Products > Test Equipment
Keysight DMM - Yay or nay?
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blackdog:
Hi,

First I want to make this clear, I can still recommend Traceless the 34465a from Keysight it is not a bad DMM!

The things I show below like this concern small deviations at the PPM level when using the Autrtocal function.
My assumption when I bought the 34465a was that if my lab temperature was stable at 26C and I used the Autocal function then the few PPM that the Autocal function corrects,
would remain stable if the temperature in the measurement environment did not change.

The corrected value of the autocal function drifts downward at two instruments I tested.
And a 34465a reference meter at Keysight's factory to which my meter was compared had the same drift after using the Autocal function.

And again, this does not make the 34465a a bad DMM, just that the Markereers at Keysight wanted an extra feature on their 34465a version to be able to sell the 34465a for a higher price.
So I paid the higher price, but the functiue doesn't work well, and I find that a bit sad.

I can still take accurate measurements as needed as I also have an Agilent 3458a standing by. but for good stability this DMM will need to be on for at least 12 hours,
I choose to keep 24 hours on and that also means the sound of an F16 next to you from the fan.  ;D

OK some pictures,
Here you can see the drift on my 34465a after using the autocal function.
This meter was paralleled to two 34461a and my 3458a DMM for verification so that I had some assurance that there were no measurement errors involved.


.
This is one of the measurements the Keysight factory made of my Unit and their reference 34465a DMM.
My 34465a is the orange line and the blue line is the reference DMM.
In the green frame that I placed in the picture is a special test from Keysight, as far as I understand it, there the Autocal function that is always present in this DMM series is disabled.

I was told that this function is not available to the users, as it is only for the fabrication or calibration process.
But the drift in both instruments is readily noticeable.
And again, the orange line is again my DMM.


.
I have no idea what the behavior of the Autocal function is at larger temperature deviations, say e.g. as a 34465a in a 19-Inch rack mounted with equipment and whether the Autocal will also drift more percentage-wise.
It may well be that the few PPM I have experienced is the "base" drift and if the ambient temperature is e.g. 25 degrees higher in a 19-Inch rack that this base drift I am measuring will cancel out against e.g. 15PPM drift that the Autocal function corrects.
I have no way to check this, I do not own a climate box to do good measurements in this regard.

Some tips
If you are going to make precision measurements keep in mind these things, among others:
Provide a measurement room with a stable temperature.
No varying Sun, no drafts.

Do not place your DMM in a place where it is heated by other measuring instruments.

Clean the inputs of your DMM regularly with alcohol and Q-Tips.

Preferably do not use banana connectors, they are slow to warm up and if you do want to use them, make sure they are gold plated and also cleaned with alcohol.
For most precision measurements use a pair from a CAT5e cable that is full copper and a pair of Q-Tips to secure this pair in the inputs of the DMM and clean them with alcohol.

Keep the wiring as short as possible, wiring always twisted!

Keep telephones and other transmitting equipment away from your test rig, and yes, also the phone in your pocket or that of your colleagues.

Also think about SMPS power supplies, these are often lighthouses as far as EMC is concerned, always think about that when you have strange outliers in your measurements, it's really not always the 1/F noise of your voltage reference or your D.U.T.
I'm sure a good number of people here on the forum can add to this list with tips.

My TEK DMM4050 meters and also my Keithley 6500 do not have an autocal function for the user, so I cannot compare my 34465a with that,
And again the 34465a is NOT a bad DMM!

I hoop this helps and if there are question, i like to hear them.  ;)

Kind regards,
Bram
Traceless:

--- Quote from: razvan784 on March 03, 2022, 10:18:32 am ---* if you are after repairability, I would skip anything new and go for a used 34401A in good condition (or even better, a 34410A if you can find one for a reasonable price). These can be hooked up to a PC via RS-232 (or even to something like a Raspberry Pi) where you can easily do logging, fancy graphs and whatnot. An old Keithley 2000 is also an option (I also like its interface better) but these are not as easily repairable - the circuitry is more complicated and official schematics have not been published.

--- End quote ---

I actually do like old DMMs and also thought about that but there are a few catches. For some reason the good brands are unobtainum in Germany. You do find stuff on ebay but:
- prices are stellar
- they're often in pretty rough shape (cosmetically) and sometimes technically have tired VFDs etc.
- you don't know if the meter has any quirks/defects
- I don't know when they were calibrated last and if calibration is still good

Since I don't have any equipment to test the accuracy and recalibrate such a meter I'll likely end up having to send it to Keysight/Keithley for calibration who will then tell me that there are a few issues with the meter that need to be fixed along with a 1000€ quote for repairs and calibration. At that point just purchasing a new one will be cheaper plus I have warranty.

Once I actually own a proper multimeter I can use as reference I can totally see me buying a damaged used one for restoration - just as a fun project/learning experience.


--- Quote from: razvan784 on March 03, 2022, 10:18:32 am ---An old Keithley 2000 is also an option (I also like its interface better) but these are not as easily repairable - the circuitry is more complicated and official schematics have not been published.

--- End quote ---

I think they are still sold new too bad though, that there is not much documentation around, but I assume that is true for most modern DMMs. If I really want good documentation I'll have to look for the HP branded stuff (which is really hard to get here).


--- Quote from: razvan784 on March 03, 2022, 10:18:32 am ---For a general-use DMM, sure, capacitance is nice to have, and stuff like continuity and diode mode are important. For a precision bench DMM on the other hand, I don't think those are relevant selection criteria. I also use a DE-5000 for capacitors and I'm quite happy with it.

--- End quote ---

Since this DMM is going to be my only (bench) DMM I'd rather not like to miss out on diode mode, my secondary meter currently is an Aneng 8009. I could live without capcitance though if I absolutely have to.
tautech:

--- Quote from: Traceless on March 03, 2022, 09:56:10 pm ---Since I don't have any equipment to test the accuracy and recalibrate such a meter I'll likely end up having to send it to Keysight/Keithley for calibration who will then tell me that there are a few issues with the meter that need to be fixed along with a 1000€ quote for repairs and calibration. At that point just purchasing a new one will be cheaper plus I have warranty.

--- End quote ---
100% !
Still for basic use the purchase of say of a DMMCheck provides a sanity check that accuracy is appropriate for your needs otherwise the Voltnut rabbit hole awaits.

You may not be aware Defpom has a website for adjusting Cal constraints for the SDM range:
http://www.thedefpom.com/siglent_sdm_calibration.php
Kleinstein:
The Keithley 2000 are no longer sold new, but there are rather similar meters as Keithley 2100 or Picotest M3500.

In Europe the situation with used higher grade DMMs is not as good as in the US.
Traceless:
Hi Bram, thank you very much for the detailed explanation and the effort to post your measurements and compiling all the high precision measurement tipps here I really appreciate that. Also I find that autocal-issue you found quite interesting and would be curious if this is a hardware design flaw or a firmware bug. Probably only Keysight can answer that question. I think you should post yor discovery in the Metrology Forum I'd love to see where the discussion about this goes.

As you say the 34465A is still a good meter despite the autocal issue, the number one question I'm asking myself now is: "Does the 34465As autocal issue make the 34461A actually the better meter just because it is not constantly drifting and recalibrating?"

Ultimately I probably shouldn't worry too much about the autocal problem because the environment conditions the meter will operate under are far from ideal. May lab is right under the roof and like in most german houses there is no air conditioning. That means that especially in the summer time I'll have temperature fluctuations from around 21-26°C in the morning up to 38°C in the afternoon, and I'll have to open the window because otherwise I'll be cooked alive ;). (Mental note to self, don't do precision measurements in summer but rather go out for an ice cream instead ;)) At that point the auto-cal issue is probably going to be negligible.

In autumn, winter and early spring the temperature will be much more stable though also not perfect (likely between 21 and 24°C). Also I usually only warm up my meters for 5 minutes or so, they don't run continously.

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