Using the Planimeter

[Paulo Issler]

I noticed that planimeters come with a piece of information related to (something like) "area of neutral circle"...
I am just starting in boat design and I bought a planimeter from an e-bay auction. It seems it is in very good condition. I am not sure, however, if the information above is critical or if I can somehow "calibrate" the instrument before using it.
Any advise is welcome.... I hope the question make sense for some of you. Keep in mind that I am not very knowledgeble in these matters...

[John E Hardiman]

There should be two things that came along with the instrument in the box. One is some indication what it reads in; usually inches^2 in the US or older UK or cm^2 elsewhere. Second there should be a gage that looks like a flat metal bar with a line scribed on the face at one end. On the other end and back side there should be a prick pin. On the face somewere between the prick pin and the line should be a punch mark with some indication of the size of the circle that will be swept out by the punch mark (My K&E 4236 measures in inches^2 and the gage sweeps a 10 inch^2 circle). Draw a line on your paper longer than the stretched length of the planimeter. Place the planimeter base at one end of the line. Place the gage prick pin on the line so that when the gage scribe is aligned away from the base, the tracer point just reaches it. Fit the tracer point into the punch mark, and zero the planimeter. Now sweep the gage in a complete circle with the tracer point, coming back to match the scribe on the gage to the line on the paper; the number read on the vernier should equal the gage marking. Without moving the setup, reverse the circle. The veriner should read zero. If the veriner reads too high, the tracer arm is too short. If the veriner reads too low, the tracer arm is too long. WARNING: DO NOT ADJUST THE TRACER ARM UNLESS YOU ARE ABSOLUTELY SURE. MOST PLANIMETERS WERE CALIBRATED AT 65-75F AND NEED TO BE WARM TO READ CORRECTLY.

Mine was given to me as a legacy. When my sister-in-laws grandfather died (he was a retired aeronautical engineer for North American), she went down and helped the other grandchildren distribute his posessions. She was the only one who knew what it was and thought I might like it. I was handed it wraped up in its box as a present, and I was told "You'll never guess what it is!". WRONG!! I stuned her when I said "It's a K&E planimeter". I've used waAAYYY to many of these in the days BPC.

[john welsford]

I have two traditional planimeters, they are both carefully packed in a big box that has among other things that I dont know how to use, several slidedrules and a big pantograph that purports to be able to convert drawings from one
scale to another.
I used to use graph paper to do my measurements, until I could find someone to tell me how to run the planimeter you understand.
I got pee'ed off with listening to my elderly friend getting more and more frustrated at trying to teach me something that he found easy and I could not for the life of me understand so I went off to a surveyors supply shop and bought a Koizumi Placom Digital Planimeter model KP-90. Best thing I ever did. Wonderful thing, does darn near all my hull calculations and changed just about every aspect of how I develop the stats for my hulls, Simpsons rule? Hav'nt used it since I got my toy.
JohnW

Originally posted by John E Hardiman:
There should be two things that came along with the instrument in the box. One is some indication what it reads in; usually inches^2 in the US or older UK or cm^2 elsewhere. Second there should be a gage that looks like a flat metal bar with a line scribed on the face at one end. On the other end and back side there should be a prick pin. On the face somewere between the prick pin and the line should be a punch mark with some indication of the size of the circle that will be swept out by the punch mark (My K&E 4236 measures in inches^2 and the gage sweeps a 10 inch^2 circle). Draw a line on your paper longer than the stretched length of the planimeter. Place the planimeter base at one end of the line. Place the gage prick pin on the line so that when the gage scribe is aligned away from the base, the tracer point just reaches it. Fit the tracer point into the punch mark, and zero the planimeter. Now sweep the gage in a complete circle with the tracer point, coming back to match the scribe on the gage to the line on the paper; the number read on the vernier should equal the gage marking. Without moving the setup, reverse the circle. The veriner should read zero. If the veriner reads too high, the tracer arm is too short. If the veriner reads too low, the tracer arm is too long. WARNING: DO NOT ADJUST THE TRACER ARM UNLESS YOU ARE ABSOLUTELY SURE. MOST PLANIMETERS WERE CALIBRATED AT 65-75F AND NEED TO BE WARM TO READ CORRECTLY.

Mine was given to me as a legacy. When my sister-in-laws grandfather died (he was a retired aeronautical engineer for North American), she went down and helped the other grandchildren distribute his posessions. She was the only one who knew what it was and thought I might like it. I was handed it wraped up in its box as a present, and I was told "You'll never guess what it is!". WRONG!! I stuned her when I said "It's a K&E planimeter". I've used waAAYYY to many of these in the days BPC.

[bainbridgeisland]

John, that was a good story. I am still chuckling and I have heard you tell it before.

By the way, the simplest thing to do if your planimiter is slightly off is to simply use a factor to adjust the measurement. As John pointed out, adjusting the arm is not usually a good idea.

By the way, I have used my planimiter to check area calculations from various computer programs. You would be surprised how poor the accuracy is on some programs. With practice I have found the planimiter to be quite accurate.

[Carlsboats]

Suggest you run a search on Google. I think you will find tons of info. there about planimeters --calibration, compensation, etc., and some articles that go through the math that proves these things work.
I have used a borrowed one, and like you, have now bought one of my own on ebay. Have had no trouble with errors or scaling problems.

[mmd]

I often use this "quick & dirty" method: I carefully draw a square of a known area to scale (if I'm working at a scale of 1" = 1'-0", I'll draw a square 3" per side, equalling 9 scale square feet) and set up my planimeter to measure it. I set the planimeter counter to zero, measure the square, and record the resulting counter reading. I reset to zero and measure the square again and record the result. Then I do it a third time. I sum the recorded results and divide by three to get the average reading. Then I divide that by the scale square footage to get a "factor" for the planimeter readings, which I write down. I can then measure the drawings I am interested in and divide the readings I get by my planimeter factor to get the scale areas. I like this method as it is independant of any planimeter arm settings and therefore not prone to inaccuracies of physical settings of the device.