Welcome to the Thursday Toolkit series, about tools for building Physical Computing projects.
(this is a bit of a side excursion, about checking the quality of a measuring instrument)
“Ah, but is it any good?” – that’s the inevitable question to ask when getting a precise instrument, right?
I’m referring to the 6.5 digit 34401A HP (now Agilent) multimeter I got my hands on, recently. This translates to: better than 1 ppm (part per million), i.e. 10,000 times more accurate than one percent!
This is the sort of thing the members of the volt-nuts mailing list ponder about, I would imagine.
In my case, with now over half a dozen ways to measure voltage here (numerous hand-held multimeters, mostly), I just wanted to know which one to trust most and to what extent.
The solution comes in the form of a transfer voltage standard – an item you can order, gets shipped to you, and which then gives a certain level of confidence that it will provide a fixed voltage reference. As it turns out, Geller Labs offer just such a thing at low cost – it’s called the SVR 2.0:
Put 15V on its input (left), wait 30 min, and the output pins (right) will produce exactly 10.00000 Volt – magic!
Each board is “burned in” (kept turned on) for 200 hours and calibrated at the temperature you specify (I asked for 21°C). You even get the measured temperature coefficient at that spot (mine is 1.7 ppm/°C), so you can in fact predict the voltage it will generate at a slightly different temperature. Now that’s serious calibration!
My bench-top multimeter will indeed go down to 1 ppm in 6-digit mode, i.e. steps of 10 µV when measuring 10 V:
And guess what – after a 30-minute warm-up (both the 34401A and the SVR), it’s spot on.
No last-digit jitter, nothing. A constant 10.000,00 readout. The current room temperature is 21.1°C, heh.
Think about it for a second: as hobbyist, you can order a precision second-hand instrument from eBay, Google around a bit to find a little voltage standard, have ‘em shipped from different parts of the planet, get them here within two weeks, hook up some wires, wait 30 minutes, and they match to 0.000,1 % precision.
Given that this instrument is from the 90′s, I’m massively impressed. This 34401A HP thing rocks!
Voltage? Current? Resistance? Game over – for me, this is more than enough precision for serious use.