Let’s go with an update of the project. Eventually I went with a slightly different approach and components. This is the final schematic:
Resistor enclosureThe container is made of copper sheet and the sides will be terminated with precut PCBs. Tin solder is used to avoid leaks. Eventually the enclosure will be covered with a thermal insulation sheet.
Heating element and temperature sensing.I decided that I’d need a heated device cause I can’t control very well the temp everywhere at my lab over the year and because I’m more interested in short term accuracy rather than long term drift. At the lower end of the oil tank I’ve placed an aluminum housed power resistor and a RTD sensor at the top. The RTD sensor is a PPG102A1. The resistor terminals cross the copper wall with a couple of Tusonix feed throughs model 4101-002, small but rated for 10A. Still haven’t decided the RTD amplifier to use. The heating resistor and the 10k adjustable resistor will be separated by a metallic mesh for EM shielding.
Series adjusting element.Eventually I went with the Vishay acccutrim trim for its smooth travel, was tempted to use a bourns wire wound though. The 1ohm resistors are wire wound from Welwyn AS21R00FTR. The trim needs an internal support so I made that with my mill in bakelite. The adjusting shaft was made in the lathe with aluminum rod and features 2 o-rings. This shaft is contained in a methacrylate cylindrical shell.
The adjustment range is as follows:
this design: 0.416 ohms
HP 11104A: claims +-30ppm, so 0.6ohms
SRX-10k: 0.666
Main resistorI chose here a well trusted and very stable wire wound from the HSP series of Ohmite, model HS520A10K00B and manufactured 1 year ago. Internally it is made of 6 sections similar to other resistors seen on this forum (standard resistors thread).
This is a 0.1% resistor so depending on the measured value I’ll have to select an appropriate ‘compensation’ resistor. With the 3458A working at a stable TEMP? 34.5º, the resistor at 19º and after proper configuration (ACAL, OHMF, APER 1, DELAY 1, OCOMP ON, NDIG 8, it reads a very stable 9999.55. After heating the resistor at 40º (planned heating temp) it goes down to around 9998.4, so this equals about 6ppm/C for this range. After cooling of the resistor and another ACAL the original value is read back again although it takes nearly an hour to do so.
Adjusting resistor
Given the value obtained after heating the 10k resistor I need to use a 1ohm series resistor, same type as before.
To be continued...