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In a recent thread http://rugerforum.net/optics/82255-lightweight-scope-10-22-a-3.html , there was a discussion about changing the parallax adjustment on a high power rifle scope to make it better suited for a rimfire ... changing from a typical corrected distance of 100 yards to a more practical distance of 50 yards. I decided to try the suggested procedure with one of my own scopes and found it works but ... it raised a couple questions ... especially about loosing the nitrogen in the scope when removing the lock collar on the front lens.
After doing some research and talking to the experts at Burris, I found out some valuable information. First, nitrogen is used to purge the normal air (which contains moisture) out of a scope and replace it with "dry" nitrogen. This procedure is done to prevent moisture from condensing inside the scope (fogging up), and to prevent oxidation of the scopes internal components ... namely when dissimilar metals such as aluminum and brass are in contact with each other, they tend to create galvanic action (corrosion) and over time will literally destroy the integrity of the scope from the inside. Further, nitrogen makes the "O" rings used to seal the scope tube last much longer.
Most scope manufactures purge the ambient air out of a scope during the manufacturing process .... good enough where the scope will at least be fog free when the customer buys it. The better quality scopes are purged with nitrogen then filled with nitrogen and sealed by "O" rings on each turret screw, plus the front and rear lens housings and any other potential leak areas such as a side dial or turret bases.
What makes a non-nitrogen filled scope draw moisture? It's a change in barometric pressure caused by changing climate conditions or a change in altitude. As the pressure outside the scope is increased (higher barametric pressure or lower elevation), the scope will literally suck outside air into the tube. Further, if barametric pressure is low or you are at a high altitude, the pressure inside the scope increases and will allow dry air to leak out. As a scope goes through several barametric pressure cycles, the dry air inside the tube will eventually be displaced by moist air, unless the tube is sealed very well. When temperature drops below the dew point, moisture will condense on the lenses and other parts inside the scope tube.
There are at least two different procedures for "nitrogen filling" ... the cheapest, easiest, and most common with cheaper scopes is to install a port ... much like a tire stem valve where nitrogen is pumped into the scope tube and allowed to vent through a turret screw hole. Once the scope has been purged, the turret screw is installed and sealed by an "O" ring, then the scope is pressurized slightly with more nitrogen. This is a very effective and probably the best way to fill a scope with nitrogen, however after the scope has been filled, the valve has to be sealed so this process is a "one time deal" and can not be used again.
Burris feels so strongly about this nitrogen filling concept that they nitrogen purge each scope 24 times inside a vacuum chamber then fill it with pure nitrogen and seal it. They also use double "O" rings at each potential place where the scope could leak. They claim their process is the best of all scope manufacturers. Of course I had to ask "why the extreme measures?" The answer was ... "if you ever had an expensive scope fog up on a hunt you would understand. Plus ... Burris scopes have a lifetime gurantee so if moisture was allowed to get inside, they will not last as long." I was also told ... Burris' main competitor (Leupold) only uses single "O" rings and doesn't go through near as much effort to purge their scopes.
My primary question to Burris was "What did I do to my Japanese scope by removing the front lens collar?" Answer: "At a minimum, some of the nitrogen leaked out and was displaced by normal air. The amount of moisture inside the scope will depend on how long you left it unsealed, the barometric pressure, and how high the humidity was at the time. It did not instantly ruin the scope but it is now more subject to fogging and its extended life has been shortened. Some scope manufacturers will repurge and fill their own brand of scopes for a fee. Burris will not do this on any brand other than their own."
"What about bagging the scope with Silgel to absorb moisture with the collar removed?" Answer: "Silgel will absorb some moisture but unless you have a way to purge the air out of the scope, it would not be effective enough to matter."
"How about testing the scope in a refridgerator?" Answer: "Yes, this will work but if your scope does fog up, it would be very difficult to remove the water droplets and may make the scope unuseable.
Last, I had to ask about changing the parallax adjustment and got this for a reply ... "Burris does not recommend this procedure for obvious reasons. The lenses in our scopes are designed for an approximate parallax correction at a given distance then precision adjusted for the corrected distance as noted in our advertised specifications. The process for precision adjustment is basically the same as you noted, however there are issues involved with changing the adjustment radically without changing the lenses. Yes, it will work but because parallax is much more critical at closer distances, if you radically change the distance for correction, you lose focus at longer distances. Burris strongly recommends buying a rimfire scope instead of trying to modify parallax correction for a high power rifle scope."
I realize some of the information I got from Burris was "company line" but they did make some very valid points ... especially about nitrogen. I don't believe I will modify any more scopes based on this information. YMMV.
After doing some research and talking to the experts at Burris, I found out some valuable information. First, nitrogen is used to purge the normal air (which contains moisture) out of a scope and replace it with "dry" nitrogen. This procedure is done to prevent moisture from condensing inside the scope (fogging up), and to prevent oxidation of the scopes internal components ... namely when dissimilar metals such as aluminum and brass are in contact with each other, they tend to create galvanic action (corrosion) and over time will literally destroy the integrity of the scope from the inside. Further, nitrogen makes the "O" rings used to seal the scope tube last much longer.
Most scope manufactures purge the ambient air out of a scope during the manufacturing process .... good enough where the scope will at least be fog free when the customer buys it. The better quality scopes are purged with nitrogen then filled with nitrogen and sealed by "O" rings on each turret screw, plus the front and rear lens housings and any other potential leak areas such as a side dial or turret bases.
What makes a non-nitrogen filled scope draw moisture? It's a change in barometric pressure caused by changing climate conditions or a change in altitude. As the pressure outside the scope is increased (higher barametric pressure or lower elevation), the scope will literally suck outside air into the tube. Further, if barametric pressure is low or you are at a high altitude, the pressure inside the scope increases and will allow dry air to leak out. As a scope goes through several barametric pressure cycles, the dry air inside the tube will eventually be displaced by moist air, unless the tube is sealed very well. When temperature drops below the dew point, moisture will condense on the lenses and other parts inside the scope tube.
There are at least two different procedures for "nitrogen filling" ... the cheapest, easiest, and most common with cheaper scopes is to install a port ... much like a tire stem valve where nitrogen is pumped into the scope tube and allowed to vent through a turret screw hole. Once the scope has been purged, the turret screw is installed and sealed by an "O" ring, then the scope is pressurized slightly with more nitrogen. This is a very effective and probably the best way to fill a scope with nitrogen, however after the scope has been filled, the valve has to be sealed so this process is a "one time deal" and can not be used again.
Burris feels so strongly about this nitrogen filling concept that they nitrogen purge each scope 24 times inside a vacuum chamber then fill it with pure nitrogen and seal it. They also use double "O" rings at each potential place where the scope could leak. They claim their process is the best of all scope manufacturers. Of course I had to ask "why the extreme measures?" The answer was ... "if you ever had an expensive scope fog up on a hunt you would understand. Plus ... Burris scopes have a lifetime gurantee so if moisture was allowed to get inside, they will not last as long." I was also told ... Burris' main competitor (Leupold) only uses single "O" rings and doesn't go through near as much effort to purge their scopes.
My primary question to Burris was "What did I do to my Japanese scope by removing the front lens collar?" Answer: "At a minimum, some of the nitrogen leaked out and was displaced by normal air. The amount of moisture inside the scope will depend on how long you left it unsealed, the barometric pressure, and how high the humidity was at the time. It did not instantly ruin the scope but it is now more subject to fogging and its extended life has been shortened. Some scope manufacturers will repurge and fill their own brand of scopes for a fee. Burris will not do this on any brand other than their own."
"What about bagging the scope with Silgel to absorb moisture with the collar removed?" Answer: "Silgel will absorb some moisture but unless you have a way to purge the air out of the scope, it would not be effective enough to matter."
"How about testing the scope in a refridgerator?" Answer: "Yes, this will work but if your scope does fog up, it would be very difficult to remove the water droplets and may make the scope unuseable.
Last, I had to ask about changing the parallax adjustment and got this for a reply ... "Burris does not recommend this procedure for obvious reasons. The lenses in our scopes are designed for an approximate parallax correction at a given distance then precision adjusted for the corrected distance as noted in our advertised specifications. The process for precision adjustment is basically the same as you noted, however there are issues involved with changing the adjustment radically without changing the lenses. Yes, it will work but because parallax is much more critical at closer distances, if you radically change the distance for correction, you lose focus at longer distances. Burris strongly recommends buying a rimfire scope instead of trying to modify parallax correction for a high power rifle scope."
I realize some of the information I got from Burris was "company line" but they did make some very valid points ... especially about nitrogen. I don't believe I will modify any more scopes based on this information. YMMV.