Australia's Science Forum

Good afternoon all,
We are having difficulty evacuating air from our bell jar (housing a ticking clock) to the extent that the clock can no longer be heard.
Any hints??
Karen - ring, ring

Are you putting petroleum jelly around the rim of the jar? Make sure that air is not entering the bell jar. I have never emptied the jar enough NOT to hear the bell completely. It is very, very quiet but you can just hear it if everyone is totally quiet.
Lada

Same here. I have taped up all the plastic tubing connections to make sure they don't leak and applied petroleum jelly around the base of the bell jar to create a seal. The difference in the sound is noteable, but the vacuum is never quite complete.

Julie

Use a mobile phone as something different instead of a bell

I never have managed to lose the sound completely.
We tell the kids to listen for a CHANGE in the volume....pretty hard when theres a noisy vaccuum pump rattling along right next to it.
Some of the sound is transmitted through vibration as well.

I had too many electrical issues with the bell(wire holes leaked air in the top of stopper)
Mobile phone didnt work. (Kept defaulting to message bank.. kids got too excited & wanted their fav ring tone etc!)

So finally bought a personal alarm from Dick Smith. It is SO loud though, that I insulated the exit holes with foam and gaff tape to make it bearable. Its still really, really loud though.
I set the alarm off, then pop it under the jar, sealing as I go.
Set the vaccuum pump going and IMMEDIATELy take a reading on the sound meter. There will come a point where its hard to hear the personal alarm over the sound of the vaccum pump.
Thats about as good as it gets for us

If it wasnt for the noise of the pump, we could most likely get away with a much quieter device, like a ticking clock.
You can also borrow a metronome from the music dept.. they are good if you have a quiet pump.

Especially if you have only a quiet device , such as a ticking clock, combined with a noisy pump, doing the expt in reverse is the way to go. As soon as the pump is switched off, IMMEDIATELY take a sound meter reading, and listen for the alarm to increase in volume again.

The sound meter(or sensor on data logger) is a good objective test in dB. The students observations are more subjective, but the meter reader confirms their findings.

Because we dont have a perfect vacuum, its a great discussion to ask why, and would this be a problem in space etc.

The tech that was here before me suspended a battery operated door bell in the bell jar and sealed it up. We just need to press the separate control pad and off she goes! No wires and minimal leakage. Still it is not totally silent in a vacuum- but close to it.

I followed your tip of listening (via a sound level meter) for the increase in sound once the vacuum was turned off - worked like a charm!!! Thanks heaps,
Karen

rainbow-sky wrote:I never have managed to lose the sound completely.
We tell the kids to listen for a CHANGE in the volume....pretty hard when theres a noisy vaccuum pump rattling along right next to it.
Some of the sound is transmitted through vibration as well.

I had too many electrical issues with the bell(wire holes leaked air in the top of stopper)
Mobile phone didnt work. (Kept defaulting to message bank.. kids got too excited & wanted their fav ring tone etc!)

So finally bought a personal alarm from Dick Smith. It is SO loud though, that I insulated the exit holes with foam and gaff tape to make it bearable. Its still really, really loud though.
I set the alarm off, then pop it under the jar, sealing as I go.
Set the vaccuum pump going and IMMEDIATELy take a reading on the sound meter. There will come a point where its hard to hear the personal alarm over the sound of the vaccum pump.
Thats about as good as it gets for us

If it wasnt for the noise of the pump, we could most likely get away with a much quieter device, like a ticking clock.
You can also borrow a metronome from the music dept.. they are good if you have a quiet pump.

Especially if you have only a quiet device , such as a ticking clock, combined with a noisy pump, doing the expt in reverse is the way to go. As soon as the pump is switched off, IMMEDIATELY take a sound meter reading, and listen for the alarm to increase in volume again.

The sound meter(or sensor on data logger) is a good objective test in dB. The students observations are more subjective, but the meter reader confirms their findings.

Because we dont have a perfect vacuum, its a great discussion to ask why, and would this be a problem in space etc.

Thanks one and all for your tips.
I ended up listening (via a sound level meter) for an increase in sound once the vacuum was switched off - worked like a charm - even the Year 10's were impressed - and that takes a lot!
Have a great easter,
Karen

Also impressive : put a 50ml beaker of water in the bell jar and watch it boil @ room temp.

Willk wrote:Thanks one and all for your tips.
I ended up listening (via a sound level meter) for an increase in sound once the vacuum was switched off - worked like a charm - even the Year 10's were impressed - and that takes a lot!
Have a great easter,
Karen

Likewise, I tend to listen for the INCREASE in volume as you slowly let the air back IN to the Bell Jar. Our bell jar has a stop cock, so when the jar is evacuated, we turn off the cock, turn off and disconnect the pump, and,while the suspended bell is ringing, we open the stop cock just a crack and listen as the air enters the bell jar. It is always convincing.

Don't do what one teacher did. She opened the stop cock WITHOUT disconnecting the vacuum pump and sucked all the pump oil into the bell jar. Now THAT quietened the bell down permanently!

regards
Ian

Xenon wrote:Also impressive : put a 50ml beaker of water in the bell jar and watch it boil @ room temp.

WOW Xenon. Never gave that a thought!

Thats going right into my notes for next prac!
Thanks

Also another neat trick is a very slightly inflated balloon inside the bell jar. As you evacuate the air, ballon gets bigger and bigger and... You can blu tac a pin on inside top of the bell jar and as the balloon grows, it will reach the top and burst (very quietly)
lada

Yep. the balloon & tack IS great favourite with our students too

I've seen them do the balloon thing, but not with the pin inside. What a great idea! I'll have to pass that one on.

As for evacuating the air from the jar, perhaps there is something wrong with the pump? Have you checked it has oil in all the right places? I don't have any other ideas... we just got a new bell jar set from serrata late last year and it works really well. The pump is reasonably quiet (I think it's loud, but the teachers think it's wonderful compared to whatever they had been using before!). I hope you get it sorted.

While we are on the topic of Bell Jars, here's an idea which our Ag teacher came up with. They were testing the growth of plants under diffferent conditions. One plant was put in the old bell jar (no way was Ag touching the new one!) which has a rubber stopper at the top. They hooked a tube up to it and connected that to an old soda stream pumper, evacuated the air, and 'buzzed' the CO2 into the jar. The control was just left on the bench next to it. They came along and re-evacuated/pumped the jar a few days later, but I think the experiment would have worked better if they had done it each day. This was the first time they had tried it.

Rosalie

rainbow-sky wrote:

Xenon wrote:Also impressive : put a 50ml beaker of water in the bell jar and watch it boil @ room temp.

WOW Xenon. Never gave that a thought!

Thats going right into my notes for next prac!
Thanks

Just saw one of our teachers do this - kids were impressed, as was I!! Got on here to tell you all, but thought I'd better do a search first and here it is already mentioned back in 2008!! I highly recommend giving it a go if you haven't seen it. The kids couldn't believe the water was still cold (even after 2 goes) and suggested taking the bell jar camping as it was a quick way to make a cup of tea!!

Our teachers also use shaving cream, just a small amount that expands greatly. They also use marshmallows.

Rosalie,
the tea idea is good if you like cold water with tea leaves floating in it. It was even suggested to boil an egg in bell jar, try and have an egg fight.

Lada

HI,
I just got my bell jar today and have no idea what sort of hoses etc I will need to make it work. All the one's I have seen on Youtube seem to be sitting on some sort of base which the pump connects to, but those jars are solid. The one I have as you can see from the pick has a stopper in the top, how do I use it? where do I connect the pump and do I need to buy one of those base things with a tap?

Hi Rowyrow

You need to buy a base with a tap. Ours allows the tap to be turned to seal the bell jar and then we can turn the pump off. Much easier to hear the sound as it increases. When you get the base you will see where the hoses go. It's best to take the stopper out of the bell jar and put silicon grease or vaseline around it before it's put back. Don't forget to use vaseline to seal against the base plate.

Thanks for that.Any idea where to get a stand from i can't find one with the supplier i got the jar from. Are they called something specific that I need to use as a search term?