Poor Man's Metrology (!) Lab - Page 1
Forum section: electronics / general

author: zhtoor
author_rank: Frequent Contributor
timestamp: February 18, 2018, 08:08:05 pm
content: -

author: LazyJack
author_rank: Frequent Contributor
timestamp: February 19, 2018, 04:00:40 pm
content: How are you going to get all this calibrated?

author: rhb
author_rank: Super Contributor
timestamp: February 19, 2018, 08:15:54 pm
content: That's what Weston cells are for.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 19, 2018, 09:14:17 pm
content: The Weston cells are a way to kind of get an absolute voltage scale and maybe for longer term. However they are not very practical for every day use - more like best if not touched at all or only maybe every month or so.So one would kind of still need a more practical (e.g. zener based) voltage source / reference.The Keithley 150 is likely mainly used as a Null-meter and thus does not really need calibration for this purpose.

author: Gyro
author_rank: Super Contributor
timestamp: February 19, 2018, 10:18:12 pm
content: The Muirhead D-801-D decade divider spec attached in case you don't already have it. Its basic spec is +/-0.1%, 40ppm/'C .....

author: rhb
author_rank: Super Contributor
timestamp: February 20, 2018, 12:29:50 am
content: Weston cells *were* the voltage standard for a long time. You can't draw any current from them, so using them is difficult and they are rather delicate. But it does provide the OP with a means of calibration. Rather old school, but I find much to admire in the effort.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: February 20, 2018, 01:31:32 am
content: As i heard , Weston cell produce are electric potential , need an extremely high resistance ( gigaohms ?) to use it , and do not kill it ... . Kinda some extra precision follower need to construct.

author: rhb
author_rank: Super Contributor
timestamp: February 20, 2018, 02:35:29 pm
content: The normal practice for sensing the voltage was an instrumentation amplifier with an input impedance of several hundred Megohms. And even then, measurements must be done quickly to avoid damage to the cell. You also need at least 3 cells to compare to check for aging and damage.With an instrumentation amp made up using JFET op amps like the TL08x the OP should be able to calibrate to the accuracy of the meter. NIST recommended annual verification of the reference Weston cell to achieve an accuracy of 0.01%.

author: MisterDiodes
author_rank: Frequent Contributor
timestamp: February 21, 2018, 12:30:23 am
content: Still with the Weston cells...and still not the most useful. UNLESS they are used as a quiet excitation source, and for that other batteries will work just as well for what you have and not so fragile.Still the trouble with those Westons is they really need temperature stabilization, and don't move them once they are measured. AS in: Don't. Touch. We've already established you don't have a good way to get those hand-carried for cal... so unless you've moved in next door to a cal lab, probably not the best.I think what you want to try to acquire next is something like a good working 731b. They are small, lightweight, cheap to calibrate and transport very well, and generally at a lower drift rate than your wet cells. They aren't the quietest but your equipment probably won't care...and then you'll have a way to bring a known absolute value volt (with much lower uncertainty than the rest of your equipment) TO your Westons. Now you have a way to monitor their discharge rate AND check other measures in your lab. Might be something to think about.

author: cellularmitosis
author_rank: Unknown
timestamp: February 22, 2018, 05:51:15 am
content: zhtoor, I enjoy reading your "poor man's" postsIf you can create a system which allows you to locally track the drift of each Weston cell relative to each other, then all you need is a transfer standard which allows you to "pin" your relative drifts to an absolute value on occasion.If you found a member of the USA Cal Club who would be willing to help a poor man out, you could do this without spending a fortune. (I would be happy to do so, but I don't have a quality history behind any of my references yet. Some day!).It looks like United States Postal Service, First Class Mail Packages from USA to Pakistan are $13.75 for up to 8 ounces (0.226 kg), and $23.25 for up to 32 ounces (0.907 kg).https://postcalc.usps.com/Calculator/ShapeAndSize?country=10343&ccode=PK&omil=False&dmil=False&mdt=2%2F21%2F2018&mdz=23%3A59&m=0&dvi=1For reference, a Hammond 1590B aluminum case is under 5 ounces, so I think it is very realistic to create a LM399 or LTZ1000 transfer standard which is shippable at under 8 ounces.

author: cellularmitosis
author_rank: Unknown
timestamp: February 23, 2018, 03:51:32 am
content: See also:https://www.eevblog.com/forum/metrology/usa-calibration-club/msg1434002/#msg1434002In your case, if you wanted to divide an LM399 from 6.95V down to 1.0183V, you'd need a voltage divider of ratio 0.146518.The "hifi" ebay seller has several z-foil resistor combinations which come close to this ratio:https://ssl.pepas.com/hifi/resistor-ratios-z-all.txtCode:0.146490097269 [1x RNC90Z 1K2500 VR Vishay RNC90 Series Metal Foil Resistors Y11891K25000VR0â€‹L] [1x Z201 8K53300 0.005% Vishay Z-Foil Series Metal Foil Resistors Y14538K53300V0L]0.146551724138 [1X RNC90Z 340R00 BR Vishay RNC90 Series Metal Foil Resistors Y1189340R000BR0â€‹L] [1x RNC90Z 2K3200 BR Vishay RNC90 Series Metal Foil Resistors Y11892K32000BR0â€‹L]0.146551724138 [1x Z201 68R000 1.0% Vishay Z-Foil Series Metal Foil Resistors Y145368R0000F0L] [1x RNC90Z 464R00 FR Vishay RNC90 Series Metal Foil Resistors Y1189464R000FR0â€‹L]0.146575342466 [1x RNC90Z 535R00 TR Vishay RNC90 Series Metal Foil Resistors Y1189535R000TR0â€‹L] [1x RNC90Z 3K6500 BR Vishay RNC90 Series Metal Foil Resistors Y11893K65000BR0â€‹L]To find the resistors on ebay, just search for the description, e.g. "RNC90Z 1K2500 VR Vishay RNC90 Series Metal Foil Resistors".edit: fixed the quoted resistors