SCUBAPRO Spring Dimensions

[NotSure]

After my recent experience with a Scubapro second stage I received that had the wrong spring installed couv suggested that I post some of the dimensions that I believe are the correct ones for a 156 Balanced Adjustable second. I decided a dedicated spring thread were others can post their findings might be easier to find for future reference. It would also be nice if we could find a DIY method of measuring the spring rate also known as the spring constant for these springs. The spring constant (k) is found using the formula k=f/x were f is the force applied to the spring and x is the displacement of the spring. This constant is the only sure way to know if you have a spring that will perform as the factory intended it. I am thinking a postal scale and some method to measure the displacement might work fine.

The photos to follow show the dimensions I got for a used spring from a 156, a used 109 spring and a new G250 spring. As one would expect the 156 and G250 springs are almost identical as they should be. If you have issues with lever height it might be a good idea to check your spring length before you start searching for a new lever, just to make sure it's close to factory spec.

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156 Spring Dimensions







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109 Spring Dimensions







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G250 Spring Dimensions

 

[halocline]

I'm pretty certain that the G250 and R195 et al use the same spring, part no. 01-020-216. It shouldn't be too difficult to find a SS spring at mcmaster carr or someplace similar that has the same dimensions and just try it out. A while back somebody did this sort of thing for the mainsprings on USD double hose regs and the new spring really helped with IP stabilization.

I've found replacing this spring on G250s and balanced/adjustables can help get more consistent tuning. I think the springs simply wear out over time and lose some resiliency. It's a little odd that your used 156 spring is a bit longer than a new G250 spring, as I suspect they are exactly the same spring and you'd expect that the older one would have compressed over time. I guess there's got to be a certain amount of variance or tolerance, and since you can simply adjust the orifice to the particular spring, it shouldn't matter much.

Anyhow, excellent project, I'll try to measure one of mine in the next couple of days. If you decide to order some springs of the same dimension from someplace, I'd be happy to buy a few and try them out. I've never had a problem buying these from a SP dealer, but it's always good to have an alternative, and there are plenty of DIYers without access to an understanding dealer.

 

[NotSure]

I tried two different methods to determine the spring rate for a G250 spring and got similar results using both methods. The first method used a c-clamp and a digital postal scale and the second method used a vertical rod pushing on the spring and scuba weights to apply force. I found that the spring rate is variable for these springs over a very small displacement. I should have expected this because they are relatively short stiff springs with only a few coils. This means that though my results might be helpful to find a similar spring as halocline suggests, I find it unlikely that any off the shelf spring from an alternate source will have the appropriate dimensions and spring rate within the very small range that is important for optimal performance. If the spring is too short, the lever geometry will be wrong and if the spring is to long the preload spring force may be too high for orifice adjustment within the range for proper lever height. If for some reason these springs became unavailable, I do believe it would be easy to get custom replacement springs made at a reasonable price.

That really wasn’t my reason for testing spring force though. I thought that a simple method might be determined to quickly check a spring to see if it still is functioning as it did when new. Determining the range of spring force that is critical for optimal performance is required if an easy test is to be found. If for example you could determine the optimal spring rate at a known spring displacement, all you would need to do is apply the resulting optimal force and measure the displacement of the spring to test it.. It sound easy, but I think the forces involved are on a scale not so easily measured with DIY tools.

If you use the orifice diameter and assume an intermediate pressure of 130 psi you get a minimum force required to hold the seat closed of about 3.31 lbs. This would be the minimum preload spring force for a 109. For a Balanced Adjustable you need to subtract the balancing force which I believe would be based on the inside cross sectional area of the balance chamber. For a rough calculation I just used the stem diameter and ignored the o-ring friction and got a balancing force of about 2.39 lbs. This results in a minimum preload spring force of 0.92 lbs for the BA at 130 psi IP. I’m sure I ignored other important factors, but just wanted to get a rough idea of the range of forces and spring displacements we are looking at. Though the minimum spring force gives us an idea of the scale of forces we are dealing with, it’s the closing force that is more important as it requires enough force to close the valve against aerodynamic forces of air flowing through orifice.

Notsure about any of this though, but it’s just too hot to go outside and do something constructive.

Orifice Diameter


Stem Diameter

 

[halocline]

I'd be surprised if you couldn't find an off-the-shelf spring that works well. I doubt that SP had these springs made to order, maybe I'm wrong, but I would think they just sourced them.

 

[NotSure]

I'd be surprised if you couldn't find an off-the-shelf spring that works well. I doubt that SP had these springs made to order, maybe I'm wrong, but I would think they just sourced them.

I'm not seeing any stainless springs close to the dimensions and spring load I need, do you think zinc plated music wire would be acceptable. I was going to order some o-rings anyway, thought I might get a few springs to try just for fun.

 

[halocline]

Just glancing through the mcmaster carr website I found one, part # 1986K84. It's 1" long, .313" in diameter, and .035 wire thickness. I'm not sure how you would compare your loading measurements to it's listed rating, but I think it's worth a look. It has closed ends, not ground flat, so you could fairly easily grind each end a little flat and end up shaving the few hundredths of an inch off the length.

Or do you think that the size is too far off? Maybe it's too skinny to accept the spring pad and fit over the poppet, it's about .020" skinnier. My guess is it's worth a try if you're ordering other things as well, it's $4.62 for a package of six. That's about 1/6 the price I pay at a dive shop.

 

[NotSure]

One possible problem I see with this spring is the inside diameter. If the outside is 0.313" and you subtract the wire thickness twice you get an inside diameter of 0.243". I measure the diameter of the balance chamber at 0.247". It might fit but it would be tight. Another possible problem with the wire size is that it is slightly thicker and would make the spring stiffer with all other things being equal. I believe if the spring is longer than stock you would want that spring to be softer if anything to offset the additonal preload force you would get with a longer spring. I would like to measure one of their 1" springs, maybe the Scubapro spring is a standard 1" spring and the closed and ground flat springs just lose some length when they are made.

Balance Chamber O.D.

 

[chrisdoc]

In addition to the dimensions you would need to determine the spring rate, (force applied/distance shortened). The idea of a postal scale or some sort of digital culinary scale might work to determine the spring constant for 2nd stage springs. Take the unloaded spring length, 1" for example, cut a small dowel or a toothpick at 3/4", weigh the spring and dowel to determine the tare,then slide the dowel inside the spring (it shouldn't bind). Stand the spring on end on the scale and measure the force needed to depress the spring to just contact the dowel end. This should give you the force to shorten the spring 1/4", x4 would give you the force to shorten 1". I haven't done any measurements yet but I was looking at this site as a possible source for mk10 spring replacement.
http://www.leespring.com/int_learn_compression.asp

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[Six2Life]

Last night I tore into the one of my 3 G250's that just didn't breathe very well compared to the others. First I replaced the brass orifice that was a bit nicked up with a new one. No difference. I was about to put in a new seat when I remembered this thread and decided to check the spring. It was slightly over 1" long, like maybe 1.008" IIRC. It did not have the ground flat edges like all of the new springs I have seen so maybe someone along the way replaced it with one of the off-the-shelf springs you guys are talking about. I installed a NOS G250 spring that measured right at .930". The difference in breathing smoothness and cracking was night and day. It is 100% better than before. Maybe if you ground .07" off the ends of one of the off-the-shelf springs it would perform well?

As a side note, I have observed some differences in orifice diameters in various 109's. The older chromed orifices with the skinny screwdriver slot I measured at about .175". The newer chromed orifices with the wide slot and all the versions after I've measured at .195" This seems like it would make a pretty big difference in the downstream bias of the reg.

 

[halocline]

I've often found that replacing the spring helps G250s and B/As work better. Actually, the spring should really be part of the rebuild kits. What are the chances of that happening; SP is now too cheap to include a plastic balance chamber in the $25 kit.