>>I've looked at the Geller labs reference before, and it's "only" accurate to 0.005% over 6 months. The new more expensive Malone reference is can do better, 0.0025%, but both of these are still more then double what the Fluke 8505A is rated for.
"I suppose what you are saying is keep sending them back for recalibration (or at least remeasuring with their 8.5digit calibrated multimeters) in order to hide their drift over time. Yeah, I guess that would work."
Since the comment quotes our published specification, it is entirely unclear what is being hidden.
But, far more importantly, the statement above completely misses the point of our product. What we are trying to do is to supply a single point calibration reference that is more accurate than is generally available to an amateur scientist or electronics hobbyist. Our SVR boards are not voltage standards, they are transfer references. For many hobbyists there are more suitable products, you should buy the product of your choice, I only write here to better explain what we are trying to accomplish. This program generally operates at a loss, there is no "got you" here to make money.
The goal of our project is to transfer the absolute voltage from our Fluke 732B, a "transfer standard", to the experimenter. Our Fluke 732B is calibrated by Fluke, usually annually (we skipped a recent period for economic concerns, it goes back this month, possibly tomorrow). A "calibrated" Agilent 3458A -alone- is not even good enough for 10 ppm and below. As others have noted, while time & frequency calibration to parts in 10 to the 12 (1e-12) is common now in amateur labs, absolute voltage calibration below 10 ppm and especially at 1 ppm is difficult. So, even though our 3458A was new out of the box (many thousands) in 2005, it cannot hold better than about 10 ppm / year. And, most 3458A users don't realize the importance of the auto cal feature just to hold that value (at the ppm levels, auto cal needs to be used every day as well as for for every one degree c change of room temperature). That is why we only use the 3458A to finally calibrate boards based on a short term measurement of the 732B 10V output.
Our short term promise is better than 10 ppm absolute, which is better than 0.001% (actually our spec is better than +/- .0005% absolute (+/- 5 ppm)). Most test runs do better. Since the chips are rated at 5 ppm / c (50 uV/c), our SVR board should be used at about the temperature where we calibrated for best transfer accuracy. Also, the board should be warmed up for about 30 to 60 minutes, and run within about 0.1 V of 15V (however, the chips perform much better than the spec for power supply sensitivity (100 uV/V). At a relatively low level of precision (e.g. 0.01%) all of these concerns are a non-issue.
To some, all these factors are taken as deceitful, however that is exactly the opposite of what is intended. The point is, is that it is very difficult to attain transfer accuracy between 10 and 1 ppm with a board that costs $35 new. That includes parts and hardly reflects assembly and calibration cost or overhead.
The 6 month spec is based on AD's 15 ppm / 1,000 hours. However, it is very conservative, since we burn in all of our SVR boards for 200 hours plus. AD reports that most of the drift occurs in the first several hundred hours, see this ADI tech note:
http://www.analog.com/static/imported-files/application_notes/301548125AN-713_0.pdf .
At over $500 / year (includes overnight shipping on battery) to maintain our Fluke 732B (not including a new battery every few years), the purchase price of our 732B, the 3458A, and testing and development of the SVR program, the $10 calibration fee does not go far. This is a service, not a "trick" to hide some hidden specification or flaw. In most normal business operations $10 is spent opening an incoming box. Then we use bench space to warm it up, check the 3458A against the Fluke 732B, carefully calibrate the SVR board, prepare a report, prepare a shipping label, pack it with a new conductive bag, over bag, bubble wrap, and peanuts in a new small box. Then we deliver it to the post office. The board can then provide another accurate voltage transfer. In the US anyway, that means the SVR program operates at a loss.
As many hobbyists have found that 0.01% is good enough, and I turn my attention to our magnetometer project, we are re-evaluating whether the program should continue. However, since we recently obtained a new batch of ceramic AD587LQ's from a power supply manufacturer that changed their design, we will continue in the short term ... with a new batch of boards for sale in about a couple of weeks.
Regards,
Joe Geller
GELLER (Geller Labs)
http://www.gellerlabs.com