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Hi everyone, I have looked on the internet to try and determine the height at which an oxygen depletion alarm should be fitted in an enclosed area containing liquid and gaseous nitrogen but not having much luck. Some sites specify between waist and knee height and some as below 1 metre. The manufacturers of the sensor/alarm we have had fitted assure us that having it fitted just above a worktop counter is ok but I am not convinced.
Anyone have experience of this?
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Rank: Super forum user 
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Martin - 1m above the floor is the approach we take. I can PM you slightly more detail if required.
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 1 user thanked JayPownall for this useful post.
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There is no hard and fast rule for this one. Nitrogen gas has a density slightly greater than that of air (which is not surprising as air is about 80% nitrogen) which means it would accumulate from the ground up if there was a leak. It would not pose a significant risk to someone (standing up) in room until it reached their breathing zone and that is where the 1 meter or there about “rule” comes from. Ideally you would have a series of sensors running from the floor upto the ceiling to monitor the flow of gas but about half between the floor and someone’s breathing zone is a good compromise.
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2 users thanked A Kurdziel for this useful post.
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Thanks Jay, I would appreciate that
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Sorry A Kurdziel the courses I’ve been on say nitrogen gas is lighter than air. The difference is quite small though.
But it’s more complicated than that since the gas evaporating from a liquid nitrogen Dewar or spill will be very cold so it will sink to the ground. If you’re concerned about a liquid nitrogen spill replacing the air then I’d agree with others that about 1M from the ground is the way to go. That’s what we have in the two sites we use these oxygen monitors. But don’t rely on them, we also only permit people to work in pairs in environments where we have these alarms. Then there’s the strict rule to get out if it goes off.
We were in the very frightening position where the first warning alarm went off whenever anything was done (18.5%). This was routinely ignored – it went off all the time. The lower alarm (16% I seem to remember) was not discernibly different from the warning alarm, especially if you were concentrating on something else.
My point is relying on the alarm is nowhere near enough to manage these situations.
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1 user thanked Graham for this useful post.
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Nitrogen has a s.p.g of 0.9669 and Oxygen a s.p.g of 1.1044. Therefore Oxygen is very slightly more dense and heavier than Nitrogen. Obviously the monitors are measuring percentage of oxygen and should be poitioned lower down. Cold vapours will of course be more dense.
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Rank: Super forum user
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How much liquid nitrogen are we talking about and what sort of leak can happen. There is difference between a slow leak from a Dewar, which builds up slowly in a poor ventilated space or a massive sudden rush of the stuff from a 2000 plus litre tank (in that case it does not really matter if it is from the ground up or not) where it probably won’t matter if it’s well ventilated or not? Yes what you will need is a risk assessment!
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Rank: Forum user 
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Apologies for bumping the thread. From what I know, and my experiences of working with liquid nitrogen...
If it is nitrogen gas you are working with at room temperature, the nitrogen gas will rise, having a lower specific gravity than air. If it is liquid nitrogen you are working with, then the vapours will fall first and inhabit floor areas primarily before warming and rising to ceiling areas. When I was filling PSVs (Pressurised Storage Vessels) from a VIT (Vacuum insulated Tank), I would wear my personal oxygen monitor in my breathing zone (left shoulder). Once I had taken the PSV into the lab, I would clip my personal oxygen meter at knee level, to warn me of vapour collecting at floor level.
The sensors we had on the fixed oxygen depletion monitors in the lab were all at floor level, but that was because we always used nitrogen in its liquid form. We had a separate storage room that was always freezing cold, because it was very well ventilated to allow any gas released by the pressure relief valves to vent to external atmosphere. This storage room also had a sensor for the fixed oxygen depletion system at floor level too. If the main problem is going to be nitrogen in its gaseous form, then I would concur it sounds logical to have the sensors at a height of 1 metre. If there is liquid nitrogen being used then, as has been suggested, you may require a couple of sensors, the second one being at floor level or thereabouts. Risk assessment is the catch all answer, however there may be extra guidance here...
http://www.bcga.co.uk/pages/index.cfm?start=1&page_id=19&showCategory=2&showSubCategory=1
... in particular CP22, CP27 and CP30 - if I recall correctly, one of them has calculations to assist in working out how badly a leak could affect the oxygen levels. If you want a good training provider, I can recommend one.
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2 users thanked andrewcl for this useful post.
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