I have a 8' rod bubble and is very sensitive. I ordered a new prism rod and was told it was a 20' bubble. Then I find out it was a 40' bubble. If I do the math correctly, the sine of 20' x height of prism (around 5') is around 0.05'. Just curious, what is the sensitivity of your rod bubble? And, what is the proper way to express the specifications of the bubble in accuracy?
I use an 8' rod bubble.
> ...the sine of 20' x height of prism (around 5') is around 0.05'....
It's more like 0.03'. I dont think that it matters if the rod bubble is 8' or 40' if it's plumbed up. Just keep it adjusted and it will work fine. Once you have it adjusted and leveled use the gun to check it for being plumb, then rotate it 90° and check it.
After that's done, offset the rod so the bubble moves to where it's edge is against the targeting circle and measure that offset error.
I may be wrong about this, I never majored in rod bubbles, but I *think* that the 40' bubble is how much angle tilt it would take to shift the bubble 2 mm. Others on this board know way more than I do about this, so I gladly stand corrected on anything I have said.
I do know that there are at least two different models of level bubbles for prism rods
Contact the manufacturer and talk with them about it. They used to be the regular pole and the precise pole, but that changed in recent years. Most salesmen believe they are all alike, so you may have to do some explaining to get thru to a tech that knows the difference.
Usually whenever I get a new rod, I will have to order the precise bullseye model and put it on and adjust it myself. Maybe $15 for that, but worth it for the degree of accuracy that you get compared to the regular bubble.
> I may be wrong about this, I never majored in rod bubbles, but I *think* that the 40' bubble is how much angle tilt it would take to shift the bubble 2 mm.
I think Paul's right about the spec: it's the angular change required to move the bubble 2 mm.
I don't agree about the sensitivity not making a difference, though. A 40' bubble has way too much slop in it for decent control work. I put 8' or 10' bubbles on all my rods, whether GPS or prism.
Good ones aren't cheap; I think I paid around $50 for the last one I got from SECO.
Thanks. I meant the sin of 40' or 0.06' error. I take it that just because one bullseye is 8' and another 40' does not necessarily mean one is more accurate than the other, just that the bubble is more sensitive?
Is a 8' bubble 5x more accurate than a 40' bubble or just 5x more sensitive.
I really want to know the relationship between sensitivity and accuracy.
rod bubble accuracy ... the precision thereof
What you gentlemen are discussing is actually the precision of the bubble as aluded to above.
Easiest way to determine the sensitivity or precision of a rod bubble is to attach it to the telescope of a transit/theodolite/total station telescope and then to tilt the telescope such that the bubble is on one edge of the circle. Read the vertical angle, tilt the telescope so that the bubble runs to the opposite edge of the circle and read the vertical angle. Difference in the angles is the sensitivity/precision of the rod bubble.
When done with a level vial, as with a striding level, the sensitivity is expressed in "pars" such that a level vial with a five second per graduation precision is expressed as ? = 5", with the greek letter, pi, being used as the symbol for par.
I have seen rod levels with two tubular vials at 90º to each other, but that was for older invar rods, certainly nothing manufactured in the past 20 years or so.
> I really want to know the relationship between sensitivity and accuracy.
Well.. What type of gun do you use? I have a Pentax 5"/1" readout Reflectorless that has two setting for the electronic leveling bubbles. They are one minute and 30 seconds. It's a lot more important to make sure the gun is leveled and in correct adjustment than worrying about an 8' or a 40' rod bubble.
If you do have a TS that has an electronic leveling feature, do this:
1. Level the gun .
2. Set the gun so the zenith angle is 90°.
3. Tilt the gun up or down about 45° or so. Observe the horizontal reading. It should not vary more than a few seconds.
4. Set thegun back to 90° zenith.
5. Unlevel the gun a bit by turning a foot screw.
6. Tilt the gun up or down again and watch how the horizontal reading goes nutz.
IMHO a 20' bubble is sufficient. If you want an 8', go for it but I think it's overkill seeing as most guns level out at 30".
Paul,
Thanks. What I really want to know is the horizontal accuracy of my prism pole. Since all I have is a level bubble to tell me how plumb the pole is, I am dependent on the accuracy of the bubble. If I have a 40' bubble that is in adjustment, what is the error range out of plub, is it +/- 0.03' or is it +/- 0.01' or is it +/- 0.005'? I know that it depends on the bubble being in adjustment and height of rod. My question really boils down to:
Is a prism pole more accurate when plumbed with a 8' bubble or a 40' bubble or a 1 deg bubble?
What is the difference in accuracies of each for a prism at 5' height?
rod bubble accuracy...Gene Kooper
> Is a prism pole more accurate when plumbed with a 8' bubble or a 40' bubble or a 1 deg bubble?
Those bubble examples each have the same thing in common.. degree of tilt it takes to move the bubble 2MM.
> What is the difference in accuracies of each for a prism at 5' height?
No clue. Gene Kooper probably knows though.