[PSUBS-MAILIST] Hatch interlock

Wed Mar 2 08:48:38 EST 2016

I should elaborate a bit on transducer construction - the sensing element is essentially just a thin diaphragm that is instrumented with strain gauges (a four gauge Wheatstone bridge). To manufacture a transducer with greater sensitivity (lower range), the diaphragm must be thinner / less stiff, but then it also has less tolerance to pressure before it is damaged. When you buy transducers, the specifications will list their range (over which they can be expected to produce a linear proportional signal), and their exposure limits (maximum tolerable pressure before the sensing element is damaged).

Sean

On March 2, 2016 6:38:18 AM MST, "Sean T. Stevenson via Personal_Submersibles" <personal_submersibles at psubs.org> wrote:
>A pressure transducer designed to measure the range of absolute
>pressures below standard atmospheric pressure.  When you buy pressure
>transducers, you have three types of available measurements: 
>
>1) gauge pressure, where one side of the sensing element is open to the
>ambient pressure, and the transducer gives you only the pressure above
>that. E.g. 100 psig is 100 psi over ambient. Your SCUBA regulator
>maintains an intermediate pressure of 100 - 150 psig, while the
>absolute pressure changes with depth.
>2) absolute pressure, where one side of the sensing element is exposed
>to a sealed vacuum reference chamber, and the transducer gives you the
>full pressure reading referenced to zero. 100 psia is 100 psi, period.
>3) differential pressure, where there are process connections to both
>sides of the sensing element, so you get only the difference between
>them. 100 psid is 100 psi difference between sides, with no regard for
>what the absolute pressure is.
>
>A vacuum transducer would be an example of an absolute pressure
>transducer, with, for example, a measurement range of 0 psia to 15
>psia, which could be expressed as negative gauge pressure, but such
>transducers are not constructed as gauge transducers because
>atmospheric pressure varies - zero does not.
>
>Sean
>
>
>On March 2, 2016 6:12:47 AM MST, hank pronk via Personal_Submersibles
><personal_submersibles at psubs.org> wrote:
>>Sean, What is a vacuum transducer?   
>>
>>On Wednesday, March 2, 2016 5:41 AM, Sean T. Stevenson via
>>Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>> 
>>
>>Doing some further design on my lockout submersible project, I came up
>>with a novel way to implement hatch interlocks, which doubles as a
>seal
>>condition monitor, and a means of establishing a preliminary seal in
>>the absence of a pressure differential without relying on the hatch
>>dogs to provide the initial o-ring squeeze.My design entails two
>>o-rings per hatch (vessel has six hatches: cabin loading / escape,
>>outer lockout loading / escape, inner lockout loading / escape, inner
>>lockout egress , outer lockout egress, and transfer). These o-rings
>are
>>concentric face seals, each residing within a half dovetail groove for
>>positive retention of each o-ring when the hatch is opened or
>>manipulated. The grooves are oriented such that the flat face of each
>>half dovetail faces the intermediate space between the two rings. This
>>intermediate volume is not isolated, but rather connected (on the
>>sealing flange side) to a vacuum transducer, and piped through
>>appropriate valving to a vacuum pump. When the hatch is closed, this
>>intermediate space is pulled to vacuum (as strongly as the pump
>>allows), then locked off, and the strength of this vacuum is measured
>>by the transducer and continuously monitored. The interlock is clear
>as
>>long as the vacuum holds, and activates the moment the seal is rele!
>>ased,instead of relying on some arbitrary movement of the hatch to
>>indicate that it is open.Apart from the obvious expense, I see a
>>potential problem with exposing those vacuum transducers in the
>lockout
>>hatches to high pressure, necessitating either a less sensitive
>>transducer that will withstand the pressure, or some means of
>isolating
>>the transducer when the pressure approaches the limit of its range -
>>I'm still working this out in my head, but I thought I would share
>>anyway.Sean
>>
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