Seal Check is an active, external vacuum monitoring system. It allows you, the user, to make a go/no go decision on housing integrity by monitoring the vacuum level in the housing.
An internal vacuum check takes this decision from you. You must trust the designer of the kit.
We opted for the external system because other factors can influence the result. Temperature changes, for example, change the level of vacuum in the housing (PV=nRT). The smaller the housing, the greater effect temperature changes will have on the system.
Also the larger Gates housings take forever to pull a reasonable vacuum with a hand pump. An electric pump is necessary.
Interesting. I spend a good part of my life in the US Navy nuclear submarine service and never encountered such a procedure except in a shipyard during overhaul and compartment testing. Now if you're snorkeling at PD and the head valve goes shut for an extended period, you do get a rapid, unintended vacuum. Was your vacuum test done by compartment or on the entire boat at one time?
JohnE: Thanks for the background. What do you define as "extreme pressure"? I assume it's beyond normal rec. depths.
Thousands of PSI. Way beyond normal diving depths.
The vacuum check idea goes back at least to 1993. Howard Hall needed a way to verify integrity of the IMAX Salido housing containing 3D camera equipment worth $2,500,000. The vacuum system was devised, and indeed resulted in a safe dry camera every time. He and Bob Cranston have since employed the vacuum checker on every underwater housing.
2003: Gates designed and built 2 custom F900 housings for Bob and Howard which included a vacuum check port. It was our introduction to the system. Shortly thereafter it became part of our standard factory test. It allowed quick identification of leaks, but more importantly it revealed a way to test for shallow water leaks a.k.a. the dreaded rinse tank failure. This experience led us to standardize 3 different vacuum and pressure checks on every housing to weed out all failure modes.
Gates 'productized' the system in 2007 with the fitting name Seal Check. It was introduced in tandem with DEEP RED, our first cinema-grade system. We felt customers would appreciate peace of mind knowing their $50K+ camera investment was safe *before* entering the water. Seal check has proven itself many times, averting disaster from mistakes, abuse and damage (like from security inspections) that we all know happens in the field. For professionals it has not only saved equipment, but their production and paycheck as well. When you have to come back with the shots, failure is not an option.
As noted by others in this thread, the obvious benefits have resulted in the vacuum check system being adopted by nearly every manufacturer and several dealers (e.g. Backscatter).
Final note: The vacuum check system may extend all the way back to the 70's. I'm asking around to find out more....
Has anyone who uses a vacuum system ever had a leak? Circumstances?
My thought is that in addition to verifying that everything is airtight before starting the dive, the vacuum increases the pressure differential to reduce the chance of ports being rotated, etc. before the dive or when very shallow. Once you get below a few feet, the vacuum in the housing in inconsequential in the pressure difference between the inside and outside of the housing. Typical internal pressure is about 0.8 bar with the vacuum compared to 1 bar without the vacuum.
If you were able to pull a complete vacuum on the housing, it would be equivalent to 33 feet / 10m of water. But vacuum checks are much lower, so your observation is correct: the pressure difference from vacuum becomes and increasingly smaller part of the overall differential as you go deeper.
However.. once an o-ring is engaged under pressure, it will only fail under two conditions:
* Extreme pressures. A housing will flex and warp under extreme pressure, changing the o-ring groove characteristics, and resulting in a leak. This will happen long before the pressure increases to a point that the o-ring actually extrudes. We have tested housings to such depths here at Gates, and have interesting examples of how things fail.
* Mechanical change in the o-ring seal. An impact, for example, to a plexiglass window, or the previously mentioned gland.
Leaving a vacuum in place after checking the seals keeps the o-rings energized. It keeps everything tight while you are gearing up and getting in the water. And also while in the rinse tank -- one of the most notorious places to incur a leak.