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Some old school instruments showing how it's done (HP 3325A and Fluke 8506a)

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SilverSolder:

--- Quote from: joeqsmith on January 22, 2021, 04:55:43 pm ---
--- Quote from: SilverSolder on January 22, 2021, 04:43:42 pm ---What do you mean by "stable"?   The zero stability is specced for 5uV / 90 days, which my examples easily beat. 

--- End quote ---

Do you have the resistance feature?

--- End quote ---

Yep.

joeqsmith:

--- Quote from: SilverSolder on January 22, 2021, 04:59:01 pm ---
--- Quote from: joeqsmith on January 22, 2021, 04:55:43 pm ---
--- Quote from: SilverSolder on January 22, 2021, 04:43:42 pm ---What do you mean by "stable"?   The zero stability is specced for 5uV / 90 days, which my examples easily beat. 

--- End quote ---

Do you have the resistance feature?

--- End quote ---

Yep.

--- End quote ---
Great.  Using an economy grade 1 ohm 4-W standard with guard attached and averaging enabled,  this meter will wander around easily +/- 0.0005.  The manual for the meter that I have doesn't seem to cover the resistance.   I'm sure some of it is temperature but it changes faster than the room temperature and I suspect most of it is just noise.   I'm curious how another meter would behave. 

If you have other standards available, I could try to compare mine with what you have.   

SilverSolder:

--- Quote from: joeqsmith on January 22, 2021, 05:48:11 pm ---
--- Quote from: SilverSolder on January 22, 2021, 04:59:01 pm ---
--- Quote from: joeqsmith on January 22, 2021, 04:55:43 pm ---
--- Quote from: SilverSolder on January 22, 2021, 04:43:42 pm ---What do you mean by "stable"?   The zero stability is specced for 5uV / 90 days, which my examples easily beat. 

--- End quote ---

Do you have the resistance feature?

--- End quote ---

Yep.

--- End quote ---
Great.  Using an economy grade 1 ohm 4-W standard with guard attached and averaging enabled,  this meter will wander around easily +/- 0.0005.  The manual for the meter that I have doesn't seem to cover the resistance.   I'm sure some of it is temperature but it changes faster than the room temperature and I suspect most of it is just noise.   I'm curious how another meter would behave. 

If you have other standards available, I could try to compare mine with what you have.

--- End quote ---

Let me see if I can find something to test.

You can always reduce noise by increasing the number of samples per reading -  E.g. press  STORE - 11 - SAMPLE   

This will set the number of samples to 2^11  (you can go as high as 2^17 if you have all day) -  default is 2^7, I believe.

It is similar to NPLC in other meters, but I believe these things take 4 readings per mains cycle so 2^11 (2048 samples) is done in 8.5 seconds, which is only 510 PLCs.

srb1954:

--- Quote from: SilverSolder on January 22, 2021, 04:43:42 pm ---

--- Quote from: srb1954 on January 22, 2021, 06:52:13 am ---[...]
I would be interested as well. My 8506A performs pretty within spec for inputs above 10mV RMS but exhibits strange behaviour for inputs below that.

For inputs from 0 to 800uV the display shows a constant 131uV. Increasing the input to 900uV the display suddenly jumps to 1.074mV and stays thereabouts until the input raises above 1.1mV after which the reading increases along with the input but with a significant error of around +10%. As the input voltage is further increased the reading gradually converges to be within spec at around 10mV input. From that point on the reading stays closely in spec until the input reaches 30V RMS where the error starts slowly creeping up again.

I am not sure if the performance I am seeing with the 8506A is normal (due to some sort of auto-zeroing function) or is abnormal. The Fluke manual is not informative as to the expected performance at such low input levels.

I have been using a Fluke 5200A as a signal source with a Gertsch RT-7 ratio transformer to accurately divide down the lower level signals.

--- End quote ---


Mine behave in a similar way.

In the manual, the Minimum Specified Level for the 100mV AC range is 12.5mV.   Accuracy falls off a cliff when you go below that - reach for an amplifier or another instrument for lower voltage measurements!

Note, the other ranges also have Minimum Specified Levels and they are quite high!  e.g. 1.25V on the 10V AC range.

--- End quote ---

Thanks. This is good to know.

I got this 8506A relatively cheap because the vendor said the instrument wouldn't zero and I was concerned that I would have to go through the incredibly complex calibration process to eliminate something that might or might not be a fault.

I have several other DMMs that perform better than the 8506A at very low levels but none can match the excellent accuracy of the 8506A over its mid-voltage ranges and over wider frequency ranges.

SilverSolder:
I have four of these units with the Ohms option.  (Two are 8505A, two are 8506A, but everything related to Ohms is the same between the two models.)

I tested all four in parallel by hooking up radial wirewound resistors on all four units, 5 ohm and  0.25%, like this:




It seems very thermally stable, I could blow on the resistors and the readings did not deviate.





Here are the results:

2 hour warm-up.  No averaging, all default settings.  Peak mode turned on at start of test period to measure deviation.

4 wire Ohms mode.

Test 1: Peak to peak deviation in 10 minutes:
Unit 1 -  0.0005 ohms
Unit 2 -  0.0005 ohms
Unit 3 -  0.0001 ohms
Unit 4 -  0.0033 ohms

Test 2: Peak to peak deviation in 10 minutes:
Unit 1 -  0.0007 ohms
Unit 2 -  0.0005 ohms
Unit 3 -  0.0001 ohms
Unit 4 -  0.0046 ohms


Conclusion - Unit 3 is almost perfect, unit 4 is broken, while units 1 and 2 are probably "normal".


I have read somewhere (perhaps on the EEVblog) that the current generator for the 4 wire Ohms can become noisy on these meters.  Maybe that is what is going on with my Unit 4.   Perhaps the time has come to look at how that circuit works, and see if it can be made nice and quiet...




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