Vacuum in Telemetry cans.

[piper]

Just had an interesting if not ackward discussion with the guys about vacuuming telemetry cans before re-installing on vehicle. What is the norm/or the right way to do it on other systems. I am not looking for, [from experience stories pls] rather an approach on a technical basis. If you dont know what a telemetry can just shut the f*&k up.

[cleeamollette]

for me it is not a bad idea even if a vaccum apply inside is quite different from a pressure apply outside. on my side, I ask myself two questions:
– what pressure do we get in there once at depht( caus the test must be based on real values otherwise…..); i would like to do the test but my sup. will never let me do that.
– why are they always pressurized? i mean when opening the vent plug there is always some air coming out.

if you have an answer, i’ll be glade to hear about..

[Martin Wareham]

Electronics in a pressure vessel generate heat (all electronics generate heat), if they heat up too much they die (or at least stop working). A vacuum (as we get in thermos flasks) is not too good at transferring heat (which is why your coffee/tea stays warm). This is a relatively bad thing in a pressure vessel for obvious reasons.

A vacuum is normally pulled (in my experience) to make sure there’s a good seal in the pod. Then after a certain amount of time it should be released (making sure you close back up the vacuum port :wink:) and then you dive.

Actually testing the pressure vessels when new should be carried out by applying external pressure, not reducing internal pressure (in my experience) because you’ll never achieve the required real world (working depth) pressure differential.

I hope that was the question 😳 it looks quite technical 😥

My tupence worth 🙂

[Mike Kidd]

Many moons ago we used to purge the cans with N2, this was to ensure the air inside was dry, then we used to use silica gel (cost!), but when we were finished, we always left a slight vacumn, this was to give an indication that something was going wrong before the dreaded water alarms went off, we all understand that heat transfer is poor in a vacumn so dont leave too large a one in the pod, Perry pods used to have fans in them but i have not worked on a perry since 2002 so you would suspect that some air must be left in to aid heat transfer, but I have not heard of any left with a pressure in them ❓

[Martin Wareham]

P1V1/T1 = P2V2/T2 😉

The P1V1/T1 is the initial pressure volume and temperature.

If the temperature inside changes, something else has to change to balance the equation, i.e. the P2V2/T2 is the final pressure volume and temperature.

I think this is why sometime there’s pressure inside and sometimes the end cap on a pod refuses to come off if the pressure isn’t equalised.

[quaich]

I was always told a vacuum is pulled for two reasons.
Firstly it establishes there is a seal after closing the pod and secondly it helps the pod seal while dropping through the splash zone.
There isn’t much keeping the pod closed if there is no vacuum.
Once at a reasonable depth the hydrostatic pressure takes over in helping the pod stay closed.
But that wasn’t really the question was it.
You asked "What is the norm/or the right way to do"
The way we do it is to suck a bigger vacuum than required, remove the vacuum pump and quickly refit the cap or tighten the vent screw (whichever is fitted) before the vacuum is lost.
You’ll obviously lose some of the vacuum but enough will be retained.
It gets a bit trickier, but not impossible, when some nugget has overtightened the swage plug and it has to be wound all the way in with a spanner.
I guess it’s progress but I can’t recall any old stories of pods opening before vacuums were pulled.

[quaich]

And I’ve always been a bit dubious about relying on vacuum pressure before water ingress sensors.
Water will have already entered the pod before you see any change in vacuum so the water ingress sensor would alarm first.
Thankfully I cannot confirm this theory.

[misiuek]

from a horrible experience once! the vaccum alarm will come on first roughly 30secs before water alarm.
and 5mins before on deck! sob sob

remember in hotter countries the vaccum pulled should just be past the indicator and not the full pressure, as mentioned previously it causes all sorts of heat problems.

[Jason Garic]

What type of system has vacuum sensors on the cans?

Generally, we pull a vacuum on a can and hold it at a set in/hg. Say 10 or 15. Hold the vacuum and monitor that the vacuum holds for 10 min’s. Back fill the can through a desicant or nitrogen.

This would tell us that the can would maintain a seal through the first few atmosphere’s. If you could make it past the first three, you were good to go.

I have no idea where the numbers come from; and, generally it shifted from system to system. And someone always asks this same question…"Why?"

It’s worked for as long as I know…..so it must be right?

Oh yeah, don’t forget the vent cap. 😯

[billyd]

My theory –

A vacuum test is carried out, purely as an indication that any recently separated; sealing areas have been re-assembled correctly and retain their integrity. This by no means, exactly simulates the real 2000m dive conditions. But given equipment usually to hand in the field this is about as scientific as we could get. Lets remember though, the external pressures felt at depth, will compress all the components and assist in certain sealing functions.

Why not pressure test? I would say safety. F=PxA!
Do the math for a 25-PSI test on your telemetry can. How is the end cap held on? Stay clear!!

The vacuum should be removed, as there would otherwise be no atmosphere inside the vessel. Nothing to circulate and set up a cooling circuit with the outer walls creating a thermal barrier. (Vacuum flask!).

As I said, only my own theory.

Toodle pip

[matt buxton]

Well as far as the water sensor vs vaccum sensor debate…….. speaking from experience I can say that a vaccum sensor is NOT a valid sensor to warn of a leaky seal.

### First hand experience………drove an active sidescan sonar towfish into a rock wall at 200m, doing 9 knots. Miraculasly the cable did not part and we got the fish back. It was the last day of the job and everyone was in a hurry to get back on dry land, so we quickly checked the vacumn on the electronics hull with a mechanical gauge…….still showed -9 in/hg. no problem right!…….well 3 weeks later when we where back in VAN after releasing the vacuum we opened the hull and drained out about 60L of nice smelly mediterranien sea water!!! along with various small bits of electronics, which had been flung at the end cap with considerable force! f@$%@$!!!!!!! #####

LESSON LEARNED……water sensor to detect leaks underwater…….Vacumn sensor to detect leaks on deck.

One more quick note on the whole vacumn topic. currently we use -7 in/hg in our pressure hulls for AUV’s……the main reason is to ease assembly and check for leaks before the "float test".

Anyhow just my two cents

[Sarcy]

The vaccum has to fail first for any water to get in. If the vaccum holds then as has been mentioned hydrostatic pressure takes over and squeezes the pods and connectors closed. I can symapthise with Miseuik on his experience as we work on the same systems – Seaeye Panthers – not a nice feeling. If water gets in with no vaccum alarm then the alarm system is faulty

As has also been mentioned it is only a partial vaccumm as the electronics in the pods need some air present to help with the cooling process.

[matt buxton]

Sarcy

no disrespect or anything but your statment "vacum has to fail before water gets in" is not entirely true.

Lets look at some quite simple physics…….

1. Boyles law PV = PV
2. Average volume of telemetry can = aprox 50 L (13 gal)
3. Average vacum in telemetry can minus 8-10 inches mercury. = -4-5 PSI
4. Sensitivity of average vacum sensor alarm probably not better then 1 PSI.
5. Now lets say that for your vacum alarm to trigger because of a water leak then you would have to drop by 1 psi. there for PV=PV becomes -5*50L=-4*X after doing the math X = 62.5L or 16.5 gal

*** This means that if you are using your vacum sensor as a water/ leak detector you would have 12.5L/3.5 Gal of nice salty sea water sloshing around on yor electronics before your alarm would even trigger. This would be a bad start to any day ***

So vacum sensors great for on deck leak detection, or long periods of dry storage. but dont hold up when the kit hits the big blue salty.

just my two cents

[spanker]

I would also ask myself a few questions about the above topic.

1. Am i really that bored when i am on shift that i think about stuff like this.
2. If i was that bored then i would go to sleep and then the end of my shift would come quicker.
3. If i remembered the above mentioned laws or went on to the net to find them, then a serious think about a career change would be necessary.
Or maybe a bit of time off!!!!!

[matt buxton]

easy now spanker

No need to start getting hostile.

Pretty sure that someone who deals with deep sea work…..AKA ROV’s. would have in there best interest to know a little about the relationships between Pressure–>Temperature–>and Volume. considering that every aspect of under sea work is governed by these rules.

And this is a very interesting topic for discussion…….because the standard answer you get when asking anyone about it……is "well thats they way we have allways done it". Now do you really think that you would be employed in this great industry if everyone around you had thought this way……… didn’t think so.
So how bout you allow a little technical disscussion about a technical subject on a technical forum…instead of berating everyone for having a little interest in something they do not fully understand. And if you cannot do this then " go to sleep and your shift will end quicker"

Just my opinion.

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