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X-Ray Chicken Wing Inspector

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plazmatron

Fri Dec 25 2009, 03:51PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Proud Mary wrote ...

I hasten to add that I have no personal experience of such powerful tubes as yours, nor of using second-hand tubes, and suggest you ask Plazmatron for his opinion.

The tube looks in fair condition to me. If it is vacuum intact, and filament intact, you should get a lot of use out of it.

Darkening of the glass is usual in second hand tubes, and has two contributing factors. A build up of Tungsten scattered from the anode, and colour centers in the glass caused by exposure to radiation.

Like with a lot of stuff I do, I tend to run tubes at the most conservative ratings I can get away with.
First off there will be a lot less radiation to shield, and secondly you will get much greater tube life.

The other consideration is cooling.

Small tubes like this, are designed to run for just a second or two, after which the heat deposited in the anode will slowly dissipate into the surroundings, be it oil or air. Running for minutes at a time, even at reduce voltage and current, may exceed the tubes thermal ratings, and cause it to fail.

I had this happen to me in the early days. I had a small dental tube running at 0.5mA and 30kV, for around 20 minutes at a time, to take radiographs. Even though the tube was sunk in oil, it failed, rather suddenly. There was no visible damage to the tube, but I guess the Glass-metal anode seal became compromised due to heating.

The good thing about using any tube in conjunction with the Fluoroscope you have, is that you can run the tube at such a low current, that thermal overload is no longer a significant problem, nor is sputtering, and filament life will be greatly extended.

Les

Proud Mary

Fri Dec 25 2009, 04:31PM

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

plazmatron wrote ...

The other consideration is cooling.

Small tubes like this, are designed to run for just a second or two, after which the heat deposited in the anode will slowly dissipate into the surroundings, be it oil or air. Running for minutes at a time, even at reduce voltage and current, may exceed the tubes thermal ratings, and cause it to fail.

The good thing about using any tube in conjunction with the Fluoroscope you have, is that you can run the tube at such a low current, that thermal overload is no longer a significant problem, nor is sputtering, and filament life will be greatly extended.

Les has raised an important issue here. You can download a short pdf file on thermal X-ray tube failure modes here:

EEYORE

Sat Dec 26 2009, 06:32AM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Thank you for the tips guys! Alot to chew on here for me. I think I may understand a few key things here now. Higher energy x-rays will be useful for penetrating thicker objects, and more current will produce more xrays. As far as I care, more xrays is not nearly as important as higher energy since I am using a fluoroscope. If using film, then yes I would want more current. Do I have this right?

So since these fluoroscopes work well with so little, I should be good keeping the current very low (say 100uA?) and just upping the voltage to a more useful value.

Before I make plans for cooling and housing and whatnot, I think I need to figure out a target voltage to shoot for. I am sure I can get away with 35-40kV out of my tripler. Would it be worth the $$$ building a VM to get me to 60-70kV? As far as targets, I would like to image things like chicken wings, lighters, key fobs, ect. I know I wont be imaging harddrives or batteries.

There is no real useful reason for me doing this other than curiosity and a desire to learn.

Now for oil. Do I need pricey and hard to find transformer oil, or is there a more simple solution?
Thanks for putting up with all my questions! The internet doesnt have a whole lot of amateur oriented information.

Matt

Fabio

Sat Dec 26 2009, 05:11PM

Registered Member #122 Joined: Fri Feb 10 2006, 12:55PM
Location: Milano Italy
Posts: 148

Hi Matt!

first of all, this tube is designed to work for really short periods (no more than 2-3 seconds per minute) hence it had a small anodic mass (thermal mass) and had a little surface for spreading the generated heat

i suggest you to build a simple heatsink; as you can see, mine is made from a screw, some nuts and some washers and have the advantage of a profile similar to the tube (so it can easily inserted inside a rounded container filled with oil)

Second, as suggested from others, playing with X-rays can be fun but can also be dangerous, never trust too much the shields, is better if you stand back and remotely control the machine; geiger counters cannot detect low energy rays so you could be exposed to a great amount of rays without detection, PLEASE PLAY SAFE!!!!.

finally, i runned this tube in open air (with heat spreader!) with a single rectified flyback transformer (grounded filament), with two unrectified FBT in series (centertapped to ground with battery powered filament) and with an unrectified ignition coil (grounded filament) always with good results.

i have a book concerning xrays, give me your e-mail if you want a free copy

********** EDIT ***********

almost all generic oils will work more than adequate for this purpose but sometimes can contains impurities, additives or moisture that can seriously degrade his dielectric strenght.

i suggest you to buy a cheap mineral oil (without additives!!)and test it with HV before using as tube coolant

Ciao!
Fabio.

EEYORE

Sat Dec 26 2009, 07:17PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Hello,
An anode heatsink will definately be in order. Now, as to insulating oils...I will use oil for the x-ray tube, but what about the HV PS? I may just go ahead and build a VM for 70kV, but might parafin wax be better than mineral oil? Since the current will be low, the VM shouldnt have any heating issues, and parafin wax is alot less messy than oil (Well once it hardens). I have a hobby store near me that sells bricks of the stuff. I may also use this for the flyback transformer as well.

thanks! (Im getting more excited!)
Matt

Fabio

Sat Dec 26 2009, 09:01PM

Registered Member #122 Joined: Fri Feb 10 2006, 12:55PM
Location: Milano Italy
Posts: 148

beware for paraffin wax, this is an excellent insulator but it usually contain a lot of moisture, if you would use it as HV insulator, expecially in really high impedance circuits (as a voltmeter probe) you probably need a preventive drying procedure.
for drying the wax you can simply heat it at little more than 100CÂ° under vacuum for few ours before pouring it inside the probe; if a less insulating agent is required (for example for HV insulation of lower impedance circuits) you can simply heat the wax until it melt and leave heated for few hours in open air; to avoid hassle, the resistor network can be simple painted with dielectrical spray paint (or polihuretane paint) instead.

Ciao!
Fabio.

EEYORE

Sat Dec 26 2009, 09:17PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Fabio wrote ...

beware for paraffin wax, this is an excellent insulator but it usually contain a lot of moisture, if you would use it as HV insulator, expecially in really high impedance circuits (as a voltmeter probe) you probably need a preventive drying procedure.
for drying the wax you can simply heat it at little more than 100CÂ° under vacuum for few ours before pouring it inside the probe; if a less insulating agent is required (for example for HV insulation of lower impedance circuits) you can simply heat the wax until it melt and leave heated for few hours in open air; to avoid hassle, the resistor network can be simple painted with dielectrical spray paint (or polihuretane paint) instead.

Ciao!
Fabio.

Ahhh...Maybe I will stick with mineral oil.

Thanks,
matt

EEYORE

Wed Dec 30 2009, 03:41AM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

UPDATE:
So I won this tube also:

I was wondering whether or not this will be a better tube to go with instead of the other? (I was planning on keeping one, selling the other)
EDIT: Okay, so while I wait for the tubes to get here, I will begin building their housings. I plan to use PVC pipe and fittings. As for oil expansion, how much does it expand? My Idea was to use T fittings on each end of a PVC section as expansion chambers. One end of the T fitting would be for terminals and the other (pointed up) would give the oil somewhere to go when heated.
Also, any ideas as to keeping the tube in place? Should I use extra stiff wires to act as both electrical and mechanical connections? (A bolt for the anode and some stiff wire for the heater/cathode)?
Matt

LutzH

Thu Dec 31 2009, 08:10AM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136

Hello:

I would keep the one with the smaller focal spot, you can test this easy by x-raying a SS mesh faucet screen on B&W film with both tubes at 30cm. Then enlarge the negatives and see which one is more clear, keep this tube. Or just hang onto it as a spare. I still think oil is the ticket, but a plastic ducted fan will help with the heatsink, if you cannot do oil. One idea I forgot to mention is to use a big Cu pipe T-fitting with a rubber cap at one, or both ends, attached with a SS screw band hose clamps. The copper will shield most of the x-rays to boot.

EEYORE

Wed Jan 06 2010, 10:28PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

Hello,
So I tried out the larger tube that I won without oil since it has a hefty piece of copper on the anode and low current....Wow!!!!!!!!!!! What a difference! The voltage couldnt have been more than 20kV and the whole fluoroscope screen was lit up a nice bright green! I was able to see straight through every object placed in the x-rays! I was even able to see through a harddrive! This is much more than I expected out of this tube at such a low voltage and current. (Maybe 300uA)

Now some issues:
1) I am using a small 12.6v (6.3-0-6.3) 1A transformer I found in a box as the filament xfmr. The seller says the filament should be run at 4V 2.5A or so. This filament draws 2.5A at 3.3v. Is this an issue? Fortunately, the xmfr cannot provide 1A at its rated voltage, so along with a current limiting resistor, the voltage drops and still allows the filament to draw its 2.5A.
2) I have the negative from the HVDC tied to the filament wire that is connected to the 0v terminal on the xfmr. This creates popping noises from the filament xfmr which I assume is HVDC arcing around the windings.

How do I put a stop to this?

Thanks guys! I will see about pictures soon!
Matt

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