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Grenadier's big thread of RÃ¶ntgen related shenanigans

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Hon1nbo

Fri Jun 11 2010, 02:26AM

Registered Member #902 Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042

just thought I'd throw this in here: a possible reason why the resistor gives a different current is because you may not have a proper resistance reading for the bulb, as it changes as the heat increases IIRC

also, for isolation, use fiber optics, it can be done for cheap, it's reliable, easy to modify and maintain (unlike an oil bath), etc

Also, if your in the US check Craigslist for things like lead shielding etc, at least here X-Ray hardware shows up at least a few times a month (sometimes a lot of listings at once!).

-Jimmy

Adam Munich

Fri Jun 11 2010, 02:45AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

1) The resistance of a half-hot bulb is 72 ohms, when the transformer draws more current, it raises the resistance to a maximum of 144 ohms. A resistor does not.

2) If i solve the voltage spike problem, i won't even need to switch the filament.

3) I've been looking. No luck.

Wolfram

Fri Jun 11 2010, 07:23AM

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

Your tube is pretty typical as far as dental tubes go, and the glass will mabe block everything under 25-30kV.

I don't think you need to worry very much about cooling. Dental X-ray heads often run the tube at close to a kilowatt without any means to circulate the oil except for convection, so I don't think your design will need any artificial way to get oil circulation either. Not even the largest medical tubes running much higher powers need fans to circulate the oil. Convection does a very good job.

Also, stop posting multiple posts in a row, it's against the rules. Use the edit button instead.

Anders M.

Adam Munich

Fri Jun 11 2010, 01:59PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Sorry.

However my tube is rated at only 75kvp@2mA. That's only 75Watts. I just don't want to break it.

Anyway, i'm livid. I got some lcd's in the mail, one for the ray, one for my geiger counter, and they're junk. The guy wants me to mail them back to china, but i'm not paying $20. I hope paypal can get me a refund.

Wolfram

Fri Jun 11 2010, 04:48PM

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

That's 150 watts, and that's the continous rating. These tubes are usually used at several times the continous power rating, as they are used for short durations. A dental exposure is usually under two seconds, unless it is a pantomographic unit, with those the exposure times can be as long as 20 seconds. The datasheets for the tubes usually give curves for maximum allowable power, but datasheets are usually very hard to find for most tubes, so I usually just find the datasheet for a tube with similar ratings and size, to get a general idea. This

should be somewhere in the same area, and should give you a general idea.

In any case, running it at 75W like you plan on doing will allow you to run it indefinitely. Taking x-ray pictures with intensifying cassettes should not require too much energy (unless you are running the tube on such a low voltage that almost all of the radiation is stopped by the glass), so I doubt you'll even be able to measure temperature rise in the oil after an exposure. This is why I think a fan to circulate the oil is largely uneccessary.

Anders M.

Proud Mary

Fri Jun 11 2010, 06:15PM

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

At a distance of 1 metre in the beam from a 75kV/2mA X-ray tube, the dose rate will be 32 Sv/hr.

We'll call this 0.5 Sv/minute to make life easy, since there is nothing very exact about the acute effects of radiation poisoning.

After 4 minutes, you will suffer 'mild' radiation poisoning, featuring nausea and vomiting within 24 hrs.

After 6 minutes, you can add general malaise, bloody stools, bloody vomit, and subsequent hair loss.

After 12 to 16 minutes, you will suffer all the previous within 30 minutes, plus high fever, vertigo, mental confusion, organ failure, collapse of blood pressure, and death in 50% of cases.

Long term consequences are to be expected among survivors of acute exposure, though some of these may take years to develop.

Repeated exposure to even very low doses is linked to various sorts of cancer in dental workers, despite their using all recommended screens, shields and aprons.

You may harm not just yourself, but innocent people and animals living above, below, and on either side of you.

And that's all I have to say on the matter.

Wolfram

Sat Jun 12 2010, 01:50AM

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

I don't think scare numbers are neccessary here, as Grenadier has already shown in the other thread that he respects the dangers of x-rays and that he has built a 4mm thick lead box to contain the whole experiment in. He is also planning on being several hundred feet away when the tube is powered, so I think any radiation exposure he will get from this will be immeasurable compared to background radiation.

Of course it's important to respect the dangers of x-rays, though most of the hobbies discussed here have very real dangers, and when the dangers are understood and safety precautions are taken, working with x-rays can be as safe as any other hobby.

Anders M.

Adam Munich

Sat Jun 12 2010, 02:11AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

@Anders
Good to know. Sorry for the silly calc mistake. I'm going to take pictures 3 ways;

1) with kodak paper and intensifying screen pressed to it.
2) Using a europium glow in the dark screen. The long lasting phosphor should "build up enough light" let a camera inside take a picture.

@Proud Mary
I'm sorry for the big pic. Idk how to shrink it.
Fwk1ux

@Anders again
Thanks for the support.

[Edit: Now click the edit button and learn how to do it]

Adam Munich

Sat Jun 12 2010, 05:47PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

I built an isolation relay. However, i'm starting to wonder if it's safe to turn on the hv before warming up the tube. I won't need this relay if my hot-bulb design works though. However i can't build it yet; my relay and other parts from bgmicro are still in the mail.

2em1i7a

Proud Mary

Sat Jun 12 2010, 09:22PM

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

Grenadier wrote ...

i'm starting to wonder if it's safe to turn on the hv before warming up the tube.

You may damage the tube by so-called 'cathode stripping' or internal flash-over if you apply anode voltage before the filament or cathode is thoroughly warmed. Further, if you are using a self-rectified circuit, damaging reverse conduction may occur.

Moreover, a tube which is either brand new, or has been out of service for some time, must be 'seasoned' before the full anode voltage is applied. The point of 'seasoning' is to re-absorb gas molecules which have cast adrift whilst the tube has been out of service. 'Seasoning' consists of the application of HT in a graduated sequence of increments over a period of time. By way of illustration only, and not recommendation, (since I know nothing of your tube, or its history) three one minute applications of 10kV with one minute between each, three applications of 15kV for a minute each with one minute intervals, three applications of 20kV, and so on in steps until you reach the full working voltage.

Different manufacturers have differing 'seasoning' recommendations, but all share the common principle of a graduated series of rising voltage steps before the full anode voltage is applied. The longer a tube has been out of service, the more thoroughly must the 'seasoning' procedure be performed. Light seasoning should be performed if a tube has been out of service for 3 - 6 months, with a longer, slower and more gradual procedure for 6 - 12 months, and so on.

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