Capacitor is a basic element of any electrical circuit. Traceability and measurement of capacitance unit of measure, Farad derived from SI Ohm, Volt and unit of time SI second. National capacitance systems realize unit of Farad which is further translated to working capacitance standards. Capacitance standards are not as common as resistance or DC voltage in hobby labs. But they are important to have for anyone who is serious into inductance/capacitance metrology, capacitance meter calibrations, AC voltage/current measurements and low current measurements.
ESI SC10 10pF capacitance standard
Here we will look inside of old ESI SC10 with nominal value 10 pF, serial number 413076.
It has interesting connector arrangement, with two GR-874 ports for actual capacitor element and five universal binding posts. Diagram on the body shows these binding posts connect to 4-wire resistance element but measurement with DMM did not show any reasonable resistance at the terminals (measurement was >1 GΩ). It might be very well broken or overloaded standard. Fifth golden post is guard labelled post, connected to metal housing of standard.
Initial disassembly is straight forward. Four screws hold top metal cover with posts to metal box.
Connections to capacitance element and GR-874 connectors done with thick Belden 8259 RG-58A/U coaxial cables.
These cables are specified to have 50 Ω impedance, 20 AWG stranded tinned copper conductor in PE insulation and with outer PVC jacket and 95% shielded with copper braid. Cable has nominal capacitance 30.8 pF/ft and insertion loss 0.4 dB/100ft at 1 MHz.
Standard itself is sealed hermetically in rather heavy metal can with massive copper top.
All connections done with large feed-thru ports soldered to lid with porcelain insulator.
One of capacitance coax wires terminated inside brass tube to provide better shielded coaxial path.
Shield braids of both coax wires connected to copper body and outer metal casing.
Whole can sits in dampening foam similar to what is widely used for shipping fragile electronics. Foam in places where cables fan out is melted away to allow space.
ESI SC1000 1nF capacitance standard
Modified: Sept. 19, 2022, 2:14 a.m.