Although still being in the metrology kindergarten I dare to present a little experiment.
Playing around with multimeters I liked to have a resistance transfer standard to transfer the 10 k and 100 k to 1 M and 10 M with moderate accuracy. Looking into the specifications of the multimeters (not very impressive in these ranges) and to my bank account I did not want to buy several SR-1010. But I thought about adapting the concept and building a low-accuracy poor-man's pocket version:
fig_1
The idea was to take leaded metal film resistors (1 M and 100 K, 1 %, TK 50, Vishay Beyschlag MBB), select them to be within 100 ppm of value with each other and assemble them in good thermal contact using a piece of aluminium. Using 25-pole sub-D-connectors as programming/shorting bars allows easy and fast switching from 0.1*R to 10*R.
How it was done:
fig_2
Taking 100 resistors, select them. Taking some some scrap aluminium and a milling machine gives the casing and will probably help to achieve somewhat homogeneous temperature and and electromagnetic shielding. A small hole in the bottom section of the casing allows the application of a temperature sensor.
Traces were cut into a piece of thin (1 mm) epoxy-pcb-material. The circuit board was glued into the casing. Copper wires were soldered to the traces to reduce the resistance of the traces. The leads of the selected resistors were bent carefully and the resistors were soldered onto the traces and to the sub-D-connector. Two contact pins of the connector were used in parallel to lower the contact resistance. Using a large area for soldering the resistor to the circuit board I tried to obtain a good heat-flow from the leads of the resistor into the casing:
fig_3
fig_4
To contact this module a "contact-box" was made. Two sub-D-connectors were "programmed" to give 0.1*R and the 10*R and connected to the 4 mm-sockets in parallel using the special very-ugly-wiring technique:
fig_5
The performance:
Playing around with a spreadsheet shows the error of the 0.1*R to 10*R ratio to be much lower than 1 ppm when using selected resistors with a tolerance within 100 ppm.
The max. error of the resistance due to self heating during measurement can be estimated to be 2,5 ppm (100 k-range, current of 100 µA, 100 µW per resistor worst case, assuming a R(thermal) of 500 K/W).
The isolation resistance was measured roughly (using an old Keithley 610C electrometer) to be approximately
75 GOhm (contact box - connector to connector),
25 GOhm (contact box with both modules installed - connectors to casing)
> 2 TOhm (modules - contacts to casing, surprisingly high).
The shocking 75 GOhm might add an error of approx. 140 ppm in the 10 M-range and approx. 14 ppm in the 10 M-range... One day I should modify the terminals... There must be some PTFE in the basement...
Finally I tested the device using a Tektronix DMM4050 (the only calibrated meter I have) in statistics mode:
Module -- Range -- T/°C R (2 wire) -- SD -- N
10k/1M -- 10k -- 27.3 -- 10.02457 k -- 1.90 m -- 128
10k/1M -- 1 M -- 27.5 -- 1.002465 M -- 769 m -- 100
100k/10M -- 100 k -- 27.4 -- 100.0285 k -- 146 m -- 101
100k/10M -- 10 M -- 27.4 -- 10.00291 M -- 73.9 -- 100 (first measurement)
100k/10M -- 10 M -- 27.3 -- 10.00290 M -- 145 -- 100 (second measurement)
100k/10M -- 10 M -- 27.5 -- 9.995030 M -- --- -- 20 (cables twisted...)
Seems to work, seems to be good enough for the designated purpose. These measurements indicate an error of approx. 10 ppm for the 0.1 : 10 ratio. Comparing these results with the specifications of the DMM4050 (1 year):
1 M -- 100 ppm of rdg. + 10 ppm of range
10 M -- 400 ppm of rdg. + 10 ppm of range
shows no severe nonsense. But I cannot find the 140 ppm-error. I have to retry the measurements when it is a little cooler (30 °C today...).
And: the twisted cables seem to have an isolation resistance of around 12 GOhm dependending how they are twisted... Good for additional 800 ppm of error...
Best regards
Marcus
