Planimeter
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Tin Falcon
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Wow how easily things can be misunderstood on the web..
I do not think anyone is implying a single use.
I am enjoying this thread.
There was actually two original questions .
how it works and what application it has for a machinist.
I think a better question it what application it has to a model engineer.
so lets focus on that.
and what does the indicator look like that draws the card.
Tin
I do not think anyone is implying a single use.
I am enjoying this thread.
There was actually two original questions .
how it works and what application it has for a machinist.
I think a better question it what application it has to a model engineer.
so lets focus on that.
and what does the indicator look like that draws the card.
Tin
rodw
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I think all of the applications referred to above are simply measuring area. It sounds like sometimes it was the area of a nomagraph for steam calculations but most of the time it is either the area on a plan or map or the area of a given cross section as in hull design or dam design. I am pretty sure the hull designer would use the same trapezoidal rule we dam designers used them for to calculate the total volume or displacement of a 3 dimensional shape.
Some of the cheap ones (which I used as a student) were calibrated so that 1 revolution was 100 square centimetres (at 1:1) but others had a vernier scale on one of the arms that allowed you to vary the calibration to agree with the scale of a map or plan. So if you had a scale of 1:5000, you could read the value based on the scale in use rather than having to do your own scaling maths.
I will say there was a bit of an art in deciding where to position the fixed base so the tool could trace the whole area under consideration and to watch out for wheels fouling the edge of the map, plan or photo.
In a real life example, I used them for water way design when surveying contour banks. Starting with aerial photos and looking at them through a stereograph (similar to binoculars mounted on a stand above the photos) , we would manipulate the overlapping photos until we got a stereoscopic view. Then we would draw in the ridgelines on one photo with a brown chinagraph pencil and the streams and waterways in on blue. Once this was done, we could site the waterways contour banks would flow into. Then we could measure the catchment area with a planimeter and calculate the estimated runoff for a given rainfall intensity return period depending on what we were designing. Once this was done, we should know the design parameters to use to design the size of the waterway .
If we were designing a dam, we could site the dam wall and measure each contour (which generally involved doing a grid survey and interpolating values to draw our own contours from) and use the trapezoidal rule to calculate the dam volume. We would also measure the catchment area determined maximum design runoff to ensure the spillway was designed to have sufficient capacity to handle the overflow without gouging big gullys or the dam overtopping and the wall breaching.
Civil engineeers might use the same procedures to size culverts under a road to handle the runoff which they would determine from nomographs. I remember while at college in the early 80's getting the bookshop to order in copies of a book of nomographs from the US. It cost us a fortune for about 50 pages. I kept this book for ages but one day I opened it up while packing up to move and could not understand even one of the diagrams so I finally gathered up the courage to ditch it.
Now everything is done with Satellite photos and GPS units.
To use a planimeter, set it on your plan with the two arms at roughly 90 degrees. Lift the mechanism and rotate the wheels to zero the scale and put the mechanism down again. Then trace your shape (which must be enclosed so you finish where you started and read the scales. If you don't know the 1:1 scaling, draw a square of known dimensions (eg 100mm x 100mm or 4" x 4") and compare the device reading with the known area. Some came with a little metal part you could trace to calibrate the device to a known standard. From memory, we would do it about 3 times and average the result if precision was required.
So the challenge now for those who have a planimeter is to find the stereoscope that was so often used in conjunction with one. See http://en.wikipedia.org/wiki/Stereoscopy and there is a picture at the top right of a small one.
Some of the cheap ones (which I used as a student) were calibrated so that 1 revolution was 100 square centimetres (at 1:1) but others had a vernier scale on one of the arms that allowed you to vary the calibration to agree with the scale of a map or plan. So if you had a scale of 1:5000, you could read the value based on the scale in use rather than having to do your own scaling maths.
I will say there was a bit of an art in deciding where to position the fixed base so the tool could trace the whole area under consideration and to watch out for wheels fouling the edge of the map, plan or photo.
In a real life example, I used them for water way design when surveying contour banks. Starting with aerial photos and looking at them through a stereograph (similar to binoculars mounted on a stand above the photos) , we would manipulate the overlapping photos until we got a stereoscopic view. Then we would draw in the ridgelines on one photo with a brown chinagraph pencil and the streams and waterways in on blue. Once this was done, we could site the waterways contour banks would flow into. Then we could measure the catchment area with a planimeter and calculate the estimated runoff for a given rainfall intensity return period depending on what we were designing. Once this was done, we should know the design parameters to use to design the size of the waterway .
If we were designing a dam, we could site the dam wall and measure each contour (which generally involved doing a grid survey and interpolating values to draw our own contours from) and use the trapezoidal rule to calculate the dam volume. We would also measure the catchment area determined maximum design runoff to ensure the spillway was designed to have sufficient capacity to handle the overflow without gouging big gullys or the dam overtopping and the wall breaching.
Civil engineeers might use the same procedures to size culverts under a road to handle the runoff which they would determine from nomographs. I remember while at college in the early 80's getting the bookshop to order in copies of a book of nomographs from the US. It cost us a fortune for about 50 pages. I kept this book for ages but one day I opened it up while packing up to move and could not understand even one of the diagrams so I finally gathered up the courage to ditch it.
Now everything is done with Satellite photos and GPS units.
To use a planimeter, set it on your plan with the two arms at roughly 90 degrees. Lift the mechanism and rotate the wheels to zero the scale and put the mechanism down again. Then trace your shape (which must be enclosed so you finish where you started and read the scales. If you don't know the 1:1 scaling, draw a square of known dimensions (eg 100mm x 100mm or 4" x 4") and compare the device reading with the known area. Some came with a little metal part you could trace to calibrate the device to a known standard. From memory, we would do it about 3 times and average the result if precision was required.
So the challenge now for those who have a planimeter is to find the stereoscope that was so often used in conjunction with one. See http://en.wikipedia.org/wiki/Stereoscopy and there is a picture at the top right of a small one.
Simpson's Rules by any chance? Would have come in handy for 2nd Mates stability when I was but a simple sailor.
Dave
The Emerald Isle
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