It is realy measuring ESR in Rs mode, in all frequencies. Rs = ESR, besides that, I checked readings agains the D of my GR1620 ( and tested it for other ranges against my standards) and it is realy ESR, not Z like on most ESR meters.
Problem is the test done by Robrenz does not prove it measures ESR. ( but it does !) But it proves the accuracy, and I do not think our meters are the only two, a compare would be nice but difficult, i o not think someone will buy two of them and I'm not Dave, the manufactures do not send me standard caps and coils or instruments as a test ;-)
A Z meauring ESR meter will also show an acurate increase because they operate at 100kHz and Xc is neglectble there for larger capacitors, ( datasheets give Z at 100 kHz, the times I saw ESR it was at 1 kHz together with D, what is stated always at 1 kHz or 120Hz) that is why many peope have so much faith in their ESR meters. But that test is not the way. Measuring DF or phase angle and transformimg that to ESR is the right way. The IET DOES measure real ESR in all C ranges and in Rs.
A very old test for LCR meters is to add 1 K in series with the capacitor, the meter should still give the same capacity. Many meters fail this test and give the capacitance based on the |Z| meaured.
If accuracy is based on the right R ratio calibration and Open/short calibration. I think there could be the huge difference between IET and other chipset users. The R-ratio calibration is done by the manufacturer and I think the resistors under the analog front end take care of that. And using cheap non precision chinese resistors or high quality precision resistors makes a big difference in performance and in cost. The fact IET only uses one one-turn trimmer and the CEM use 2 multiturns also is remarkable, multiturns are a cheap and more unstable way to avoid expensive precision components or hand-picking components.
Besides that there are things like routing, connectors, quality of pcb material, proper use of guarding ( very important) and things like Leaking current due to dirty boards or chips ( that is why i never open this sort of instruments unless for repair) and that could be the reason for the potting on the IET chipset too !
Bob Pease once has written about how you ruin a precision chip by touching it with your fingers. The leakage current can increase with a factor as big as 100. )
In my Fluke 332 are pcbs that have a sticker not to touch them because they are ultrasonic cleaned. My Fluke 720 was out leakage specs at 1000V by a factor of 1000, after two times cleaning using ipa and measuring it hanging on a rope i got it good again.
I designed and build a fF meter ( meaures impedance but ESR is neglectble for the result in those regions) with 1 fF usable resolution. It had 6,5 digit readout using a TRMS meter. The last 2 digits were unstable until I added guarding and cleaned everything with ipa, even cleaning between the front connctors improved performance.
If it was so easy, just using a chipset, every multimeter using a LM399 would be the same too ?
I think cyrustek makes a hell of a lot mor LCR chipsets as for instance AD makes LM399 or LT makes LTZ1000 Vrefs. And both those you can buy in grades so why not the cyrustek chipset ? Also things like burn in time by the manufacturer of the meter make a difference, maybe here too. There also is a crystal. There is a big difference in stability between cheap and good crystals, and a stable frequency is important for accurate measurements. And IET also offers NIST calibration for the DE-5000 as an option ( but that takes 2 weeks and a lot of cash extra)
I also do not understand why the masstek needs 12V and does not work on 9V if it used the same chipset.
An other strange thing, i can not find real datasheets of this chipset. Only the (end-user based) sales brochure they call datasheet, so it could be possible there are different versions. If they statet accuracy < 0.3 % It also can be they can deliver chipsets with accuracy < 0.3 and not all chipsets are better then that. It can be possible they also have 1 % or worse sets. We will never know, and assuming is the mother of all.......
So all these factors make the meter too, but buyers only look, "hey, it is cheap, they use that chipset too, so it must be as good as the IET", even advise to use the IET manual. yes, sure, dream on, like with many multimeters, it uses the same cabinet, or some dare to say, almost the same, so it must be the same meter. The chinese are more clever then some buyers. if I build a car out of wood and cardboard, looking like a RR with a RR engine in it, it is still not a RR ;-)
It even could be possible some chipsets are fake ( I do not say it is, but there are plenty examples of fake Chinese components and instruments so it is possible)