00352) / 99.00352 X 10^6? =? - 258 ppm??
?Measuring 20K is a 2/3 FS? reading on 30K range:? ? ? ? ? ? ? ? ? EXPECTED? max. 2/3 FS error <=? 170 ppm? ? ? ?
?Measuring 98.975K?is a 1/3 FS? reading on 300K range:? ? ? ? ?EXPECTED? max. 1/3 FS error <=? 180 ppm? ? ? ??
Both of your readings are low by about 260 ppm which is out of range.? The Red Devil resistors have a TC of around? [+/- 5 ppm/C]? to? [+/-10 ppm/C]? so 25-50 ppm might be due to temp drift from your environment compared to mine ( a 5C change).?
Of the 260 ppm error,? around 210 ppm is non-temperature related (50 ppm max due to 5C temperature change) which tells me the Ohms reference resistor R201 may have shifted upward by about 210 ppm over time since two ranges seem to be out about the same amount.? This is a bit out of the maximum expected range(170 - 180 ppm) by about 30 - 40 ppm but not worrisome.
You could bring the Ohms into compliance by lowering the 40K Ohms reference resistor, R201, by 210 ppm to 39.9916K Ohm by paralleling it with a 190 Meg Ohm resistor without calibrating the ranges.? It might be interesting to try this.? If all ranges come into compliance, then R201 has shifted.? A huge improvement would be to replace the 40K Dale resistor with a 40K Vishay S102 precision resistor.
------------------------------------------------------------------------------------------------------------------------------------------------------------
ANSWERS:? ?
?(1)? I am letting the DVM run for about 30 minutes prior to doing the Zero cal procedures, as recommended in the manual. Since you did not mention this not sure it is only needed if further range calibrations would be done, but I assume it won't hurt.
The reason for powering the meter for 30 minutes before calibrating is to insure the internal components reach a stable temperature.? I did not mention it because I just assumed you would be calibrating with the meter having been on a long time.? ?I never turn my meters off -- the references only get better with time while being powered up.
(2)? Since you pointed out earlier in this thread that one cannot calibrate the DVM to both 2W and 4W ohms due to lack of storage space for the constants, I figured I Zero calibrate the 3468 for 4W, and the 3478 for 2W, so I have one of each calibrated measurement modes. . . . Went out of cal mode, and now reading in 4W with both jumpers in, and bridged together still reads 000.000. If I go to 2W I get a reading of 000.059 ohm. I guess this small error is to be expected as the calibration was performed on 4W.
This is really not a good idea.? You should always cal in 4-W mode.? If you insist caling one meter in 2-W and the other in 4-W, cal the 3478 in 4-W since it has one lower range in OHMs - 30mOhm range.? The 2-W offsets due to input wiring and lead resistance in 30mOhm range will be a nuisance.? The offset of 59 mOhm you read between 4-W and 2-W is reasonable.? There is about 75 squares of trace resistance on the PCB plus some wiring to the input terminals from the where the DC voltmeter senses the point where the current from the ohms converter enters the input.? The traces on the PCB are 1oz copper clad 20 mils wide having 0.508 mOhm per square with 74 squares of resistance which results in about 38mOhm of resistance plus the wiring of 3.5" of 20AWG from the PCB to the input terminals (20AWG:? 10.15mOhm/ft.@ 3.5"? =>? 3mOhm).?Therefore, the total resistance unsensed in 2-W mode is about 41mOhm so your reading of 59 mOhm is reasonable for there may be some other offsets due to thermals and contact R.?
(2a)? If you plan on caling the instruments in 2-W Ohms, then do so with your test leads that you intend to use connected to the meter and shorted together at the probe tips to ZERO out the lead resistance.? Nothing is to be gained shorting at the meter inputs and performing a ZERO calibration in 2-W mode.? Your test lead resistance will still show up and be an annoying offset to your readings.? Cal out all the measurement lead resistance and not just the internal residual wiring of the meter.
(3)? In any case would it be possible to use your two resistors to at least get the 30K and 300K ranges more closely calibrated?
Yes you can use those but get as close to 22C as possible with the environment containing both the meter and resistors.? Keep in mind that the 99.975K resistor is caling the instrument at 1/3 FS and the 20K is caling at 2/3 FS so it is more susceptible to errors.? We designed the instrument to cal at FS of 300,000 counts, 2/3 FS at 200,000 counts and 1/3 FS at 100,000 counts keeping in mind there are standards out there for Fluke that has 200% over range and others at the cardinal values of 1.000000 etc.? Our meters 3468A and 3478A have 300% over range reading capability but can be caled at? FS, 2/3 FS, or 1/3 FS to accommodate the various standards out there in the world.
(4)? Pushed in both your jumpers and wrapped around them tightly a shorting wire at about the same position on both jumpers to hopefully force the same potential.
It is not necessary to create a very low ohmic short between the source and sense jumpers.? All that is needed is a jumper between them of reasonably low resistance to force them to the same potential.? No current flows between them so you could even connect them with a 100 ohm resistor and the cal would be good.? Just use a short jumper wire with alligator clips to connect them.
---------------------------------------------------------------------------------------------------------------------------------------------------------------
When HP closed down its component division that made the precision resistors for standards and the instruments, I purchased the remaining inventory of Red Devils and Kelvin resistors.? I just looked over the inventory and decided I can lighten the inventory and will make sets available to viewers in the near future.? I need to inventory all the values and will post them here in the future and offer them at a very low price for what they are.
The video for the design and use of the 3468A / 3478A has stalled but hope to finish it soon. UP to 4 hours now.? I will cover in more detail all that I have discussed in these pages and more regarding the specs and measurement techniques.
Stay tuned.
George? ? April 18,? 2019? ?8:28am
