James,
Let me backtrack on something I said earlier. If you had enough
water enter the sub that it caused a higher pressure reading on the altimeter,
you'd already be in a world of hurt. I don't yet have any detectors in
mind for that situation. I'll keep anything on that in a separate post in
order to stay on topic.
Jim
In a message dated 3/27/2012 12:38:07 P.M. Central Daylight Time,
JimToddPsub@aol.com writes:
James,Alec also brought up an important function of the altimeter when he mentioned setting the midrange as 0. If you have an internal air leak such as from an O2 or air tank, or if you have water coming in from the outside, the air pressure in the sub will increase and the altimeter would so indicate. An over-pressure alarm would be a fine thing to have onboard. Malfunction of your scrubber or O2 feed rate could also cause a rise in cabin pressure.I think I'll add an onboard checklist for possible causes if the alarm goes off. This brings up the question: What level of cabin pressure increase should activate the alarm?I'd like to hear some answers on that one.Thanks,JimIn a message dated 3/27/2012 12:05:27 P.M. Central Daylight Time, JimToddPsub@aol.com writes:Hi, James,I plan to go to a certified aircraft instrument shop at a general aviation airport and see what they have available. For my purposes the altimeter will not have to have been re-certified within the required time frame. I've never had to be concerned with reading an altitude of over 20,000 feet, and now I don't even recall the maximum altitude mine were capable of displaying. It won't be an issue for actual dive operations, but I might need to come up with something with a wider range for shop tests to maximum obtainable vacuum.Don't get an altimeter out of a pressurized aircraft since its design has to read from a port outside the aircraft. Here's the ebay link in the U.S.:I just saw Alec's posting and skimmed it. Sounds good.The altimeters I've dealt with can easily show a 10-foot change, so if you're holding one in your hand, you can stand on a chair or ladder and hold the instrument above you head, then get down and hold it at floor level. If the reading doesn't change, it's broken. If you happen to be in a two-story building, even better.Let us know what you come up with and what issues you encounter along the way.Cheers,JimIn a message dated 3/27/2012 11:25:50 A.M. Central Daylight Time, jamesf@guernseysubmarine.com writes:Hi Jim,Yes, altimeter sounds good. i understand what you mean with the test. Was going to do exactly that myself. It was just the gauge I was being a dope about. Its obvious now I've thought about it.Had a quick look at altimeters. Loads of different ones you can get. Cheapies, wrist watch ones, aviation, skydiving etc. What make of altimeter are you using? I do have a proper custom made caison gauge as well which came out of a diving re-compression chamber. That might be ok but it will need to be re-calibrated.Anyway, plenty of other things to do first before I need to fit that.RegardsJames
On 27 March 2012 15:26, <JimToddPsub@aol.com> wrote:
James,To clarify: The reading of 5,000 ft. is what I've chosen for pre-dive tests when the pilot is inside and the hatch has been latched. When testing the vessel for leaks in your shop with no one inside you want to take it as far as you can.Even if you could pull a complete vacuum with a pump, the maximum difference you can achieve is 1ATM or about 14.7 psi. which equates to a depth of 33 feet or so in water. In the shop test I'll go as far as the pump and the altimeter can handle (maybe 100,000 feet above sea level). If it passes all that, the next step is dunking it in water to the appropriate depth(s) or testing in a pressure chamber.JimIn a message dated 3/27/2012 8:40:27 A.M. Central Daylight Time, JimToddPsub@aol.com writes:James,I'm planning to install an aircraft altimeter for the same purpose. An altimeter is simply a barometer with a wide range, so you could use that instead. It has the sealed air chamber (aneroid) which expands or contracts in response to changes in external pressure.With the hatch closed and latched you can pump air out with an external vacuum pump, turn the pump off, close the valve, take a reading on the altimeter, wait a few minutes, then take another reading to see if any air leaked back into the sub. This can even be done with the sub unoccupied as long as the altimeter is positioned where you can read it through a viewport.In most cases you can use the adjustment knob to set the altimeter to 0 for the first reading. I plan to run the vacuum pump until the gauge reads 5,000 ft. altitude, shut the valve, and wait 5 minutes. If the altimeter reading drops to 3,000 feet for example, I know a seal is leaking inwardly. One caution: If the sub is sitting in strong sunlight, you could get some increase in internal air pressure just from heat build-up.This test is on my pre-dive checklist on every dive. During a dive the pressure reading should change very little if your scrubber and O2 inlet are in balance.
![http://upload.wikimedia.org/wikipedia/commons/c/c3/Sens_alt_components.PNG]( "http://upload.wikimedia.org/wikipedia/commons/c/c3/Sens_alt_components.PNG")
Comments and criticisms are most welcome.JimIn a message dated 3/27/2012 7:36:00 A.M. Central Daylight Time, jamesf@guernseysubmarine.com writes:Hi All,Mr Numpty here.I kind of know the answer to this already, but i thought someone may be able to clarify my mistake.I was very pleased when i spotted just the sort of gauge i was after mounted on the back of a sewage tanker. A pressure gauge but one that reads a negative pressure as well as a positive. So it starts at 0 and can go to -1 or +1 bar. Idea was that it would go inside the cabin to monitor internal pressure. (I'd like it to be able to measure a negative pressure as well as i intend to drop the internal pressure to test for penetrator seal leaks).I was just thinking about mounting it up and ive suddenly realised that its not going to work is it? It must measure the port pressure relative to the ambient pressure, which if i just mount it on a bulkhead will always be the same no matter what the pressure actually is.I suppose i'd have to make a pressure proof box sealed at surface pressure and pipe the port to the inside of the hull? So its kind of like a caisson gauge.Just thinking aloud really.James
