Calibrating an old HP 3478A Multimeter with a DIY uncal calibration aid. - Page 1
Forum section: electronics / general

author: dazz1
author_rank: Frequent Contributor
timestamp: May 31, 2025, 10:19:04 am
content: HiI wanted to attempt to calibrate my old HP 3478A multi meter to a level that would provide me with confidence that the measurements were trustworthy.Getting the meter calibrated by a certified test house would cost more than the meter is worth.Rob from Tautech was kind enough to lend a 6.5digit Siglent multi-meter to compare with my 5.5digit HP 3478A.I was able to compare voltage and current measurements from ordinary power supplies directly between the two meters. This suffered drift and noise pickup that made comparisons to 5.5 digits "challenging" but sufficient to gain confidence in the HP 3478A.For measuring resistance, I made up an aid. The cal process requires resistors in the ratio of 1 to 3. eg. 10k, 30k.I used vero board to mount strings of resistors with a ratio of 1:1:1. Eg. 3x resistors 10k, 10k, 10k.I used random tolerance resistors pulled from my stock.This worked surprisingly well for measuring resistances from 1k/3k and 1M/3M.Below 1k thermal effects caused significant resistance drift.Above 1M, it seems noise pickup caused drift.My meter measured within tolerance for all VDC, VAC, IDC, IAC ranges. It measured out of tolerance for almost all ohms ranges, but not my much. I was not confident enough in my ohms cal aid to make any adjustments to the meter cal settings. I am working on the assumption that the meter is within tolerance for resistance, even though it measured out of cal.If I do this again, using the same comparison method, I would make sure I had a low drift, low noise adjustable power supply. I would also make up a ohms cal aid that was shielded and used high stability resisters for the low value resistors. Good enough to do a confidence check.I don't know when the HP 3478A was last properly calibrated, but it appears to be still within spec. Most of my electronics only requires 1 decimal digit accuracy so 5.5 digits is a luxury I don't actually need. It is nice to know I can rely on the meter if more digits are needed.Going through the cal process by hand was tedious and time consuming.A good DIY project would be something to help control and automate the process. Never seen one published. Commercial systems are way beyond my budget and needs.

author: Harfner
author_rank: Contributor
timestamp: June 03, 2025, 04:25:49 am
content: How did you use the 1:11 divider?

author: Gertjan
author_rank: Frequent Contributor
timestamp: June 03, 2025, 06:00:54 am
content: Hi dazz1,I do the same thing. Once a year I have a 3458A on loan for a month, and I will check the calibration of my DMM'sYes, it can be challenging (and great fun) to create all stimuli as stable as possible.Trying to do the best with your existing equipment is very educational: You will learn about the properties of your kit, and better understand the subtleties of a measurement, and how to avoid errors.More challenging are stable high voltages and currents, especially AC.For your resistor measurements: use 4-wire measurement, and put the resistors in a cookie tin. The cookie tin is for screening. The next challenge is where to connect this screen to: ground, guard, minus terminal of your meter?Use stable connectors, think about the Seebeck effect. So wait until your measurement is stable (at least 5 minutes!)Keep the room temperature stable, and all doors & windows closed. (Why?)regards & good luck, Gertjan.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 20, 2025, 08:29:12 am
content: HiMy voltage source was a Chinese power supply. It has limited resolution and significant noise. For the lower calibration voltages, I used the 11:1 divider. The divider increased the set point resolution and decreased the noise.For calibration, I did not need to set a specific value, I needed to know the value set.

author: dazz1
author_rank: Frequent Contributor
timestamp: June 20, 2025, 08:40:06 am
content: HiThis was an experiment in figuring out the capabilities and limitations of a very budget DIY setup.In my setup, the factors that most affected calibration quality were:short term power supply stability,thermal stability of the resistors, especially the lower value resistors,probably power supply noise.So if I do any upgrades, I would start with the voltage source, both AC & DC.