Questions about CRT Safety Glass

[KeepItSimpleStupid]

you can put a 1/4" piece of plexiglas in front of the tube to reduce the implosion hazard.

Use Polycarbonate, not plexiglass. I know first hand what happens with Plexiglass.

 

[unclejed613]

Use Polycarbonate, not plexiglass. I know first hand what happens with Plexiglass.

good point, although i've used plexiglass before for various uses, and it is pretty durable when it comes to fairly light objects moving at moderate speeds... i've seen CRTs implode and the fragments don't travel very far..... but polycarbonate is a lot stronger...

 

[crutschow]

I cringed as soon as I read that!

Me too.
I put it in the same category as defusing a bomb..

 

[unclejed613]

and hit the neck of the tube with something heavy.

the technique i was taught was to break the vacuum nipple under the socket... it took longer for the tube to fill with air, but the chance of implosion was about nil

 

[Nigel Goodwin]

I was breaking one in a metal dustbin at work once, the dustbin was old and very battered - and I was struggling knocking the nipple off the end of the neck. All of a sudden the CRT imploded, making a really loud thud/bang, and straightened out most of the dents in the metal bin

 

[schmitt trigger]

I worked for several years at a CRT manufacturing place (Sylvania), and saw first hand the effects of an implosion.

Jus think of it this way: a 25” diagonal 4:3 CRT is 20” by 15” or 300 square inches.

Atmospheric pressure is 14.7 lbs per square inch.

Do the math: that front screen is withstanding 4410 pounds.
You don’t want to release all that force in an uncontrolled manner

 

[Nigel Goodwin]

I worked for several years at a CRT manufacturing place (Sylvania), and saw first hand the effects of an implosion.

Jus think of it this way: a 25” diagonal 4:3 CRT is 20” by 15” or 300 square inches.

Atmospheric pressure is 14.7 lbs per square inch.

Do the math: that front screen is withstanding 4410 pounds.
You don’t want to release all that force in an uncontrolled manner

Must admit, I'd never thought to do the maths - and having seen that figure I'm glad I didn't

 

[KeepItSimpleStupid]

At one time, I wondered why so many bolts around a 8" vaccum flange. I think I calculated over 400 PSI.

 

[crutschow]

that front screen is withstanding 4410 pounds.

I was amazed when they were able to make a flat-screen 32" tube.
I don't see how they could make the front strong enough to stand about 7000 pounds of force and still have a tube that you could lift (albeit probably by two people).

 

[KeepItSimpleStupid]

it's like wow for the strength of glass: https://en.wikipedia.org/wiki/Strength_of_glass

 

[RayRay1132]

maybe make a new one out of plexiglass? I've never seen a set with a safety glass except for my RCA model 730. Most of the newer crts that I have don't seem to have any safety glass on the tube.

 

[AnalogKid]

and straightened out most of the dents in the metal bin

By sucking the walls inward?

ak

 

[AnalogKid]

I worked for several years at a CRT manufacturing place (Sylvania),

Alrighty ... You seem to be someone who might know this ...

How complete is/was the partial vacuum inside one of those old fat color CRTs? Or any "big" CRT, say 19" diagonal and up? IOW, any numbers on the air pressure *inside* a working CRT? Of course it wasn't zero, and of course it had zero oxygen in it. So, what was it?

ak

 

[RayRay1132]

I was breaking one in a metal dustbin at work once, the dustbin was old and very battered - and I was struggling knocking the nipple off the end of the neck. All of a sudden the CRT imploded, making a really loud thud/bang, and straightened out most of the dents in the metal bin

Did they really make that strong of a bang? I've seen plenty of crt sets here in the US, from the mid-80s and beyond without any safety glass. Could it have been built into the front layer of the tube? Just asking because apart from my old RCA, I haven't seen any with safety glass.

 

[crutschow]

Of course it wasn't zero, and of course it had zero oxygen in it. So, what was it?

Why do you think it's not very near zero?
The vacuum needs to be as good as possible, since any gas molecules will interrupt the electrons going to the screen.

Could it have been built into the front layer of the tube? Just asking because apart from my old RCA, I haven't seen any with safety glass.

Yes, I believe they made the safety glass as part of the front of the tube.

 

[KeepItSimpleStupid]

Yes, I believe they made the safety glass as part of the front of the tube.

The safety glass was bonded to the front of the tube. The rest of the tube generally had a metal shield around it.

 

[Nigel Goodwin]

By sucking the walls inward?

And then carrying on out the other side - an implosion is very little different to an explosion.

 

[Nigel Goodwin]

Did they really make that strong of a bang? I've seen plenty of crt sets here in the US, from the mid-80s and beyond without any safety glass. Could it have been built into the front layer of the tube? Just asking because apart from my old RCA, I haven't seen any with safety glass.

Any remotely 'modern' CRT's have safety glass built-in the front of the CRT - it was only the really old antiques that had separate safety glass.

I would imagine any CRT from the late 60's/early 70's were made that way.

 

[Nigel Goodwin]

Alrighty ... You seem to be someone who might know this ...

How complete is/was the partial vacuum inside one of those old fat color CRTs? Or any "big" CRT, say 19" diagonal and up? IOW, any numbers on the air pressure *inside* a working CRT? Of course it wasn't zero, and of course it had zero oxygen in it. So, what was it?

As already mentioned, it was VERY low - it had to be for the CRT to work, and contamination greatly reduces the life and operation of the CRT. This is why they had 'getters', you suck the absolutely maximum possible out, and then fire the getter to burn up and remaining molecules.

 

[schmitt trigger]

This is from memory from 40 years ago, some details escape me.

To achieve an ultra high vacuum level, the tube was attached to a mechanical pump in series with diffusion pump (google the term) while simultaneously heating it in a convection oven at 250 Celsius for several hours. This roughly de-gasses the tube.
Then utilizing a RF induction heater, the gun’s metal components would be heated cherry red, with the pumps still attached evacuating any
gassy emissions.

Then the heater would be subjected to 2 or 3 times it’s rated voltage, it would glow incandescent white for a few moments. Again the pumps would evacuate any additional gas.

Only then the stem would be sealed, but the process was not yet complete. With another induction heater, the barium getters would now sputtered, producing a marvelous purple glow while depositing the silvery metal inside the tube. The barium would capture any leftover molecules.

The final pressure before the stem was sealed was measured in tenths of mili torrs but can’t recall exactly the value. A milli torr is already very small: 1/760,000 atmospheres.

I never measured the final vacuum after the getter had been sputtered, but was told it would be reduced 10 to 100 times lower. A high vacuum indeed!

By then, the Japanese manufacturing had already upgraded to turbomolecular pumps, and was anecdotally told that they had 20X lower vacuum. One of the several reasons that those tubes lasted so long.