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Should my x-ray tube be under oil?

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LAN_403

Download

Mon Nov 15 2010, 11:10AM

Registered Member #561 Joined: Sat Mar 03 2007, 02:46AM
Location: Adelaide Australia
Posts: 230

So I recently bought a Belmont dental x-ray head of eBay. Rather than use it as a complete unit I chose to dismantle it and separate it so I could use and operate the 70kV/10mA transformer within. Because it was a 110v transformer and I live in Australia were we have 240v I was a bit stuck at how to run it without a expensive step down transformer. I eventually worked out that I could split the two primary's that were running in series and run them in parallel so it would take 55v in, the same voltage my AC arc welder puts out. This works very well and it makes some of the biggest arcs I've ever played with. I had a small problem were I had an arc jump out across the surface of the oil and arced to the other primary when I didn't put a load on the transformer but other than that it works fine.

The tube that was in the head is a Toshiba D-088. I found a spec sheet that said it was good upto 90kV with a cooling rate of 180 watts, the filament is fine and it runs on 2v at 4 amps. The tungsten and copper anode is nearly spotless with a small blemish on the focal point that you can see in reflected light.

Basically I want to know if it would be safe to run this tube in air without the risk of flashover from the 70kV transformer. If I have to I can buy some more oil but I'm tying to save money at the moment

Proud Mary

Mon Nov 15 2010, 12:36PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

X-ray tube oil immersion has both an electrically insulating and thermal cooling function.

Dental X-ray tubes are designed with a low duty cycle in mind. Exposures are generally less than half a second, and even for a full mouth 18-film series, there will be long gaps bewtween exposures in which the tube can cool down. Your exposures are likely to be much longer - due to inverse square law effects - unless you are to make only small dental film-sized images and have an intensifying screen.

From your description, I assume that your tube is set up to self-rectify, and will conduct 50 times per second. If your anode gets over-heated, and starts to thermionically emit electrons on the inverse half cycle, there is a risk of it striking back, and going into funeral mode

So cooling will be an issue for you.

Wolfram

Mon Nov 15 2010, 01:21PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Proud Mary wrote ...

From your description, I assume that your tube is set up to self-rectify, and will conduct 50 times per second. If your anode gets over-heated, and starts to thermionically emit electrons on the inverse half cycle, there is a risk of it striking back, and going into funeral mode

This was mostly an issue with the old radiation cooled tubes that ran with the anode white hot. Modern conduction cooled tubes will most likely fail way before this, I suspect the glass-metal seal will fail first, or maybe the glass envelope itself.

I fully agree with Proud Mary, the tube should be run under oil, but for low voltage low power tests it can be run in air. Be sure that the tube envelope is very clean though, and that you don't run it at full voltage. A spark across the glass could be very bad for the tube.

A.M.

Proud Mary

Mon Nov 15 2010, 04:41PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

By quick mental ready reckoning off the top of my head, I'd estimate your maximum exposure time for 70kVp @ 10mA, positive half cycles only, and waveform correction factor 0.71, your 180W /second cooling rate will be exceeded after about 3/4 sec before your tube starts to overheat.

Download

Mon Nov 15 2010, 08:53PM

Registered Member #561 Joined: Sat Mar 03 2007, 02:46AM
Location: Adelaide Australia
Posts: 230

Alright then, so I'll have to get some more oil. I don't see why it wouldn't be possible to run it for 3/4 of a second then cool for a minute then run it for 3/4 of a second again so I don't damage the tube. No problems with this?

Proud Mary

Mon Nov 15 2010, 09:16PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Download wrote ...

Alright then, so I'll have to get some more oil. I don't see why it wouldn't be possible to run it for 3/4 of a second then cool for a minute then run it for 3/4 of a second again so I don't damage the tube. No problems with this?

You'll be fine doing either that, or some other kind of pulse service that keeps the duty cycle low enough to prevent you having a heat log jam.

Do you know about the need to 'season' X-ray tubes that have been out of service for more than six months or so? If not, I will explain it.

Download

Tue Nov 16 2010, 06:28AM

Registered Member #561 Joined: Sat Mar 03 2007, 02:46AM
Location: Adelaide Australia
Posts: 230

I don't know about it. I presume is has something to do with running it at low power?

Download

Tue Nov 16 2010, 06:31AM

Registered Member #561 Joined: Sat Mar 03 2007, 02:46AM
Location: Adelaide Australia
Posts: 230

Also, I'm thinking I should put a full wave rectifier on it in case I don't give it enough filament current for it to self rectify. Good idea?

klugesmith

Tue Nov 16 2010, 07:24AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Download wrote ...
Also, I'm thinking I should put a full wave rectifier on it in case I don't give it enough filament current for it to self rectify. Good idea?

1. You shouldn't "double post" here. Edit the previous post instead of creating a new one.

2. With or without fullwave rectification, you need to heat the filament enough to emit the desired amount of electron current. Other things being equal, the average anode current with fullwave rectifier would be twice as much as in self-rectified mode. You could get the same effect (doubling the anode current) by increasing the filament voltage by about 10%.

3. I think a significant benefit of fullwave rectifier is to preserve zero DC current in the XRT secondary winding. In halfwave (self rectified) operation, the secondary winding carries a DC current equal to the average anode current. That tends to saturate the core (it runs pretty close anyway, as in a MOT) unless you have a properly designed diode/resistor network in series with the XRT primary winding.

Proud Mary

Tue Nov 16 2010, 06:51PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Download wrote ...

I don't know about it. I presume is has something to do with running it at low power?

'Seasoning' or 'ageing' of X-ray tubes that are either brand new, or have been out of action for some time, is a simple process to re-absorb gas molecules that have seeped from the tube's internal structure, and might lead to destructive plasma discharge, arcing, or other instability.

Some authors state that seasonong should be performed after the tube has been idle for one month, others say three months, and others yet again six months.

There are no very definite rules about the incremental voltage and current steps of the seasoning process, so I suggest you have a look in
the X-RAY EQUIPMENT MAINTENANCE AND REPAIRS WORKBOOK Module 2.1 pp. 48-49, and simply scale down the seasoning regimen in proportion to the voltage and current needs of your own tube:

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