My TRIAX cable shielded box project for Keithley Instruments

[texaspyro]

I even found a 3M double sided tape that sicks well to the Teflon.

Using adhesive tape anywhere remotely near a high impedance device is a VERY bad idea.   Tape has a very high stored electrical charge.  As part of the manufacturing process they zap the stuff with what amounts to a Tesla coil  and it produces what amounts to an electret.   

You can generate fairly strong (medically useful) x-rays by uncoiling a roll of tape in a vacuum chamber.  I have a couple of 3M static field testers that you calibrate using a piece of adhesive tape... these testers use a tritium source to provide ionization.

[ManateeMafia]

Thanks for showing your ideas and results. I am starting a teardown repair on a Keithley 6517 and was considering a similar design. The only change I was going to make was a guarded plate or box. I did something similar with a 1G resistor with a box within a box mounted to a sheet of PTFE. It was stable when moving my hands nearby.

Since you added 2mm connections for the guard ring, do you plan on adding a guard to reduce leakage?

[Vtile]

Really nice... Those small and cheap "safeboxes" might also act as a good starting point of these kind of chambers.

Are you sure it the box is leaking while you touching it. Not the Triax cable or ground drifting from induced emf. I can measure up to about 10 volts (I assume even more) of stray AC voltage from my finger with my modest 100meg (DC .. AC is something less 10 megs iirc ) needle movement (with mains power or from floating battery operation) at my workbench with all sorts of household appliances running in poor day. I can also measure with the same meter my own movement (meter upwind) in my "wilderness" fishing cabin (no electricity network in about 5 km radius) from over 2 meters distance. The wind itself is makes the needle move a few millivolts DC (read with binoculars from 15 meters away). In there the AC range gives zero no matter what I try.

 

Edit1.
Under the powerlines 110kV I assume, I can measure 75Vac at the level of my head and 3 uA current to ground, with totally unscientific method just turning the meter on. Low freq. EMF litter everywhere in urban environments.

Edit2. Also that melamine coating of the lab table is like propably also a really good generator of troubles.

[plesa]

Nice box! BTW why did you choose  B2987A  instead of 6517B?
Also interested in tear-down, I can post some internal photos of 6517B, but not internal pictures of input stage, sorry, too sensitive

[HighVoltage]

I even found a 3M double sided tape that sicks well to the Teflon.

Using adhesive tape anywhere remotely near a high impedance device is a VERY bad idea.   Tape has a very high stored electrical charge.  As part of the manufacturing process they zap the stuff with what amounts to a Tesla coil  and it produces what amounts to an electret.   

You can generate fairly strong (medically useful) x-rays by uncoiling a roll of tape in a vacuum chamber.  I have a couple of 3M static field testers that you calibrate using a piece of adhesive tape... these testers use a tritium source to provide ionization.

OK, I must admit, that I have not even thought in this direction, since one side of the electric tape is "grounded" to the metallic box.

If there will be residual charge, I should be able to measure it in the nano volt range between the box as a ground and the 4 different PTFE plates, right?
Very interesting and thanks for pointing this out.

[HighVoltage]

Thanks for showing your ideas and results. I am starting a teardown repair on a Keithley 6517 and was considering a similar design. The only change I was going to make was a guarded plate or box. I did something similar with a 1G resistor with a box within a box mounted to a sheet of PTFE. It was stable when moving my hands nearby.

Since you added 2mm connections for the guard ring, do you plan on adding a guard to reduce leakage?

Yes, this was one of the thoughts I had for the next improvement.
I also wanted to place my high ohm resistors in in a separate small aluminum box with 2 mm banana plug receptors.

Please show us the inside of your 6517 repair.

[HighVoltage]

Really nice... Those small and cheap "safeboxes" might also act as a good starting point of these kind of chambers.

Are you sure it the box is leaking while you touching it. Not the Triax cable or ground drifting from induced emf. I can measure up to about 10 volts (I assume even more) of stray AC voltage from my finger with my modest 100meg (DC .. AC is something less 10 megs iirc ) needle movement (with mains power or from floating battery operation) at my workbench with all sorts of household appliances running in poor day. I can also measure with the same meter my own movement (meter upwind) in my "wilderness" fishing cabin (no electricity network in about 5 km radius) from over 2 meters distance. The wind itself is makes the needle move a few millivolts DC (read with binoculars from 15 meters away). In there the AC range gives zero no matter what I try.

 

Edit1.
Under the powerlines 110kV I assume, I can measure 75Vac at the level of my head and 3 uA current to ground, with totally unscientific method just turning the meter on. Low freq. EMF litter everywhere in urban environments.

Edit2. Also that melamine coating of the lab table is like propably also a really good generator of troubles.

Interesting thoughts indeed.
I will further investigate.

[Vtile]

Really nice... Those small and cheap "safeboxes" might also act as a good starting point of these kind of chambers.

Are you sure it the box is leaking while you touching it. Not the Triax cable or ground drifting from induced emf. I can measure up to about 10 volts (I assume even more) of stray AC voltage from my finger with my modest 100meg (DC .. AC is something less 10 megs iirc ) needle movement (with mains power or from floating battery operation) at my workbench with all sorts of household appliances running in poor day. I can also measure with the same meter my own movement (meter upwind) in my "wilderness" fishing cabin (no electricity network in about 5 km radius) from over 2 meters distance. The wind itself is makes the needle move a few millivolts DC (read with binoculars from 15 meters away). In there the AC range gives zero no matter what I try.

 

Edit1.
Under the powerlines 110kV I assume, I can measure 75Vac at the level of my head and 3 uA current to ground, with totally unscientific method just turning the meter on. Low freq. EMF litter everywhere in urban environments.

Edit2. Also that melamine coating of the lab table is like propably also a really good generator of troubles.

Interesting thoughts indeed.
I will further investigate.

Note that I'm noob. So it is only rather uneducated guesswork. As an one example of electrically charged membranes: http://www.sciencedirect.com/science/article/pii/S0924424799002691

[plesa]

I even found a 3M double sided tape that sicks well to the Teflon.

Using adhesive tape anywhere remotely near a high impedance device is a VERY bad idea.   Tape has a very high stored electrical charge.  As part of the manufacturing process they zap the stuff with what amounts to a Tesla coil  and it produces what amounts to an electret.   

You can generate fairly strong (medically useful) x-rays by uncoiling a roll of tape in a vacuum chamber.  I have a couple of 3M static field testers that you calibrate using a piece of adhesive tape... these testers use a tritium source to provide ionization.

OK, I must admit, that I have not even thought in this direction, since one side of the electric tape is "grounded" to the metallic box.

If there will be residual charge, I should be able to measure it in the nano volt range between the box as a ground and the 4 different PTFE plates, right?
Very interesting and thanks for pointing this out.

In your case it should be not big deal, I suppose, because it is between insulation and ground. If you needs to make sensitive measurement create quard box inside insulated by PTFE.

[plesa]

Thanks for showing your ideas and results. I am starting a teardown repair on a Keithley 6517 and was considering a similar design. The only change I was going to make was a guarded plate or box. I did something similar with a 1G resistor with a box within a box mounted to a sheet of PTFE. It was stable when moving my hands nearby.

Since you added 2mm connections for the guard ring, do you plan on adding a guard to reduce leakage?

What failed in 6517? I saw one unit on ebay which does not power on.

[HighVoltage]

Nice box! BTW why did you choose  B2987A  instead of 6517B?
Also interested in tear-down, I can post some internal photos of 6517B, but not internal pictures of input stage, sorry, too sensitive

After many weeks of looking at both units in detail, the Keithley 6517 and the Keysight B2987A, I decided in favor of the B2987A because:
1. Statistical analysis directly at the display 
2. Integration of the instrument in to BenchVue

The Keithley 6517 intrigued me a lot, I must say and I was almost at the edge of buying one.
Yes, please show us some tear down pictures.

I will probably open up the B2987A but feel the same as you about the input stage, although that would probably be the most interesting part.
Plus, I am not sure if the unit will have the Keysight calibration seals over the screws.

[plesa]

Nice box! BTW why did you choose  B2987A  instead of 6517B?
Also interested in tear-down, I can post some internal photos of 6517B, but not internal pictures of input stage, sorry, too sensitive

After many weeks of looking at both units in detail, the Keithley 6517 and the Keysight B2987A, I decided in favor of the B2987A because:
1. Statistical analysis directly at the display 
2. Integration of the instrument in to BenchVue

The Keithley 6517 intrigued me a lot, I must say and I was almost at the edge of buying one.
Yes, please show us some tear down pictures.

I will probably open up the B2987A but feel the same as you about the input stage, although that would probably be the most interesting part.
Plus, I am not sure if the unit will have the Keysight calibration seals over the screws.

I have the same dilemma and decided to buy 6517B At the end the input bias decided. Keysigh sent me B2987A  for three weeks and I found quite difficult to use everything from remote panel.
In my setup I'm using Python and Raspberry, so statistic in menu and BenchVue is not added value for me. BTW 6517 is integrated into KickStart SW.

[razberik]

Dont be afraid and be a hero ! Open that input stage.

[plesa]

Dont be afraid and be a hero ! Open that input stage.

Hero != dumb I found the two SMD capacitor near by enclosure seems to be over-pressurized. SO warranty can apply soon

[razberik]

What a pity. Is that shield cover soldered to board ? I would open it anyway. I don't think that opening it would screw it if I follow sensitive rules and precautions.
There are A version internal photos on xDevs and nothing special there. Few resistors, capacitors, relays and some semiconductors.

[plesa]

What a pity. Is that shield cover soldered to board ? I would open it anyway. I don't think that opening it would screw it if I follow sensitive rules and precautions.
There are A version internal photos on xDevs and nothing special there. Few resistors, capacitors, relays and some semiconductors.

There is Kapton tape only. It is not soldered. I'm familiar with 6517A units there is cover similar to cover in K6485.

[HighVoltage]

After many weeks of looking at both units in detail, the Keithley 6517 and the Keysight B2987A, I decided in favor of the B2987A because:
1. Statistical analysis directly at the display 
2. Integration of the instrument in to BenchVue

The Keithley 6517 intrigued me a lot, I must say and I was almost at the edge of buying one.
Yes, please show us some tear down pictures.

I will probably open up the B2987A but feel the same as you about the input stage, although that would probably be the most interesting part.
Plus, I am not sure if the unit will have the Keysight calibration seals over the screws.

I have the same dilemma and decided to buy 6517B At the end the input bias decided.

What about the input bias of the 6517B did you like better to make this decision?

BTW 6517 is integrated into KickStart SW.

That one I missed! Thanks for pointing it out.

It would be nice to have both instruments next to each other for a real comparison.

[HighVoltage]

I just came across this nice video by Keithley, explaining that Teflon (PTFE) indeed is a good choice for the insulation in such a shielded box.
Watch the video to the end.

[HighVoltage]

Today I build a little aluminum box for the 1 Tera-Ohm resistor with a connector to guard the resistor box.
What a difference!
Suddenly the value measured is very stable!
Here are the first pictures:

[HighVoltage]

It seems with these settings and 50V Source Voltage, the final value settles after about 10 min to 992 GOhm.

[plesa]

The input bias current is on Keithley 3fA and on Keysight 20fA. The price is same.
Try to contact Keithley they should have demo unit.

In your box I will replace the banana socket or place them in insulator like PTFE. The banana insulation can affect the measurement.just try it.

And thanks for video, this is the low measurement hack video with alu box, which I was looking for for long time:)

[HighVoltage]

In your box I will replace the banana socket or place them in insulator like PTFE. The banana insulation can affect the measurement.just try it.

OK, good advise.
I ordered some Teflon material to make insulators for the 2mm sockets.

Today the same 1TOhm resistor settles at a little higher value.

[Conrad Hoffman]

Not sure if it matters, but there are some differences between virgin Teflon and "mechanical" grade Teflon, which contains some recycled material. Hopefully the vendor is clear on what they're supplying. Teflon is the bee's knees, but I've also used polypropylene for insulators to good effect.

[HighVoltage]

Not sure if it matters, but there are some differences between virgin Teflon and "mechanical" grade Teflon, which contains some recycled material. Hopefully the vendor is clear on what they're supplying. Teflon is the bee's knees, but I've also used polypropylene for insulators to good effect.

Yes, that is a good point for some young players.
I had some good experience in using PTFE over the years for high voltage applications.
And at one time, the PTFE was so conductive, that nothing worked.

I also had bought some "conductive" polypropylene many years ago.
Best to check first.

These days I use medical grade PTFE for all my high voltage applications and now also for this insulator.

[HighVoltage]

Today is the third day in follow that I have been measuring this very same 1 Tera Ohm resistor.
And every day the graph looks a little different.
I wonder why.

Humidity and temperature has been the same all all three days.
My turn on procedure has been the same.

May be it is a residual charge of sort, that was left in the resistor, when it was turned off yesterday?

Here are the pictures from all three days in a row.
All three have 50 Volt source voltage