To Andreas (reply #55) and Dr. Frank
I've compared the data sheets of RhoPoint Econistors and the Neohm UPW series, from the descriptions, they both use essentially the exact same bobbin designs, absolutely nothing proprietary there, materials and encapsulation. Also, they both specify 0 +/- 5 PPM/°C TCRs, then say 0 +/-3 PPM/°C is typical. Depending on where you look, the 5 PPM/°C spec may be left out, naughty. The power ratings are a bit different between the two considering they both are the same, I think the UPW50 is a bit over rated in power for its size and is going to run rather hot above 0.33W. The derating curves are also different, the RhoPoint curves are more realistic.
Dr. Frank, you are mostly right about the thermal conductivity, because the epoxy bobbins have very low thermal conductivity, as does the silicone rubber coating and shell, some of heat is 'pulled' out of the resistor by the leads and whatever they are soldered into. If they are using an unfilled epoxy the thermal conductivity is ~0.00047 cal/sec/cm/°C, for a filled epoxy, ~.00123 cal/sec/cm/°C, quite low indeed. Depending on just what kind of epoxy these guys are using, the resistor's characteristics will be different, mostly evident in the linearity of the TCR line.
Given enough time, the thermal 'lag' of the resistor will catch up to the change in temperature and will stabilize at that point, until that point is reached, any readings will be in error and could show up as a 'hysteresis' error. Once stabilized, the resistance readings should be consistent, if they are not, some other form of 'error' is evident, possibly within the resistor itself if all else remains the same. Unfortunately, you cannot directly read the internal temperature of the resistor without a temperature probe (such as a NTC) in close contact with the resistor's windings, quite impractical. At best, you can only come in close contact with the resistor's shell and try to compensate for the thermal lag by waiting a sufficient time. Indeed, TCR measurements have many ways of cobbing things up.
However, compared to the other type of PWW resistors which are molded in DAP or alkyd, the thermal conductivity is much higher but the molding process imparts a severe strain on the resistor's windings which cannot be completely relieved by any stability bake, the resistor will continue to drift at an unknown rate over its lifetime.
Obviously, oil filled resistors are probably going to have a shorter heat lag as mineral oil has a higher thermal conductivity than epoxy but the same situation still applies here, sufficient time must be given for temperature stabilization or the readings are inaccurate.
While I'm thinking of it, in the postings on stability curves, I still have not seen any references to reading uncertainties, while I've seen some very well done stability measurements, the readings are consistently inferred as accurate, such as 0.2 PPM/°C, while that number may be consistent, the actual value could be anything within +0.5 PPM/°C to -0.1 PPM/°C (the uncertainty I chose is a ballpark figure, it could be a bit less or even a bit more). No calibration lab would ever give an measurement number without an uncertainty figure of the measurement. The measurements I have seen here do indicate a good stability figure but do not represent an accurate one and there are uncertainties in regression calculations as they are only as accurate as the data being used at best. Another source of error (and this one is fairly big) is that, at a minimum, the instruments and references must be calibrated/accurate to a known figure that is at least 5 times better, preferably 10 times better than the measurements being attempted. None of the equipment nor references used here have specifications good enough to even come close to accurate readings; linearities, resolution (big one), ect., are no where near good enough, the uncertainties are very significant.
My own instruments, the ESI 242D, in a direct comparison to my calibrated SR-104 would be on the ragged edge of sub-PPM measurements and the uncertainties makes the absolute measurement of those quantities rather suspect. If I can make repeatable measurements within a given time period, that only means that the uncertainties are also stable with time but that does not improve the accuracy of the measurement. This is quite acceptable if one is attempting to match resistor TCRs, the uncertainty is still there but the matchup of the resistors remains very accurate, only the absolute accuracy is suspect.
If you are working with a resistor that has linear TCR qualities, the best way to improve the accuracy of the TCR measurement is to measure it at several temperatures over as wide a temperature range as possible, a 100°C range is typical if the resistor can stand those temperatures. If the TCR curve is nonlinear, then it becomes much more difficult without very high grade laboratory equipment and if accuracy is important, a 4-wire Kelvin bridge is really not up to the job. A much more complex setup is required, primary calibration labs usually employs such a bridge for very precise measurements or a DCC bridge if the resistance in question is below 10K ohms.
Dr. Frank's setup has shown very good stability over a given time period, he has done an impressive job in that respect but I still caution readers here to not accept any readings as anything more than a figure of stability which may, after all, be the only characteristic intended.