Good Question Don
For those that might be interested in the DA/Density Altitude variations that Gene and Wilbur might have faced during their tuner experimentations last week...............the Weather Underground at the Odessa Municipal airport, for 2-06-08 Wednesday, the first day of tuner testing reporting, showed a variation of 1412 DA at 9am to 3754 DA at 6pm, for a daily swing of 2342 DA.
I think in previous postings Gene has indicated that his tuner required changes at 500 DA increments to maintain optimal accuracy, so, 4-5 changes should have been required during a full days testings, quite alot of variation.
I am not sure that Genes closed tunnel would see the same DA readings as the open air readings at the airport...............would be interesting to hear what the actual tunnel readings were for that day.............Don
Don, you bring up a good question and I thought I should address it because others have asked the same thing.
When I first started experimenting with tuners in the tunnel, I had some question about where to take the DA reading; outdoors, in the tunnel itself, the expansion room, the firing room, where? The DA readings taken outside in the parking lot were always somewhat higher than those taken in the firing room and expansion chamber. This makes sense because densitity altitude is pressure altitude corrected for temperature and we do not experience the same temperature variations indoors as out. It was decided that the DA reading should be taken in the same room, near the rifle.
Outdoors on a typical day here in West Texas, temperatures usually vary about 20 to 25 degrees from early morning to the warmest part of the afternoon. This results in about 2000 to 2500 change in DA which is very close to what you found when you checked the weather service records. BTW, that was a good piece of detective work on your part.
The smallest change in DA for which I will make an adjustment is 250 feet. When tuning with my tuner, this equates to 1/8 turn.
Expressed in velocity, the nodes appear at 120 fps intervals.
Expressed in density altitude, 2000 feet intervals.
With the Beggs tuner, one revolution.
Armed with this information, it soon becomes obvious that regardless of the load, DA or tuner setting, you can never be more than a half turn out of tune when you go to the firing line. Rather than worrying about what the correct tuner setting should be based on density altitude, formulas, etc., why not just ignore DA altogether and fine tune the rifle each time you go to the line? That's the easy, foolproof way and there is plenty of time.
With my tuner, you can remove the bolt from the rifle, slide the rifle back off the rest and place it on the bench beside the rear bag. This gives easy access to the tuner, which is loosened with two small three inch tommy bars. The reference mark on the tuner is reindexed to the desired setting, tightened and you're ready to shoot again. With a minimum amount of practice this can all be accomplished in about thirty seconds.
Hope this sheds new light on the situation. BTW, is Don, Don in Redondo and Don Jackson one and the same?
Later,
Gene Beggs
