Range Report JBM Range Card

[Bigwheels]

I was printing off a range card the other day, & noticed that the card gave a higher correction for a higher temp. ie. given the same 1000' elevation with temps of 40/70/90 deg. the correction went from 23.9 @ 40* UP to 24.3 @ 90*. This seemed backwards, so I did the same with their trajectory calculator, using a given pressure, & changing only the temp, & it showed a lower(flatter) trajectory. When I contacted them about it they replied that, the card was right, as the temp goes up with a given density the drag , & mach also goes up. They continued to say that it is unusual to see a density stay the same with an increasing temp. I agree with this as everything I've read tells me that air density goes down with a higher temp. Can someone please explain?

 

[Rob01]

Re: JBM Range Card

I don't use their range cards for that reason. I make my own using their standard calculators, drop it into Excel and then delete everything but elevation, windage and mover, which I put in the first three rows. Easy to do.

 

[DIBBS]

Re: JBM Range Card

Wheels- as far as your original question, this is what I came up with: I cant explain it though

Air density decreases with increasing altitude, as does air pressure. It also changes with variances in temperature or humidity. The addition of water vapor to air (making the air humid) reduces the density of the air, which may at first appear contrary to logic.This occurs because the molecular mass of water (18 g/mol) is less than the molecular mass of dry air (around 29 g/mol).

I've also been experimenting with JBM the past few days, and with the help of some old posts was finally able to get output in a useable form. I cut and pasted and deleted the columns I didn't want as Rob has done. However, one of my scopes .1 mil elevation values are ~0.42" instead of 0.36". I know there's a simple way to correct my initial mil values, but it's been 30 years since my last math class, and I cant seem to come up with a formula.

What I have in mind though, is something like
2.8 mills/28clicks)is to .36" as x mils/clicks is to .42". Am I on the right track, ie my mil adjustments for elevation need to match the adjustment value on the scope? I better go back and <span style="color: #FF0000"><span style="font-weight: bold">re-read</span></span> Lindy's article on optically checking a scope. MAYBE it will sink in. Any mathmeticians that can help?

 

[tigerhawk]

Re: JBM Range Card

Dibbs, 28 clicks at .36"/click will be 28x0.36/0.42 clicks at .42"/click, or 28x6/7. To convert any number of clicks at 0.36" to the number of clicks at 0.42", just multiply by 6/7, or by 0.857.

 

[jbmillard]

Re: JBM Range Card

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Bigwheels</div><div class="ubbcode-body">I was printing off a range card the other day, & noticed that the card gave a higher correction for a higher temp. ie. given the same 1000' elevation with temps of 40/70/90 deg. the correction went from 23.9 @ 40* UP to 24.3 @ 90*. This seemed backwards, so I did the same with their trajectory calculator, using a given pressure, & changing only the temp, & it showed a lower(flatter) trajectory. When I contacted them about it they replied that, the card was right, as the temp goes up with a given density the drag , & mach also goes up. They continued to say that it is unusual to see a density stay the same with an increasing temp. I agree with this as everything I've read tells me that air density goes down with a higher temp. Can someone please explain? </div></div>

As I said before, the density altitudes uniquely define the density. Yes density depends on temperature, but it's in the density altitude. Any instrument that measures the density altitude would temperature correct it.

The temperature is required so that the range card can correct for changes in muzzle velocity.

So if you run a range card with a density altitude of 0' and three temperatures of say 50, 70 and 90 degrees, you're asking the program to calculate a trajectory at the standard density at sea level and three different temperatures, all with the same density.

Brad

 

[Bigwheels]

Re: JBM Range Card

OK. I think I get it now. The program uses DA & the DA stays the same when the temp changes. So If I got this right the program sort of automatically changes the pressure to account for the change in temp so the DA will stay the same. That sort of explains it. I think.
Also just so you know I'm not bashing the product, just not fully understanding how the outputs corresponds to the inputs.

 

[Lindy]

Re: JBM Range Card

The temperature parameters in that card exist for only one purpose: to compensate for the change in muzzle velocity with temperature.

If you don't know how much your muzzle velocity changes with temperature, you might want to use a figure of 1 fps/degree, i.e., if the temperature goes down 20 degrees F., then your muzzle velocity drops 20 fps.

You might find the explanation here useful: Making Ballistic Cards Using Density Altitude

 

[Bigwheels]

Re: JBM Range Card

Thanks Lindy. You always have the right stuff.

 

[jbmillard]

Re: JBM Range Card

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body">The temperature parameters in that card exist for only one purpose: to compensate for the change in muzzle velocity with temperature.
</div></div>

Two purposes actually. One as stated above and the other to find the speed of sound. The mach number of the bullet (speed divided by the speed of sound) is used to find the value of the drag function at the bullet's speed. This is what is causing the issues the OP was seeing since he didn't have any velocity correction (same velocities for high and low).

Brad

 

[Lindy]

Re: JBM Range Card

Thanks, Brad!

Added: And that points out a limitation in accuracy: The temperature, as noted by Brad, is a partial determinant of the drag function - yet there are many temperatures which, combined with the pressure and humidity, will yield the same density altitude. The effects across most realistic temperature spreads should be small, though.