Knowing that Gene B. , Bart, Billy Stevens and many other great shooters are pilots, why use DA instead of plain ole barometric pressure. The two are directly correlated and it would seem easier to obtain the barometric pressure than the density altitude. Also, the amount of air inside the bore of a typical 6 PPC is very miniscule and a change in density altitude (or barometric pressure) would change the characteristics of this column of air very little. Maybe that small change is enough to adversely affect the tune (dunno), but it seems unlkely to cause radical changes. I am all for experimentation and congratulate those like Gene Beggs who help us all by their efforts. Keep up the good work. James Mock
Why density altitude??
- Thread starter JD Mock
- Start date
JerrySharrett
Senile Member
James, Density Altitude is a direct measurement of atmospheric thickness. In the aviation application of DA is the direct determinate of how much weight an airplane can lift. Barometric pressure is just the pressure exerted on an object that is in its realm. BP is a component if DA but not the same thing.
DA, since it is used in race car engine performance tuning has created an availability of instruments to measure it. That availability of instruments led some rimfire shooters to relate DA to ammunition velocity. That is probably the origin of trying to use DA in centerfire.
I have been measuring DA for about 3 years. Like Bart and others I don't think DA is the sole determinant of accuracy tuning. I have a feeling DA is a component but not the determinant of when to click up or down. If DA is a component, it may not even be a linear determinate.
DA, since it is used in race car engine performance tuning has created an availability of instruments to measure it. That availability of instruments led some rimfire shooters to relate DA to ammunition velocity. That is probably the origin of trying to use DA in centerfire.
I have been measuring DA for about 3 years. Like Bart and others I don't think DA is the sole determinant of accuracy tuning. I have a feeling DA is a component but not the determinant of when to click up or down. If DA is a component, it may not even be a linear determinate.
Pete Wass
Well-known member
I realize that this is a - -
Plum and tangerine sort of comparison; me shooting 30 Cal HBR rifles but I think I notice that temp. swings have more to do with tune than any of the other things. I recently bought a Kestrel to do the checking thing but I think I know for sure that when the temp goes up, say 20*, tune changes. I too have found it difficult to believe that the ambient inside a barrel was somehow the culprit but what do I know?
I can tell you that once an HBR rifle is heated up, accuracy goes away and when the ambient gets above 80* my rifles don't shoot like they do at 70*. Of course living in one of the Refrigerators of the US, we don't get much 80* and damn little 90* weather to even test in.
Plum and tangerine sort of comparison; me shooting 30 Cal HBR rifles but I think I notice that temp. swings have more to do with tune than any of the other things. I recently bought a Kestrel to do the checking thing but I think I know for sure that when the temp goes up, say 20*, tune changes. I too have found it difficult to believe that the ambient inside a barrel was somehow the culprit but what do I know?
I can tell you that once an HBR rifle is heated up, accuracy goes away and when the ambient gets above 80* my rifles don't shoot like they do at 70*. Of course living in one of the Refrigerators of the US, we don't get much 80* and damn little 90* weather to even test in.
bryan
Bryan Armatys
Da
DA is determined by barometric pressure, temp, humidity, dew point.........
But temperature has the biggest influence. So temp alone could be used for most tuning changes. I'm at work and do not have the specs in front of me, but if memory serves, a .10 change in baro------28.70 to 28.60 for instance would change DA by about 93 feet. 5 degrees of temp changes it by about 200 ft. and the relative humidity is minimal. I think RH may have a more direct affect on certain powders, but not so much on DA itself.
Sure, race cars monitor DA as a main source of tuning data.........Top Fuel cars use 7 to 1 compression ratio till they come to Denver, where they switch to 8.5 or 9 to one CR to get the extra air they need.
One needs to only look at a hot air balloon to see the relationship.
I'm no authority, but barring a MAJOR change in baro, I will be watching and monitoring temperature as it relates to tuner position this year.
BA
DA is determined by barometric pressure, temp, humidity, dew point.........
But temperature has the biggest influence. So temp alone could be used for most tuning changes. I'm at work and do not have the specs in front of me, but if memory serves, a .10 change in baro------28.70 to 28.60 for instance would change DA by about 93 feet. 5 degrees of temp changes it by about 200 ft. and the relative humidity is minimal. I think RH may have a more direct affect on certain powders, but not so much on DA itself.
Sure, race cars monitor DA as a main source of tuning data.........Top Fuel cars use 7 to 1 compression ratio till they come to Denver, where they switch to 8.5 or 9 to one CR to get the extra air they need.
One needs to only look at a hot air balloon to see the relationship.
I'm no authority, but barring a MAJOR change in baro, I will be watching and monitoring temperature as it relates to tuner position this year.
BA
J
J. Pendergraft
Guest
Some very respected shooters are saying it can't be used to tune a rifle. I guess time will tell.
Because it was a long winter, and just having a *tuner* topic wouldn't be enough.
Gene Beggs
Active member
James, if barametric pressure alone determined air density, we could dispense with density altitude computations, but air density is also greatly affected by temperature. For example, here in my hometown of Odessa, Texas, we are 3000 feet above sea level; that never changes. If the altimeter setting is say, 30.02 and we set that in the window of the instrument, the hands will read 3000 feet. That would hold true whether the outside air temperature was 32 degrees or 110F, but air density would be far greater on the cold day. On that cold morning, density altitude might be as low as 1500 feet and the takeoff roll of an aircraft might be cut in half or less. On the day when temp is 110F density altitude could be as high as 7000 feet and takeoff roll might be tripled. You may be wondering what flying has to do with shooting, but trust me, the two have a lot in common.
James, you and many others as well, question whether the column of air inside the bore of a typical 6ppc could affect internal ballistics in such a way as to alter bullet exit timing. I assure you it can and does. It is not the 'weight' of the air in the barrel itself that we are concerned with but rather the 'weight' of the atmosphere against which the bullet must push in order to exit the muzzle. The bore is a long, tightly sealed cylinder; the bullet is a piston that accelerates from zero to around mach 2.7 in the length of the barrel. The air trapped in the barrel cannot escape as fast as the bullet is accelerating and is compressed against the weight of the atmosphere. The greater the air density, the greater the resistance.
Hope this helps shed some light on what is going on.
Gene Beggs
Butch Lambert
Active member
Gene,
As fast as some shoot won't the barrel have hot dry air in it?
Butch
As fast as some shoot won't the barrel have hot dry air in it?
Butch
Gene Beggs
Active member
Yeah, I guess so Butch, good question. As fast as Ratigan shoots, I'll bet his barrel has a vacuum in it between shots; huh?
Butch Lambert
Active member
Bryan,
I thought DA was the subject. Temp. is a big factor and it ain't gonna get back the same right now.
Butch
I thought DA was the subject. Temp. is a big factor and it ain't gonna get back the same right now.
Butch
Let me clarify....
After I read my own post on the subject, let me apologize to Gene Beggs. I don't use a tuner on my barrels, but that does not mean that I don't believe that they work. Gene has spent many hours and many dollars experimenting for the sake of improving the game that we play.
When I saw a tuner on Charles Huckeba's rifle at Raton (NBRSA Nationals), I knew that they worked. Charles is a VERY serious competitor and he proceded to win the 3-gun and 4-gun at that match. If Charles has a tuner on his rifle .......just let me say that it is not there for decoration.
Charles is a frequent visitor to Gene's tunnel and he and Gene try things until they prove to themselves that they either work or they don't. If Charles uses a tuner during a match, you can bet your sweet bippy that it works.
As Forrest Gump stated...."That's all that I have to say about that".
James Mock
After I read my own post on the subject, let me apologize to Gene Beggs. I don't use a tuner on my barrels, but that does not mean that I don't believe that they work. Gene has spent many hours and many dollars experimenting for the sake of improving the game that we play.
When I saw a tuner on Charles Huckeba's rifle at Raton (NBRSA Nationals), I knew that they worked. Charles is a VERY serious competitor and he proceded to win the 3-gun and 4-gun at that match. If Charles has a tuner on his rifle .......just let me say that it is not there for decoration.
Charles is a frequent visitor to Gene's tunnel and he and Gene try things until they prove to themselves that they either work or they don't. If Charles uses a tuner during a match, you can bet your sweet bippy that it works.
As Forrest Gump stated...."That's all that I have to say about that".
James Mock