Hi there,
I've been reading the measurement section of this forum for a while and I've never felt the need to have a meter with more than 3.5 digits but now I think I was contaminated and you are all responsible for this
Last week I got my first bench meter. It's a Solartron 7150, it is marked as "Enertec/Schlumberger" and has all inscriptions in French but it looks identical in every aspect to the Solartron for which I found the manual on this forum. I got the meter from a craigslist-like ads site for 60 euros.
It was a good surprise when I saw that it got its latest cal in 2010. From the stickers it looks like it has been used at an Alcatel lab since 1994. I don't know if it has been manufactured in 1994 or it was moved to that lab in 1994 since the 7150 has been around since the 80s.
I have read here that 7150 have a nasty mains filter with capacitors that can leak and spill their contents inside the meter. Do you think I should replace them now ? Will opening the case and fiddling with the board have any impact on the calibration ?
Unfortunately I don't have any precise references to tell if the device is really in cal. I've tried comparing its readings with my other less precise meters and at least they all agree within their respective specs. It might still be off, but less than the two last digits.
The closest thing to a voltage reference I've found in my parts bin was two LT1021CCN8-5. I've made two identical very simple boards powered by 9V batteries and with a trimming circuit exactly as described in the datasheet page 9. They talk about using a "cermet" type multi-turn trimmer but I only had old 10 turn trimmers from a mysterious brand (that google doesn't know about) so I hope that they are stable enough. I let the meter warm up for several hours and I trimmed the two references as close to 5.00000 Volts as I could. Then I've been re-measuring them every day since Friday to see if anything drifts.
After building the references I realized that 5V is a bad value, since the first digit goes only up to 2, but I don't have a stable enough divider to get 1 Volt. I use the meter in FILTER mode, in this way it takes many measures and averages them to get better reading, it also displays one extra digit so I have a 10 microvolts resolution with my 5 Volts source.
All in all everything seems pretty stable. When the meter has just been turned on, it is about 20 microvolts off for both references. After an hour it gets to 5.00000 Volts and starts slowly wandering between 4.99998 and 5.00002 Volts, which is 4 ppm. At this stage both the source and the meter are within spec but I really wonder if the slow variations come from the LT1021 or from the meter.
Here are the pictures of my reference board, feel free to tell me if I have done anything bad in terms of precision here. I have really no experience with precision electronics.
What would you advise me if I want to build dividers to get 1 and 0.1 Volts ? Do I need a chopper stabilized op amp to buffer the output of the divider ? Also, maybe the next step is to get a few LM399 and see how stable they read ?