Some first measurements after keeping the SDM3065 running for now 480h.
But first a disclaimer to avoid unnecessary discussions:
- It's my hobby, 35 years ago I was involved in measurement topics at university but this is long, long ago
- a real measurement needs at least a 7.5 digit DMM fully calibrated, I can only compare relatively to my HP34401a (20y+ old)
- for some days I got the strong desire for appropriate Voltage references but reading thru the threads here convinced me that it needs
a lot of effort and funding to get something good enough for 6.5 digit DMMs and again access to a much better calibrated DMM
- all measurements were done with the help of HKJs great software TestController
https://www.eevblog.com/forum/testgear/program-that-can-log-from-many-multimeters/- temperature was always 23...24 C in my lab.
- all readings done with Hi-Z (10Gohms) and 100 PLC
My observations:
First surprise: The readout of the SDM is 3 digits more than displayed see photo but the last 2 make no sense to me
May be software is still prepared for a upcoming SDM3085X
(joke)
In photo 2 you see the warming up of the HP34401a against the running SDM3065 for a 13h period. Voltage source is the 9 volts battery (block) in a double styropors box.
I was surprised how sensitive at this microVolts level the battery is for mechanical influence, sitting on my bench and do some soldering was enough for drifts.
In photo 3 the same data normalized for the first 2 hours. I've done some mathematical voodoo in Excel to keep the noise levels of both devices visible.
It took 2 hours for the HP34401a to warm up and as we will later see there is a little movement during the next hours.
Plz be aware that the readings are 7.5 digit from both devices but it's clear that the SDM is a little bit noisier.
Last picture shows the measured difference of the (HP-SDM) at 9.5 volts starting at 120 min till 720min.
It's obvious that there is another "settlement" of the HP during the next 3 hours.
As mentioned before the SDM is now for 20 days 30...40 Microvolts (0.03...0.04 mV) in the 20 V range lower than the HP (in 10 Volt range). There was no additional drift
during this "always on" period.
Looking into the specs of both devices and doing some rough calcs (assuming 10 volts reading):
SDM: In 20 V DV range the drift in 90 days can be 0.0030% of measured value + 0.004% of range = 1.1 mV
HP: In 10 V DV range the drift in 1y (there is no number for 25 years
) can be 0.0050% of reading and 0.0030% of range = 0.85 mV
Assuming both devices are on the lower limit of this span the HP shows (for 9.5 volts) 9.499,15 volts and the SDM 9.498,9 volts. (250 uV difference)
The measured difference of about 30...40 uV is much better.
I'm pretty happy with this results but this is of course "amateurish", further measurement in other ranges and of linearity can be made.
But I have now enough confidence in the SDM and can use it for my projects and I'll avoid the "volt nuts" rabbit hole.
Next week I will do the measurement of the SDM's warming up against the running HP.