IP drop as performance indicator

[halocline]

On my MK15, which I just rebuilt for the 1st time, the IP drops a good 15-20 psi on purge or a quick deep breath. It also did this before the rebuild. My MK5 drops 10 PSI max on full purge, and my MK2 drops 10-12. I tested all the 1sts with the same 2nd, a well tuned balanced/adjustable.

The MK15 IP is solid at 125 PSI, although by sipping a little air occasionally I can get it to stop at 120-122 and creep to 125. I read that the slight creep was indicative of a new seat, and would probably go away as the seat breaks in. The MK5 and MK2 IP is 130.

The question is, does the larger IP drop indicate that something is wrong with the MK15, or is the extra 5-10 psi drop just part of the newer design? Could it indicate higher flow? I read in the Harlow book that less of an IP drop was an indicator of higher performance. Any thoughts?

Thanks!

 

[Luis H]

Well, first impression, it does seem that the Mk-5 is higher performance than the Mk-15, but that seems a bit strange.
I am familiar with the Mk-5 and your data for it is very reasonable. I can’t comment on the MK-15. But, in either case the max flow would be to some degree restricted by the second stage.

The actual flow on the second would be affected by the actual IP so your MK-15 is not keeping up as well as the MK-5.

Have you checked that the Mk-15 filter is in good shape? You can try a different one or for testing purpose try it with no filter.

Were you using similar tank pressure? A balanced first stage static IP is not affected by tank pressure (for the most part), but the tank pressure will affect flow rate and pressure drop while air is flowing.

Something seems to be restricting the flow and it could be something simple. Was the tank valve open the same amount in all cases.

This may seem kind of basic things to check, but it has happen to me that I was doing all kinds of testing on a regulator and after a while the IP was just low. Well, I just lost track of the tank pressure and it was below what the IP was set to.

In any case I would adjust the IP a bit higher. I like around 140 psi in all my regulators. I am not sure about the MK-15 again, but the MK-5 you can change the seat to the thinner one to get a higher IP (and / or add spacers under the spring). I suspect the MK-15 is the same.

 

[redacted]

There may be more variables that need to be controlled to make a meaningful measure of IP drop. I suspect you want a gauge connected inline with the 2nd stage inlet to control any venturi effect variable on the gauge. You might also need a gauge that records the continuous reading rather than the eyeballing of a very transient needle. (And look at the total area under the curve rather than the max drop.) But the dirty filter idea is interesting. I may have to play with that one a bit. I have noticed that IP drop appears to be greater when testing with lower tank pressure - which seems to make sense if input pressure is a regulating parameter for some range of tank pressures. Plus I suspect that variables like filter cleanliness and tank pressure would interact.

If you look at a WOB record, there are fairly large jumps which should be reflected in the dynamic IP reading.

I don't think I would worry too much about the difference between a 10 and a 20 psi drop unless there were some other indication of a problem.

 

[halocline]

I have wondered about the accuracy of the test set up. It's the peterbuilt IP guage on a LP inflator hose. I did use the same tank, same 2nd, same LP hose, for both. The filter in the MK15 is new. The tank pressure is around 2000.

Interestingly, I went to oceanic's site, and their BP 1st lists a higher IP drop than their NBP 1st, as if the greater drop were an indication of higher performance.

I'm going to add another shim in the MK15 to bring the IP up to 130, see if that does anything. There is another thought that I had, which was maybe the HP section of the MK15 is a different volume than the MK5, which might explain a slightly longer reaction time for the IP lockup. That's just a guess. In any event, it's not something I'm worried about because the performance of the MK15 has always been really high. It's more of a curiosity.

 

[DA Aquamaster]

The Mk 15 has more in common with the Mk 5 than the Mk 10 as they went back to the larger piston head diameter in the Mk 15 - which theoretically is more responsive.

The flow rate is significantly higher for the Mk 15, due to larger air passages so if you control for everything else (valve, tank pressure, second stage, measurment erro, etc), the IP drop should be less.

I agree with boosting the IP to the 140 range if possible, but there ie enough performance surplus wuth the Mk 15 that it is no critical.

The Mk 15 and Mk 5 are not perfectly balanced as the area of the knife edge itself is not accommodated as the piston stem is constant in diameter so you will probably see a 4 to 6 psi difference in IP between 3300 and 300 psi.

The Mk 2 is not balanced so a 20 psi drop in IP between 3300 and 300 psi s normal. This also means however that it is vital to set the IP as close to 140 to 145 psi as possible at 3000 psi so that you will still have an IP of at least 120 psi at 300 psi.

 

[halocline]

I added 2 shims (I had to use Mk20 shims which are part #01-060-219, the MK15 shims are 218) and brought the IP to almost 135 at 2000 PSI. Although the IP does lock up well, there definitely is a larger needle swing with the MK15 than with either the MK5 or the MK2. This is using the peterbuilt tech-diver IP guage, same tank, 2nd stage, etc. I plugged all HP ports to minimize any variables. It's visible on purge, lightly tapping the purge, sipping air, etc. I have no idea if this signifies anything, it's just not what I expected.

Thanks for the replies!

 

[Luis H]

I know you mention that you are using a new filter. I would try it without any filter just for a moment just for testing purpose. I am assuming you have a good, clean, trusted air source.

 

[halocline]

Luis, I tried it without the filter, it may have made a slightly perceptable difference but not enough to say that the filter was a problem.

Oh well, somewhat of a mystery. If anyone with SP balanced piston regs has any inherent interest in this, and has access to an IP guage, I'd be very curious to know IP drop for Mk5s, MK10s, MK15s, and MK20/25s, especially the latter. That would be IP drop with a normal breath, full purge, and just "tapping" the purge several times. You can get a pretty good idea of the average drop. If it turns out that the newer BP regs (w/bushings) have a higher drop than the older, simpler style, that would be very interesting to me.

Thanks to Luis, Awap, and DA for replying!

 

[halocline]

A small update; I figured that an interesting test would be IP drop under higher flow, so I tried two 2nd stages, both under full purge. The MK5 dropped almost 20psi, the MK15 dropped about that much for an instant, then leveled at 10-12PSI.

 

[PhilEllis]

Actually, I don't think that IP drop in the range you are talking about (20psi) is any indication of regulator performance, negative or positive. The IP pressure MUST drop. Otherwise, the regulator would not work.

Dynamic measurement of IP drop, inhallation effort, and flow rate will tell you something. But not IP drop by itself.

Phil Ellis