DIY calibrations, and keep track of drift. - Page 1
Forum section: electronics / projects

author: george graves
author_rank: Super Contributor
timestamp: April 20, 2014, 09:39:17 am
content: As I'm starting to collect more gear - I'm wondering what is a responsible/sensible way of keeping track of calibration, and measuring drift.Half of my gear is old stuff and half Chinese new stuff. It's a mixture of stuff like a HP 3478A(old), fluke 87-5(new), Uni-t 61-e(new), and a LDC-832s counter(really old). Throw in a rogil 1052e(new-ish) and a really old function generator(old). Add in some Korad power supplies(new). I'm happy with the little ebay deals I've cobbled together. Decent lab set up for under $2000For a resistance standard, I use some expensive mouser resistors - low ppm. For a voltage standard, a ebay AD584LH 4-Channel 2.5v/7.5v/5v/10v High Precision Voltage Reference. Meh it's ebay...but it's tested to 5-6 digits.I'm sure I'm not the only one that mixes new and old gear....My question is....what's a responsible/sensible way of keeping track of calibration, and measuring drift?It is as simple as a spread sheet, and keeping track of things over time/temp? What do you use? Stickers on each device? Or what?

author: G0HZU
author_rank: Super Contributor
timestamp: April 20, 2014, 12:55:33 pm
content: There's no 'one size fits all' answer to this but I would suggest that the vast majority of hobbyists can just trust their DMM(s) to make basic AVO type measurements. Of course, the meter might go out of spec or go faulty and so it helps to have more than one meter.For those that have need for greater reassurance they can (like you have done) buy reasonable quality voltage references from Analog Devices for less than the cost of a beer. Or precision resistors can be bought for peanuts.RF is my area of interest and I use a combination of techniques to cross check my RF test gear here at home up to a few GHz. Usually using noise and/or precision step attenuators and a few precision signal sources.But I don't need or expect to achieve great accuracy in my calibration checking. It pays to have a good appreciation of overall measurement uncertainty when making any kind of measurement at RF and the system uncertainty can often be much higher than the accuracy of my home calibration checks. So I try and keep my calibration requirements realistic.I do keep a record of my cross checks and I have a few 'gold standard' pieces of test gear that I use to kick off the whole process. I'm not particularly interested in being able to accurately measure DC voltage/current/resistance beyond a few digits but this puts me in the minority on this forum I guess...In terms of calibrating your counter then maybe you can use the method outlined by W2AEW in this youtube video if you can pick up WWV on a radio.I check my various frequency references in my test gear using offair techniques here in the UK using the Droitwich transmitter on 198kHz and it works well enough for my needs. The transmitter aerial system at Droitwich is currently being serviced so the service is only available at certain times of the day at the minute but I've been using it as a frequency checker for about 20years now.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 20, 2014, 01:42:30 pm
content: I've used a spreadsheet in the past to keep track of drift when doing drift measurements, but mostly I just go through a cal procedure for a given instrument, then record it in a folder kept for the purpose. It's been about decade, so it's probably time to do it again! I've found once-a-year calibration of quality HP, Fluke and some other gear is a waste of time. My HP3455 never changes, nor do my counters or ancient Fluke 77 DVM. I've found it best to keep a voltage standard powered up and handy for spot checks before doing anything important. Sudden failures or changes scare me far more than slow drift.

author: skipjackrc4
author_rank: Regular Contributor
timestamp: April 20, 2014, 03:01:35 pm
content: I don't keep records of anything--just periodically check the equipment against my "standards" and against other pieces of equipment. For low voltage, I have a Krohn Hite voltage source, and an HP 740B for higher voltages (up to 1000V). For resistance, I picked up a set of resistors out of a closing cal lab, along with a set of standard capacitors. For time/frequency, I compare between a rubidium oscillator and a Trimble GPS disciplined oscillator.Basically, if I'm going to be doing a critical measurement, I check it out first. Keeping track of drift is very difficult when your standards may be drifting as well, so I just don't worry about it.

author: Andreas
author_rank: Super Contributor
timestamp: April 20, 2014, 03:58:43 pm
content: Quote from: george graves on April 20, 2014, 09:39:17 amHello,depends what you know already of your instruments and what you intend to do.A spreadsheet with dayly measurements over at least 1 year with tracking temperature + humidity will give you many informations of your instruments.I use this for characterizing the stability of my self built instruments. (Ageing, humidity response, noise + temperature (slope) drift).But this effort usually will not be necessary for a 5,5 digit instrument. And also a band gap reference (AD584) will have a similar stability (at least after some years of ageing) than the instrument itself. So you will probably never know wether the reference or the instrument is drifting. (I would trust the well aged DMM in this case).My "standards" 2 LM399 + 2 LTZ1000A references I try to verify at least once per year against a freshly calibrated instrument. So together with the calibration history of those instruments and according correction factors I get a good picture of the drift of my "standards". For my standards I use stickers where the calibration data is "stored".With best regardsAndreas

author: David Hess
author_rank: Super Contributor
timestamp: April 20, 2014, 06:12:12 pm
content: Quote from: skipjackrc4 on April 20, 2014, 03:01:35 pmI do the same thing. Where I have multiple instruments from different manufacturers and ages, I check them against each other periodically and when making specific measurements as a sanity check.Once this caught an issue I was only peripherally aware of with my best multimeter, an HP3478A. I was making some precision high voltage DC measurements using a high voltage probe which naturally expects a 10 megohm termination. The HP3478A however is high input impedance on its low voltage ranges and you cannot change this behavior. When I saw the discrepancy which was outside of any possible accuracy limitations, I immediately suspected what had happened and confirmed it by doing the math which showed no calibration issue at all between meters.