Is pressure increase linear?

[rarick123]

Apologies if this is the wrong forum...

If someone was to provide the pressure levels at two different measures of powder, would it be a standard linear progression between the low measurement and the high?

For example...

39.5 gr of powder X = 43,700 psi
44.0 gr of powder X = 60,500 psi

Is it safe to assume then that 41.75 gr (half way between 39.5 and 44) would be 52,100 psi (half way between 43.7k and 60.5k)?

 

[jlow]

A picture is worth a thousand words. This from Bill chevalier's "The ABC'S of Reloading". Be careful out there!

 

[Tommie]

Not in my experience.

 

[scott in ohio]

NO NO NO ???

 

[kelbro]

It can... or maybe it doesn't. Lots of 'depends' in the equation. Propellant volume to container volume percentage is one big factor.

Like Dave said above, in between two knowns is not scary. Getting outside those knowns is where it gets hairy.

 

[rarick123]

For what it's worth, I wasn't considering going above a known maximum value, just curious how close of an estimation it might get me in between a starting load and a maximum load. That graph is kind of what I was expecting... it's not quite logarithmic, but it's certainly not linear.

 

[barkingsnake]

Exponential with linear powder changes (looks like that from the picture).

There are a couple of good books on the subject. Vihtavuori's loading manuals are very good. I think the other might have been Vectan's (French firm, English text) which is also better than you might expect. I can't recollect the same level of detail from the likes of Speer or Hodgdon.

Basically, it's a controlled bomb but go too far and it'll do the bomb stunt very well.

I'd not try cutting the working up process by interpolating.

 

[Lapua40X]

... would it be a standard linear progression between the low measurement and the high?
...

No

 

[jlow]

With all due respect there were some serious liberties taken with that graph. The first three powder charges were every bit as linear as the velocity. It was the authors choice to draw it as a curve.

A common load development technique before pressure measurement systems were available to mere mortals was to measure velocity as a function of charge weight and when increasing the powder charge resulted in less velocity increase than the linear graph indicated one had reached "the point of diminishing returns". More powder, no more velocity. I will stand by my belief that interpolation of the two data points you currently have will be a close estimation of the pressure and velocity. Of course I'm kind'a new at this.

Dave,

The author did the correct thing. He basically used ALL the data and not pick points. This is what you have to do if you are going to analyze data. It's a bit like shooting for goups and not ignoring "flyers"... Besides, I am sure that this is just an example and the relationship is well known.

 

[TC260]

The author did the correct thing. He basically used ALL the data and not pick points. This is what you have to do if you are going to analyze data.

Yep. Looks like a normal regression line to me too given all the data points from 25-40 grs. If the range was 20-37 grs then the regression would be more linear. I'm not a mathematician but it seems to me both of yall are correct depending on the data range being analyzed.

 

[jlow]

Remember that is an exponential curve which means depending on where you are. it might go up BIG time in pressure for a small change in charge….

I don’t know about you fellas but when I am dealing with something that can blow up next to my face, I don’t make any assumptions unless I am 110% sure of my facts. Thread very carefully OP.

 

[Laurie]

I'd agree with the line taken in most posts - ie that it is often - but not always - linear. There are two or three factors I reckon should be taken into account.

1) the case-fill ratio, or how much of the available space in the case has been used. Most smokeless cartridges perform best at 80% or higher values, but as you approach then reach 100% and finally move into compressed loads, this can change the powder's behaviour.

2) smokeless powders only work efficiently within a certain pressure band. Go outside it and behaviours can change considerably. It's always said that when you hit or exceed a certain pressure level with some powders in some cartridges that increases the speed of burn and you get a feedback loop enter the equation. I don't know if that's true or not, but it seems to apply to pistol and revolver cartridges using very fast-burning powders where very small increases past a safe level cause a disproportionate pressure spike.

3) how well suited the powder is to the application. If it's optimal for the cartridge and bullet weight, it probably responds to charge weight changes more predictably than if it's marginal or unsuitable.

Years ago Sierra used to recommend load development steps equating to 1% of the recommended maximum charge weight as a good measure, so 0.4gn steps for a ~40gn ultimate load, 0.5gn for 50 and so on. I've generally found that a good dictum to follow, and as I approach what I think may be maximum safe / sensible charges (nowadays usually based on QuickLOAD estimations) I'll drop the step-size further, usually to 0.2gn.

 

[Beau]

The faster the powder, the faster the pressure rises with charge weight at maximum charges.

That is why you usually use slower powders with larger capacity cases/heavy bullets. For instance, if you use a fast powder in a 223 with 80gr bullets, you might go from a nice, brass friendly load to popping primers in only .3gr. Whereas, if you had used a slower powder like Varget, it may take .6gr to do that. The faster powder can seem to skip the intermediate excess pressure signs and go straight to cartridge failure.