Electronics > Metrology
The Current Cost of Chasing PPM's (Fluke 732b price list)
MisterDiodes:
I know a lot of people here on EEVblog will know this already, but this might be interesting for others new to Voltage Reference design.
Since I just helped a client setup a critical test procedure and required equipment list on a critical semiconductor test process, I thought I post here current prices from Fluke for 732b voltage reference units.
The method below would apply to a reasonably profitable business or very serious hobbyist, of which we have a few on EEVBlog.
Background:
For most people, if you're a student of LTZ1000 circuits, one of the more economical solutions is having a bank of 732b's and possibly a good 7 decade Kelvin-Varley Divider and a battery-operated null meter (Keithley 155 / Fluke 845ab / similar) will give good measurement results down into the sub 5ppm range, and sometimes even into the sub 3ppm range - but anytime you're going after about 2ppm or better gets exponentially harder. The advantage to NOT using a 3458a DMM here is that you can get an absolute voltage measure while running the entire test system completely on battery power - and we'll even switch over to battery powered DC lab lighting for attempting an accurate low-ppm measure. Any switching power supply in the vicinity can sometimes cause a noise / drift problem with critical measures. Also realize that a LTZ1000 - especially the non-A version - can pick up any vibration or board stress in the vicinity of the part. So really keep an eye on the test environment and shielding is key.
Generally, single 3458a will not give you a full story on how an LTZ1000/a circuit is performing for drift. It will tell you how your circuit is drifting compared to the Vref in the 3458a, but you won't have a lot of low-ppm confidence in the absolute voltage value. Basically if your working with an LTZ circuit and 3458a, you're comparing two of the same reference circuit. Depending on where your 3458a is very recently calibrated, you can get good ballpark results in the ppm range, but even a 3458a is not considered a true transfer standard for voltage. If you have several 3458a's that can help - the late Jim Williams would measure into the low PPM range with usually 3ea or 5ea 3458a's to increase measurement confidence. Good, properly kept, aged and calibrated 3458a's can typically have drift rates as good or better than 3458a's with -002 or -HFL options, so we rarely worry about that unless you need that low drift performance on an infant "new" unit.
Comparing a Voltage Reference to at least a few 732b's is one accepted way to get an accurate measure of of an absolute voltage in the lab to better than 5ppm. The 732b will have a similar zener diode inside, but it's drift characteristic is presumably (not always) a bit different than an LTZ circuit, and more than one 732b on hand will reduce uncertainty. A 732a that is well aged and working properly (not easy to find on the used market) can even be more stable than a 732b, but be aware there are quite a few drifty 'A units in circulation now, and most seller's haven't got a clue of what they are selling. The 732a / 732b does NOT like to be powered off for long periods of time - if they've been sitting in a hot / cold warehouse for years they can take months to stabilize when powered up - or never stabilize. A 732a/b that has a drifty zener or oven assembly is basically useless since the critical zener / transistor and surrounding PWW / laser trimmed resistors are a carefully matched set, and Fluke isn't going to ever hand those replacements out, especially for 732a. Sometimes a 732a can be repaired with the simpler things like caps in the power supply, new batteries, etc. But sometimes they are 100% useless, so beware.
SO: If you're into LTZ circuits as a profitable business, probably the most direct, time-saving way is to buy a new or off-lease 732b units from Fluke, with a warranty. Even on the new 732b's we've seen are having to be returned to Fluke for repair, so getting a warranty on these units is a valuable time & money-saver. Being acquired by Danaher has done nothing for Fluke quality, at least from what we're seeing.
PRICES:
Current prices quote direct from Fluke as of December 2016 (there will be a price increase for January but I wasn't able to get that in time for this essay). Prices in USD$, do not include shipping:
732b - Basic unit, shipped to you cold (power off), no calibration: $8,500
732b/h - Basic unit shipped to you hot (shipped on battery power) with basic calibration certificate (not accredited): $9,525
732b/c - Basic unit shipped to you hot, with basic calibration certificate and a 90 day drift rate characterization: $10,705
Basic calibration for 732a or 732b runs about $550. If you want accredited Z-540 documentation that'll run around $1,632. That will get you within +-2ppm when your unit is shipped. You can pay for tighter tolerance calibrations as well if price is no object.
Most ordinary cal labs are not equipped to calibrate (and perhaps adjust) a 732b to low ppm uncertainty. Fluke is. You typically will never find a cal'd unit on eBay or any used equipment auction.
HEAD'S UP: If your 732b goes without power for long enough for the batteries to die (about 72 hrs with good batts), then your calibration certificate is immediately invalid with Fluke. It will need to go back in to get a new cal certificate at that point. Keep them powered up and have a good lab backup power plan.
NOTE that a 734A is really just a metal frame for 4ea. independent 732b's of your choice, and some built-in power sockets at the back. The idea here is that to comply with NIST guidelines you always have at least 3ea independent voltage references (with no common point of failure) to inter-compare references, and the 4th unit is for shipping into the field for adjusting field equipment OR for shipping back to Fluke for calibration. When the 4th unit returns from calibration or field test it is immediately compared against the other 3 units on a 734A bank to make sure it is still in-spec and no damage occurred.
Price for 734A is basically price of the 4ea 732b's of your choice plus another $1250. So you're looking at around $40k for some 732b's and maybe another $10k or so for each 3458a for a well-equipped lab that maintains a good and accurate "Volt" with low uncertainty.
Of course when you talk to Fluke sales they may offer you a deal on refurbished lease-returns or loaner units.
So a 734A set of 4ea. 732b's can get you in the ballpark for some accurate low PPM measures, and that's about what it takes if you're really checking high-end (LTZ and similar) voltage reference performance accurately.
Some interesting reading - How many references do you need, and how often do you need to calibrate to maintain a 0.3ppm lab "Reference Volt" (this would allow to to measure accurately to say about 1ppm at any time with confidence)?
http://download.flukecal.com/pub/literature/aug96pp2.pdf
I know there are other methods to maintain an accurate reference lab Volt, but the method above is probably the more popular and relatively economical way. If you have an unlimited budget, the other way would be to maintain an Josephson Junction Array, but that is generally much more expensive. You can call these guys for a current price quote:
http://www.hypres.com/products/voltage-standard/
Vgkid:
Thanks for this, it is always neat to read about what is involved in maintaining the upper echelons of voltage references.
retrolefty:
Volt-nuts!
I really wonder what the total U.S. market is for such a set-up? How many publically listed manufactures order/need this level of standard? Does Fluke publish sales volume by product?
MisterDiodes:
--- Quote from: retrolefty on January 03, 2017, 08:39:47 pm ---Volt-nuts!
I really wonder what the total U.S. market is for such a set-up? How many publically listed manufactures order/need this level of standard? Does Fluke publish sales volume by product?
--- End quote ---
You'd be surprised. I don't know the actual number but at semiconductor fabs plants they will typically have a calibration room where the "good" references are kept - and you'll see quit a few 734b sets and racks of 3458a's on the production line test jigs. For one outfit I was in where they do very high reliability laser diodes (where every diode produced has a serial number) - they cycle the production test jig 3458a's into the cal room often (at least every 30 or 45 days) to get a touch up - because these devices being produced come with a 30yr warranty, and so this company are absolute sticklers for detail, and if anything is drifting it is pulled out of service right away. Plus there is that Dept. of Defense traceability requirement as well for every piece of equipment used to build a component.
For these places, they don't worry much about 1yr drift on an LTZ circuit, they are more concerned with the reliability of 30 or 90 day drift and the low noise you get with several LTZ's running in parallel on a production line test apparatus.
In this case the Fluke standards are kept in cal often with Fluke mobile cal service, and both Keysight and Fluke have their own offices in-plant to do detailed inspections and repairs, and many pieces of equipment are kept in stock as hot spares. If a test engineer or production manager needs a 3458a or whatever, they can go get one of several already warmed up off the shelf and put it in service right away - the production line has to run 24/7. So an application like that will keep quite a few 732b's busy as reference checks and as artifact sources for other gear.
So you know, on the price list above, there is about a 1 to 2 week wait on the basic 732b's, the 732b/c has about a 6 week wait. Which sounds like they have several in the pipeline at any time, which means Fluke probably has a fairly steady supply of orders.
VintageNut:
I visited a cal lab last year that has 4 x 732B and has had this group of references for many years. The drift is about 0.03 ppm/month in the up direction.
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