I bet if you get 3 same models they will have up to 15% difference in readings. BEst to make a lightbox and calibrate to a known good reading. That way you can eliminate the factory offset almost completely.
At least you can measure the relative difference from 1 to another (light source). That's still usefull!
Hmmm... the DX meter you listed is the one I have ordered, only from an eBay vendor. It would be interesting to see how my light readings compare to yours. I have a similarly driven XM-L C8 and TR-1200 to compare...
A pity that the results would not matter. Too many variables... driver, battery, emitter "born" quality, lens...
Tried before. Don gave me some readings to compare to and eventually found myself with a pretty number generator and nothing else. Regarding lux that is.
Indeed. :/ I borrow one particular XTAR TZ-50 which was measured accurately by a friend to exactly 617 OTF lumens with 2 full batteries. That is my calibration light for my lumen boxie. Actually i used it only once for such purposes. Have to make myself a better integrating something instead of the smelly fish styrofoam box i have made. It does work but it is so damn ugly and smelly that i almost do not wish to use it.
Even with that flashlight one cannot be truly certain up to a few lumens but generally the accuracy is still reasonable.
Comparing lux readings is only for comparison between different flashlights. Anything else it is just useless. Perhaps fun but useless.
It's well known that the DX meter under-reads 10-20%, many cases in CPF. I apply a 10% correction factor. Not sure how's the variation within the batch though.
But a lux meter is a better indication for lumens output in both throw and OTF lumens compared to tail readings, i mean when used as a supporting number as a basis for performance it is much more useful than a DMM. ie uncorrected my Ultrafire U80 is only 17k, you would not be mistaking that as outthrowing a DRY triple XM-L which relatively gets me 26k uncorrected. (DRY is 50% more than U80...unless if anybody gets > 26k from a C8 or similar 40mm reflector light)
My 980L gets like 18k only uncorrected. LOL!
No big issue about 3.17m if you are doing "relative".
eg, with a particular DMM I have 3 leads. A 10A stock leads which comes with the Uni T UT-58E, this gets me say 3 amps. A 10A stock leads from another DMM which gives me 3.4A. A 10cm long stock lead from the UT-58E which gives me 3.7A. This is not counting the DMM accuracy itself or that what voltage drop inherent in DMMs etc....
Also, a DMM does not show you how a flashlight reacts to heat. I have some lights like the P60 which drops like a rock after turn-on. If you use such lights for > 1-2 mins, really no point.
BTW, also for that Cer CT1330B DX meter try to use the "middle range" of the range and not over. Eg 200 lux try to use 100 thereabouts, and 2000 try to use 1000 thereabouts.
Try it : If you get a 80 lux reading, it shows 80 on 200 and 2000 range.
But if you have a 190 reading on 200 lux range, it does not show 190 on 2000 range.
Something to do with circuit overload or inaccuracy at that high range probably. Same goes for 2000 range and 20000 range, if you get 1950 on 2000 range, it does not show 1950 on 20000 range. I'd guess the photosensor is not good for high intensity measurements.
So basically you are just accurate for that 100 lux, or 1000 lux values or below. So you vary your flashlight distances accordingly. For thrower (high intensity), you have no choice but to move it farther.
Regarding the current measuring, when the tailcap and the DMM resistance is the same (and the DMM is accurate) you will get the right amperage. There are some methodes (and equipments) to measure very low resistance. If you have a milliohm meter, you are the winner. If you have 2 DMM and a lab power supply with voltage/current limiting, you can measure milliohm.
If you use thinner/thicker longer/shorter wires, you can equalize the resistance of the DMM, but you know that. :)