What’s the deal... SP 108 HP verses 156BA

[buddhasummer]

The .218 in later G250V models replaced the .216. That part number, .218, is currently listed as being for both the G250V and A700 but not the G260 which still lists the .216, go figure.

I've used both, not surprisingly I can tell zero difference.

 

[buddhasummer]

Apparently the .215 spring found in the C370 is the same as a .216 but "coated", with what I don't know but apparently it's more corrosion resistant or it could just be magic SP fairy dust.

 

[buddhasummer]

As mentioned by halocline, spring for 108HP is the 01-020-216.

Thank you. Then yes, I do.

 

[halocline]

I have a flowmeter, vacuum pump and magnehelic gauge. I’ve only had the 108 in a 15 foot pool with no free flow, but to be honest I don’t remember what it was tuned to at the time.

How did your 108 at depth compare to your BA when it was working stable?

As I mentioned, I would not place a lot of stock in the numbers from those machines for comparative purposes, unless the numbers are so different that it's obvious one stage is wildly outperforming another. I think 2nd stages need to be evaluated by diving with them and treating it more from an educated subjective perspective. I think the cracking effort measured by a magnehelic gauge can be useful for tuning.

The 108s that I am familiar with (not that many) have a deeper, rounder case than the 109s. This means (I think, I've never actually measured it) that there is more distance between the point where the diaphragm/lever make contact and the exhaust valve. It's this distance that determines case fault geometry, and as a result, positional stability and free flow resistance during inhalation. I found that my 108s, if tuned very lightly, would start to free flow much more easily when looking down (maximum depth differential between diaphragm and exhaust valve) and would free flow more easily with a strong inhalation, due to more sensitivity to venturi effect. At greater depths, the venturi effect is more pronounced because the air flowing through the reg is more dense.

But, I have had 108s that breathed very well. I just tend to not use them because I like adjustable 2nd stages, particularly as alternates where I can tighten them down when they're just hanging around my neck. (I always use bungeed alternates on a short hose)

 

[Open Ocean Diver]

As I mentioned, I would not place a lot of stock in the numbers from those machines for comparative purposes, unless the numbers are so different that it's obvious one stage is wildly outperforming another. I think 2nd stages need to be evaluated by diving with them and treating it more from an educated subjective perspective. I think the cracking effort measured by a magnehelic gauge can be useful for tuning.

The 108s that I am familiar with (not that many) have a deeper, rounder case than the 109s. This means (I think, I've never actually measured it) that there is more distance between the point where the diaphragm/lever make contact and the exhaust valve. It's this distance that determines case fault geometry, and as a result, positional stability and free flow resistance during inhalation. I found that my 108s, if tuned very lightly, would start to free flow much more easily when looking down (maximum depth differential between diaphragm and exhaust valve) and would free flow more easily with a strong inhalation, due to more sensitivity to venturi effect. At greater depths, the venturi effect is more pronounced because the air flowing through the reg is more dense.

But, I have had 108s that breathed very well. I just tend to not use them because I like adjustable 2nd stages, particularly as alternates where I can tighten them down when they're just hanging around my neck. (I always use bungeed alternates on a short hose)

As soon as the pool opens I’m gonna test it again and if it free flows I’ll tighten it up and I’m planning to go to Key Largo and will test it in the open ocean and do a subjective test see how it feels at depth.

I know the two have different valve designs but it’s interesting one of the reasons to balance the second stage is to use a lighter spring yet the 108 HP uses the same spring as the BA.

I measured the 108 from lever point of contact and the exhaust valve is approximately 1 inch, and the 156 is 5/8 in the downward position so I guess theoretically the 108 would free flow.

 

[Angelo Farina]

After tuning for the same cracking effort, I find no difference between a 108, a 109, a 129 and a 156.
The only big advantage of 156 is that it seems to require servicing at longer intervals. This is possibly due to the lighter spring, so the seat does not degrade so much.
Balanced vs unbalanced has little effect with a balanced first stage. I never owned an unbalanced first stage, so I cannot testify if in that case the balanced second has benefits.
I own a total of 8 second stages, only one is a 156. And only this year I converted all my 109 to 156, they were working substantially the same with the unbalanced poppet.
Last point is the knob. Usually I tune the reg to work perfect with the knob all out, so you typically do not use it all. However, the knob becomes tremendously useful when, after many years without servicing, the reg starts free-flowing slightly. Then you close the knob by half a turn, and you stop this subtle hiss (and you save the dive).
Without the knob, you are in trouble if the reg starts hissing...
So in the end my favourite remains the 109 (or the 129, when you want the hose on the left). Better chrome finish, and better solid knob than the 156. You can convert internally to 156 if you want the second stage balanced.
My oldest 109 was manufactured in 1976 and still works perfectly. I do not think than any modern reg will work so well after almost 45 years...

 

[Open Ocean Diver]

[QUOTE="The only big advantage of 156 is that it seems to require servicing at longer intervals. This is possibly due to the lighter spring, so the seat does not degrade so much.

I thought the108 used the same spring as the BA, wouldn’t that make it equally as far as longer service intervals?

Do you know how to decode the date codes to figure year manufactured?

 

[halocline]

[QUOTE="The only big advantage of 156 is that it seems to require servicing at longer intervals. This is possibly due to the lighter spring, so the seat does not degrade so much.

I thought the108 used the same spring as the BA, wouldn’t that make it equally as far as longer service intervals?

You do understand that the spring is used very differently in the two designs, don't you? In the downstream valve, it's quite compressed and therefore exerts a lot of force on the soft seat when un-pressurized. This is what causes the wear. In the barrel poppet balanced regs, the spring is much more relaxed and exerts much less force on the seat when the regulator is un-pressurized.

 

[Open Ocean Diver]

You do understand that the spring is used very differently in the two designs, don't you? In the downstream valve, it's quite compressed and therefore exerts a lot of force on the soft seat when un-pressurized. This is what causes the wear. In the barrel poppet balanced regs, the spring is much more relaxed and exerts much less force on the seat when the regulator is un-pressurized.

Thanks for pointing out the obvious. They’re both under tension, what do you mean? “quite compressed” have you been able to measure the tension? They are assemble in different ways with the downstream you have to compress the spring manually to get the nut on with the barrel you use the knob to compress the spring.

 

[halocline]

Thanks for pointing out the obvious. They’re both under tension, what do you mean? “quite compressed” have you been able to measure the tension? They are assemble in different ways with the downstream you have to compress the spring manually to get the nut on with the barrel you use the knob to compress the spring.

If you wanted to quantify the difference in force that the spring is under in the two regs, I guess you could measure the length of the space that the spring occupies in each reg, then take a spring and compress it to each of those lengths while using some sort of scale to measure it. Maybe there's a complicated equation to determine the ratio of force to compression for springs, I really don't know. But what I do know is that the more you compress a spring, the more force it applies in the direction of expansion. I don't know, and don't really care, if it's a linear relationship or what the actual amounts are.

I'm sorry if I pointed out the obvious to you, but you did make a comment that suggested you did not understand the way in which these regs wear or the forces that the seat is under in each case. On the 156, because there's a hole in the seat connected to a balance chamber, there is air pressure pushing the seat against the orifice when the reg is pressurized. That's what makes it balanced. (Again, sorry if I'm pointing out the obvious, but this is the answer as to why the regs don't wear equally) So you have a combination of air pressure and the light spring pressure pushing the seat against the orifice. Obviously, that pushing force is far lower when the reg is unpressurized. Hence, less seat wear.

On the 108, and basically all unbalanced 2nd stages, the mechanical spring provides all the force keeping the seat against the orifice, and that force is constant regardless of whether the reg is pressurized or not. So there's a far higher force on the seat while the reg is in storage, which likely constitutes 95% of it's life.