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Forced Revision DMMCheck Plus

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J-R:
There's no good reason to attack me.  My ribbing was clearly in jest and in my opinion high post count users shouldn't expect the gentle redirect that new members get.  There are plenty of video, written and forum reviews of the DMMCheck Plus as well as the Technical Information page that can provide tons of details about the product.  I can understand if you landed on the Shop pages there is little information provided, but a single click at the top of the page or a search engine would have solved that.  Couldn't you have just apologized for being in a hurry or whatever, and moved on?

The "games" I was playing with my DMMCheck Plus are not useless and I spent a lot of time running the calculations and actually testing it all with half a dozen DMMs before posting.  100mV is a very common DMM mV range and the 1mA current source with the 100 Ohm resistor provided solid results and in my tests were within 2uV based off of two calibrated 6.5 digit DMMs.  The voltage divider as stated is less than ideal, but everything can be calculated for and again the numbers were excellent or I wouldn't have posted the idea.  I wouldn't have any concerns doing this with a 50,000 count DMM, which is going to be a common market for the DMMCheck Plus, but it also provided some decent info for my 6.5 and 7.5 digit DMMs.

To dig a little deeper into my results, we can consider that Russ uses a calibrated 8.5 digit DMM but provides quite a bit less on the calibration sheet.  In my case, I can introduce some very basic uncertainty to the calculations and add or subtract from the LSD on the calibration sheet and get 100.170mV for the high, 100.159mV for the exact calculated value and 100.148mV for the low.  In my case I got 100.160mV on my 34461A and 100.161mV on my 34401A which is a really great result (both using high impedance mode).

KungFuJosh:
Please don't take it personally, I meant specifically useless for my needs, not intended as an attack on you. Sorry if I wasn't clear enough there, I wasn't quite awake yet when I replied.

I understand that we each have different needs and use cases, and that was my point. For my needs, it has to be a specific device calibrated in the low mV range. I'm annoyed that I have 2 calibrated bench DMMs, and in the mV range they're far off enough that a 3rd calibrated device only agrees with one of them. The problem is, I was trying to see if the 3rd device was in spec or not, and one DMM says yes, the other says no.

The DMM that disagreed is going to be sent out for calibration, but I still need a calibrated device to test different voltage ranges, primarily low mV.

CatalinaWOW:

--- Quote from: KungFuJosh on January 26, 2024, 07:58:13 pm ---
--- Quote from: alm on January 26, 2024, 06:27:58 pm ---Actually most voltage references offer just one proper output and are used together with other ratio devices to cover other ranges. With good metrology practices, like a Hamon divider, you can transfer between ranges (e.g. 5 V and 500 mV) with a low uncertainty.

--- End quote ---

Yes, but the important factor here is that whatever is in the circuit is permanent, and calibrated. Without the specs on paper, there's nothing to trust.

--- End quote ---

It needs to be traceable, but that doesn't mean that each and every part used be calibrated.  You can match resistors to arbitrary accuracy using bridge type measurements (It may take a lot of bootstrapping, but it can be done).  Match large enough resistors and you can bring the load within the specified capability of your voltage source and you can eventually get a low voltage reference with accuracy numbers that you know. 

This process is long and tedious, requiring great attention to detail, and side tests to assure that nothing is happening that is unpredicted or controlled.  Buying a calibration box saves most or all of this labor.  But a box that meets a wide range of voltages (mV to kV for example) is very, very pricey.  The DMM check is actually quite inexpensive for what it does.   It would be nice if it did everything, but it meets a need at a price point. 

Perhaps you would like do develop one that meets your specs and sell them at a comparable price.  I am sure you will get substantial sales, particularly after the market determines that your device performs as claimed.  Whether you will make money is much more questionable.

KungFuJosh:

--- Quote from: CatalinaWOW on January 27, 2024, 05:38:25 am ---It needs to be traceable, but that doesn't mean that each and every part used be calibrated.  You can match resistors to arbitrary accuracy using bridge type measurements (It may take a lot of bootstrapping, but it can be done).  Match large enough resistors and you can bring the load within the specified capability of your voltage source and you can eventually get a low voltage reference with accuracy numbers that you know. 

This process is long and tedious, requiring great attention to detail, and side tests to assure that nothing is happening that is unpredicted or controlled.  Buying a calibration box saves most or all of this labor.  But a box that meets a wide range of voltages (mV to kV for example) is very, very pricey.  The DMM check is actually quite inexpensive for what it does.   It would be nice if it did everything, but it meets a need at a price point. 

--- End quote ---

Not each part calibrated, the values need to be calibrated. Say for example it does 2.5V, 5V, 7.5V, 10V like a lot of standards do. Each point is calibrated as a (hopefully) trustworthy value. Now add in 1V, 500mV, 250mV, 100mV 25mV or something like that with calibrated values, that's what I need.

I'm well aware I could mess around and set other arbitrary values. The issue is that I would have to trust my meters to tell the truth, and I don't own a calibrated 8.5 digit DMM. Since I don't trust my DMMs like that, it's impossible to work with a reference that's not externally calibrated in the value ranges that I need.


--- Quote ---Perhaps you would like do develop one that meets your specs and sell them at a comparable price.  I am sure you will get substantial sales, particularly after the market determines that your device performs as claimed.  Whether you will make money is much more questionable.

--- End quote ---
I'm working on a version of the Elektor reference in another thread. My issue is that it's quite expensive (to me/for what it is), and once complete, will need to be sent out for external calibration anyway. I would be super happy if there was already a device under or around $200 that included the ranges I need and calibration certification.

I'd love to develop my own at some point, but the development alone is expensive, then I either need to suffer from TEAS and get some higher end calibrated DMMs, or send them out for 3rd party cal. Who knows. It could happen. 😉

Thanks,
Josh

alm:

--- Quote from: KungFuJosh on January 27, 2024, 03:46:48 pm ---Not each part calibrated, the values need to be calibrated. Say for example it does 2.5V, 5V, 7.5V, 10V like a lot of standards do. Each point is calibrated as a (hopefully) trustworthy value. Now add in 1V, 500mV, 250mV, 100mV 25mV or something like that with calibrated values, that's what I need.

--- End quote ---
The main value in the product like the DMMCheck Plus in my opinion is the external calibration. From there you can transfer to other values yourself. If you follow the traceability chain for the lab, then probably at the top of their equipment is something like a Fluke 732B DCV standard that gives 10V (I'm not sure anyone ever uses the other outputs). All other DC voltages are derived from there using ratio techniques like the Hamon divider I linked to above. As CatalinaWOW mentioned, such a divider can be built without any external calibration using bridge techniques where the only requirement is short term stability of the components involved. In exchange for a second mortgage, Fluke will also sell you a box (autocal multi-function calibrator) that uses the same techniques to generate voltages from mV to kV from a 10V standard (and with a resistance standard also resistance and current).

Of course it is fair to say you don't want to do that work. But then I'd expect to spend a lot more than for the DMMCheck Plus. Generally in calibration you pay per point that you want measured, so if they would have to measure a few dozen points, the calibration would become a lot more expensive. The PVDS2mini (well above €200 and not currently available) might be the closest to what you want, though it may not meet your traceability requirements and the specifications like INL appear to be not very tight.

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