Siebe Gorman Mk II

[merlinman]

I thought some of you might be interested in the SG Mk II,a surface demand version of the Mistral complete with harness and navy issue full face mask.

[luis]

I like the mouthpiece inside the mask.

That curved lens is also weird. I wonder how much distortion it has and how prone they are to break.


Thanks again for the pictures.

[merlinman]

Hi Luis

The lens on the mask is in fact flat, only curving at either edge to provde side vision, there is no visible distortion when in use.

[SeaRat]

I am curious about the 90 degree air intake elbow. The R/D (radius divided by diameter) of this elbow looks to be about 1.0. If so, according to my industrial ventilation book, the elbow loss would be somewhere between 0.21 and 0.45, depending upon the aspect ratio. Why did they put those elbows on this regulator?

John

[luis]

searat said:

I am curious about the 90 degree air intake elbow. The R/D (radius divided by diameter) of this elbow looks to be about 1.0. If so, according to my industrial ventilation book, the elbow loss would be somewhere between 0.21 and 0.45, depending upon the aspect ratio. Why did they put those elbows on this regulator?

John

The published losses that you are seen is the loss coefficient (also known as the K or Cd value or multiplier) used in Bernoulli’s equation.

The term Cd (C subscript d) is used instead of K for drag coefficient, which is basically a similar term.

The basic equation is very simple, but a bit hard to write without Greek letters and in a single line. I will try:

Pressure drop (DeltaP):

DeltaP = K * (0.5) (fluid density / g ) * (fluid velocity)^2

Notice the fluid density has to be divided by “g”, gravitational constant if you are using English units.

Notice that the pressure drop is a function of the velocity square. The air velocities in this system are relatively low. Since the pressure drop is a function of gas velocity square, the most important factor is to keep velocities down. This is accomplished by keeping the flow area large. Flow velocities increases on any constrictions.

The loss coefficient for a sudden contraction (like the transition from the can to the horn) can be easily as high as K=1, for square edges as found in most regulator can to horn transition. Therefore, a K value of 0.21 to 0.45 is significant as compared to other piping “minor” losses, but you can see that it is less than just the contraction. The published data I have has similar numbers as what you are showing for different elbows.

Another thing to consider is that the bend is going to happen in a smooth elbow or a corrugated hose. The bend radius is larger in the hose, but the losses due to friction on the corrugations may just balance it out.

The pressure drop is linearly proportional to gas density and the density is increasing with depth, but again if the velocities are kept down with good design the pressure losses are reduced to minor effects.

When I have more time I should put some actual numbers behind the flow equations. It is not very difficult; I just have to make some instant flow assumptions. In reality it is transient, constantly changing system.


Note: Loss coefficients (like all coefficients) are a unit less multiplier. The units in the equation have to be consistent to obtain correct results. One of the largest sources of error with our unit system is related to the mixture of unit we use.
Velocity in: feet per second
Density in: pound “mass” per cubic inch
Low pressure changes in: inches of Water Column (sometimes in pounds “force” per square feet).
High pressures in: psi (pounds “force” per square inch)
Etc.

[YankDownUnder]

The mask you have on your Siebe Gorman MkII 'hookah' appears to be the same as the one issued with the RN & RAN Avon EAN60/O2 CDBA and the O2 UBA. I sold my Avon, so I cannot be sure if the mask was marked on the body or on the spider, but it is an excellent full face mask. I used it while diving the rebreather and found it to be very comfortable. The rebreather came with a pendulum hose and the mask for long period use.
See: www.nobubblediving.com/cdba1.htm

The plastic mouth piece connectors were used on some early Siebe Gorman regulators, before they bought Heinke in 1962. After that Siebe used the Heinke designed one piece, on the Merlins and dropped the Mistral. Siebe was the first company to be licensed to make the Mistral and sold in the Commonwealth Nations. They had great difficulty meeting early orders and most of their production went to the Royal Navy first.

You have an interesting find. You could use it as a chest mount regulator, attaching a reduction valve on a tank to the supply hose, much like the Sydney, Australia Lawson Lung.
See: www.classicdiver.org/hdsseap1.html (Mel Brown article) The Lawson Lung was made because Australians could not buy La Spirotechnique regulators because of the licensing agreement and the difficulty Siebe Gorman had trying to meet demands.

Congratulations on a great find!

[merlinman]

Yes your quite right, the mask is the Royal Navy model.