Electronics > Metrology

Fluke 720A KVD oil seal

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texaspyro:
My 720A developed a small leak at the resistor can oil seal.   The seal does not seem to be in the manual, so I contacted Fluke.   They seemed a bit baffled about what it could be.  I sent them a couple of photos and they finally came up with:

171959   EA   23.70
Item Desc :SEAL,BUNA,3/8 EXT THREAD,COMPOSITE,STEEL RETAINER,CADMIUM PL,.750 OD,.063 THK

So, only 24 bucks for a 50 cent bolt and 10 cent O-ring.

I don't think enough oil leaked to make a difference, but I also asked them about what oil to use and they responded that they don't sell the oil, but to use "Grade 5 mineral oil"... I assume that is viscosity 5.    Does anybody know of a suitable oil available in small quantities?    I'd prefer not to go dumping in some no-name goop of unknown quality/purity/virginity from the local hardware/grocery/etc store.

MK:
two choices, vacuum pump oil or medicinal parafin are suitable alternatives, some mineral pump oil is straight parafin to cope with oxidisers in the vacuum stream, a choice of viscosities available there too.

SeanB:
Buna rubber, so I would go with a vacuum pump oil. Simply remove 10ml of the existing oil and place in a glass jar, and introduce 10ml of your intended oil to it as well, and seal the jar and leave for a few days to see if there is any interaction, which will show as either separation of the oils or one turning to gel. otherwise mineral refrigerant oil ( which I use as vacuum pump oil, make sure it is mineral and not POE or even worse PAG which is conductive) will also work, or just go to the hardware and get transformer oil.

All the oils of course will need to be vacuum dewatered and degassed before use, preferably also heated to 70C as well during this, to get them as dry as possible so you do not introduce any moisture into the sealed system.

texaspyro:
It looks like most vacuum pump oils are rather viscous.  I am assuming when Fluke said "Grade 5" mineral oil that they meant 5 weight (rather thin).  What leaked out (maybe a tablespoon) was thin (like baby oil).

I don't think enough leaked to matter, but I dont know about the earlier history of the machine.  This is what Fluke said:

"The oil is a grade 5 mineral oil: we don’t get it in small quantities he has to get it from somewhere where it is sold in a quart or small quantities. Why does he need to change the oil? And has he lost a lot of the oil He does not need to have it completely full. If it is visible where it is up to the internal PCB in the can that is more than sufficient. As when he ships it or sends somewhere when it is on a plane the pressurization will cause it to leak if it is too full."

I have not tried looking into the can.  The way it is mounted may require some disassembly to see into the hole.

floobydust:
I've had products with o-ring seals leaking and given MecE's an earful about it.
Thermal expansion of the oil and can, and seal-interface, altitude/barometric pressure differences were found to be the causes. Mostly o-ring compression was not enough.
Now I understand why car headlight fixtures use Gore-tex vents instead of trying to absolutely seal them.

I would replace the o-ring possibly AS568-016, local o-ring shop charges $1 for a bunch.
Not sure if mineral oil stays homogeneous after years or the lighter distillates rise up. The thin oils might have seeped out.

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