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Peter

Tue Mar 21 2017, 06:33AM

Registered Member #61521 Joined: Mon Mar 20 2017, 11:58PM
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Posts: 6

Hi All, First post but long time reader.

I purchased a Tek P6015 HV probe that unfortunately had a damaged BNC connector. I lived with it for a while before finally replacing the connector with a â€œmore commonâ€ male panel mounting BNC connector.

The new BNC has a 3/8â€ diameter thread with 32 threads per inch. I was able to tap the end of the original plated brass connector assembly (has several parts) and simply screw the new BNC into it.

Happy with the repair result and though my solution might be useful to others.

I have been using butane to pressurize the probe but lately I have started thinking about other gases. Looking at the manual it looks like the probe uses a 100Meg 4W resistor. With short duty cycle pulses the probe voltage rating doubles. Using helium or hydrogen as a pressurizing gas the 100Meg resistor cooling would be about 10 times greater. A French style bicycle valve is 6mm in diameter and that should fit next to the female BNC inside the probe body but with almost zero extra space.

Food for thought,
Peter

Sulaiman

Tue Mar 21 2017, 08:13AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I have not needed to refill my P6015 with butane for several years now,

I would be concerned about the leakage rate of your proposed gasses,
as butane has a large reserve in liquid form,

I have not yet felt the need to 'upgrade' my P6015 ... I prefer it to be working

Peter

Tue Mar 21 2017, 10:05PM

Registered Member #61521 Joined: Mon Mar 20 2017, 11:58PM
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Posts: 6

Wants become needs

The butane phase change is a nice fix.

I have the same concerns about â€œimprovementsâ€ and the risk vs. reward. Doubling the probe DC voltage rating would be useful for unstable load measurements.

The probe calibration after my repair was a bit fiddly but well worth the hour of effort. Yellow trace 1 is the 6015 output and the blue trace 2 is the input waveform to the 6015.

1490133372 61521 FT179355 P6015 Calibrated

A bicycle valve is a small modification that could be reversed, however there is plenty of opportunity to screw up. Drilling and taping the probe or later testing the extended range. The standard valve makes it easy to measure the pressure when one feels the need.

The other approach is fabricating a voltage divider from HV resistors and capacitors. Not a trivial task with 75MHz of bandwidth but if you do have an accident you might not destroy your scope.

Sulaiman

Wed Mar 22 2017, 10:05AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

two separate requirements, voltage and bandwidth
e.g. look at the voltage vs frequency limits for the P6015

unless you are cooking yourself with em radiation
(10's kV rms @ 75 MHz)
for this I would use a capacitive divider.
e.g. 1pF: 1nF

Peter

Sat Mar 25 2017, 06:30PM

Registered Member #61521 Joined: Mon Mar 20 2017, 11:58PM
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Posts: 6

I found a big old 1976 gov document on cooling with helium and air mixes.
This graph shows it all. With an 85% helium mix cooling is 4 times better than 100% nitrogen or air.

]helium_page_19_graph_3445605506342.pdf[/file]

Tubeless aluminum presta valves are manufactured by American Classic and probably many others. I like this outfit because they have an O ring seal against the bicycle rim.

Fill with a couple atmospheres of helium and then a squirt of liquid butane to regulate the probe pressure.

For the adventurous the 15% of other gas could be mostly SF6. Not certain if 15% SF6 would actually do anything positive.

For KISS advocates two stainless spheres (garden mirror stuff) a fixed distance apart makes a good HV DC blocking capacitor for measuring AC signals with the probe. One on the probe tip and the other on the HV signal wire. I did a quick low voltage calibration test and the waveform rise time is fine.

Peter

Peter

Wed Mar 29 2017, 11:51PM

Registered Member #61521 Joined: Mon Mar 20 2017, 11:58PM
Location:
Posts: 6

I found some information on dielectric strength.

Helium and hydrogen have a lower dielectric strength than nitrogen, darn.

Also somewhat concerning is the dielectric strength of butane.

The 2nd document has one of these gases in the list.

]dielectric_strength_of_insulating_materials_- crc_handbook_of_chemistry__physics_.pdf[/file]

Putting it together it looks like butane has a dielectric strength about 10% better than nitrogen and it is used at a slightly higher pressure in the probe.

The bad news is the original probe gas R-114 is 3.2 times better than nitrogen.

My take is the probe voltage is substantially reduced when pressurizing with butane.

Sulaiman

Thu Mar 30 2017, 05:59AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

From that graph,
Butane breakdown at c4.4 kV at p.d = 50 cm.mmHg
at 760 mmHg (atmospheric pressure) d = 50/760 cm = 0.658 mm
electric field gradient = 4.4 kv/0.658mm = 6.688 kV/mm
e.g. 10mm path length >= 66 kV breakdown.

Peter

Fri Mar 31 2017, 04:41AM

Registered Member #61521 Joined: Mon Mar 20 2017, 11:58PM
Location:
Posts: 6

My take on the information is not quite as optimistic.

Because the probeâ€™s internal geometry is different than the electrodes used to create the graph you canâ€™t use the numbers directly from the graph.

Comparing butane and R114 to nitrogen (or air) within the probe the operating voltage does not look as high as with R114.

Methane (3.0 on the graph) has the same breakdown voltage as nitrogen (on the list in the 2nd attachment). N-Butane (or Butane) is 4.4 on the graph or by the ratio 1.47 times higher breakdown voltage than nitrogen (incorrect number in my previous post). R114 is 3.2 times better than nitrogen and it is also better than Butane by 3.2/1.47 = 2.17 times.

Butane saturation pressure is 1.17 times higher than R114.

For Butane at its higher saturation pressure the R114 advantage over butane is reduced to 2.17/1.17 = 1.85. The reciprocal is 0.54 so letâ€™s call Butane half of R114.

Crunch the numbers for yourself maybe I have missed something or you can find a better breakdown voltage table. I would suggest a reference to air is the most convincing comparison.

BTW I picked up that French bicycle valve and the size/fit is questionable. I think the valve needs some â€œadjusting on a latheâ€ and the supplied external nut has no clearance, so the nut needs two notches for the BNC and the plastic probe handle. Not worth the effort unless you intend to pressurize with SF6

Sulaiman

Fri Mar 31 2017, 07:40AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I do not suggest that butane is the best choice, just a cheap easily available one.
(but potentially explosive)
With air as dielectric my P6015 arcs internally along the front clear plastic part,
with butane it does not ... with a c33 kV peak flyback, I've not tried higher.

If anyone finds a better, easily available dielectric, please tell me.

Conundrum

Fri Mar 31 2017, 08:16AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4059

Probably why He is used in hard drives.

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