As you know, USMS passed a rule that will require us to measure any pool in which we hold a meet, IF we want the times from that meet to count for USMS Top Ten. I'm trying to figure out the best way to do this - what sort of tape to buy, the best method to use, etc. (Can you tell that I'm an engineer?)
I'd like to hear from those of you who have measured pools. What brand of tape do you recommend? What length? (60 meters?) Any idea where I can buy such a tape? Any suggestions for how to get a good (accurate) measurement, especially over a 50 meter course?
FYI, below are the instructions on how to perform pool measurements, taken from the USMS Pool Length Certification Form.
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"A completed form must be on file or submitted with an application for sanction or recognition to your LMSC Sanctions Chairman. For pools utilizing bulkheads, completed forms from each session must be included with results sent to the Top Ten Chairman if the results are to be included in Top Ten submissions.
Measurement procedures:
--Measurements must be conducted using a measuring device with the minimum measuring quality of at least ± 0.005 m (±.0.20 inches or 0.016 feet) over the nominal distance. A laser based measuring device may be used but it must be as accurate as required for steel tapes. Tapes may not be combined to perform the measurements.
--Measurements may be reported in either the English system (feet/inches) or the metric system (meters/centimeters).
Instructions for pool length measurements:
--Measurements must be conducted for each lane. Inspect each end of the pool to insure that it is vertical. If there are any protrusions from the wall into the pool between 0.8 meters (2.5 feet) below the surface to the top of the touch pad or wall or up to 0.3 meters above the surface whichever is higher, you must use the protrusion as your measurement point. Otherwise, measurements must be taken in the center of each lane at water level.
--The measuring device shall be moved up/down and left/right at least 12 inches (unless you are on a protrusion) during each measurement and the minimum distance for each measurement recorded.
--Measurements may be conducted with or without touchpads in place. However, if touchpads are used the pool must still meet the length requirements with them in place.
--Permanent courses need only be measured once unless structural changes have occurred since original measurement.
--For pools with a moveable bulkhead, after each session a course measurement must be confirmed by a responsible person. Only the outside lanes and a single center lane need to be measured to confirm the integrity of the bulkhead and its placement."
Mary:
You are correct on the .005 distance. Sometime I am going to get the dimensions straight between centemeters and millimeters. (of course my favorite dimensions is furlongs per fortenight - but that is in the English system).
But being a gluton for punishment, lets revisit the co-efficent of expansion for a moment.
"Steel tapes are standardized at 68 degrees Fahrenheit. The coefficient of expansion on most steel tapes is .0000065 ft per ft and average 0.01ft every 15 degrees over a 100 ft distance. If your water/air temperature were around 83 degrees, you would experience a stretch of your tape of 0.01 ft using a 100 ft tape or 0.02 ft with a 200 ft tape. Thus your pool may appear to be shorter than it really is. Record the measurement without correction for temperature. Also record the temperature. Only if your pool measurement comes up short would the temperature factor need to be considered."
We felt this too technical to include in the final version, since the stretch of the tape at higher than the 68 degree standard would not have much impact. The greatest source of measurement error with a tape is sag. "
Lets assume as your example the temperature is 83F, then the tape would expand about 5 mm. If you already have a tape that is 5 mm long (the degree of uncerntainty of tape and still meeting spec at that length- this would be on the long side) your 50.00 M pool will measure out at 49.99 (it fails).
If I am thinking correctly (and there is no guarantee about that), sag would introduce an error that would make the pool appear to be longer than it is.
In a thread on the USMS forum titled "The Losers" on page 4 of discussion, I wrote about trying to calculate the true length when there is sag.
"Rob Copeland wrote back "a true 25 meter pool is measured with a 0.1 meter sag (about 4 inches), in the tap, the length on the tape would read 25.001066663 meters"
According to Rob, the formula used to calculate the segment of a parabola is
s = sqrt+* ln)/(c/2)]
Where: s = tape distance, h = midpoint sag and c = true pool length."
To me it appears that the sag introduces less error than expansion, but the errors are going the other way from the coefficient of expansion. (co-efficent of expansion errors make the pool appear shorter than it is (if the temperature is warmer than 20C.) sag errors make the pool appear longer than it is. It might be that the errors cancel out each other. I should also say that my head hurts when looking at the equation and I did not recalculate Robs findings (so if I am wrong, I am blaming Rob).
I think in the final analysis, there are going to very few pools that are going to come so close to the 50.00M min length. However, I would think that it would be good if we asked for the ambiant temperature when the people make their length measurement (as you seem to suggest), so that we can recalculate a pool that is short by 1-2cm. This may only affect one or two pools in the country, but better to have pools where we can legitimately swim for records than not have them because of bad measuring protocols.
To change the subject a little
I have not seen seen us provide guidence on the setting up of a pool with a bulkhead. The distance should be measured before the meet begins and it must be measured at the end of each day.
If someone is setting up a 50 M pool that has a bulkhead and two standard 8mm touch pads (pads at each end), the pool is being measured with a laser that has a 3mm + or - . What should be the distance from the edge of the pool to the beginning of the bulkhead?
I would suggest the distance should be 50.19M. This allows for both pad (0.16M) plus the measurement error. just a thought
michael
Mary:
You are correct on the .005 distance. Sometime I am going to get the dimensions straight between centemeters and millimeters. (of course my favorite dimensions is furlongs per fortenight - but that is in the English system).
But being a gluton for punishment, lets revisit the co-efficent of expansion for a moment.
"Steel tapes are standardized at 68 degrees Fahrenheit. The coefficient of expansion on most steel tapes is .0000065 ft per ft and average 0.01ft every 15 degrees over a 100 ft distance. If your water/air temperature were around 83 degrees, you would experience a stretch of your tape of 0.01 ft using a 100 ft tape or 0.02 ft with a 200 ft tape. Thus your pool may appear to be shorter than it really is. Record the measurement without correction for temperature. Also record the temperature. Only if your pool measurement comes up short would the temperature factor need to be considered."
We felt this too technical to include in the final version, since the stretch of the tape at higher than the 68 degree standard would not have much impact. The greatest source of measurement error with a tape is sag. "
Lets assume as your example the temperature is 83F, then the tape would expand about 5 mm. If you already have a tape that is 5 mm long (the degree of uncerntainty of tape and still meeting spec at that length- this would be on the long side) your 50.00 M pool will measure out at 49.99 (it fails).
If I am thinking correctly (and there is no guarantee about that), sag would introduce an error that would make the pool appear to be longer than it is.
In a thread on the USMS forum titled "The Losers" on page 4 of discussion, I wrote about trying to calculate the true length when there is sag.
"Rob Copeland wrote back "a true 25 meter pool is measured with a 0.1 meter sag (about 4 inches), in the tap, the length on the tape would read 25.001066663 meters"
According to Rob, the formula used to calculate the segment of a parabola is
s = sqrt+* ln)/(c/2)]
Where: s = tape distance, h = midpoint sag and c = true pool length."
To me it appears that the sag introduces less error than expansion, but the errors are going the other way from the coefficient of expansion. (co-efficent of expansion errors make the pool appear shorter than it is (if the temperature is warmer than 20C.) sag errors make the pool appear longer than it is. It might be that the errors cancel out each other. I should also say that my head hurts when looking at the equation and I did not recalculate Robs findings (so if I am wrong, I am blaming Rob).
I think in the final analysis, there are going to very few pools that are going to come so close to the 50.00M min length. However, I would think that it would be good if we asked for the ambiant temperature when the people make their length measurement (as you seem to suggest), so that we can recalculate a pool that is short by 1-2cm. This may only affect one or two pools in the country, but better to have pools where we can legitimately swim for records than not have them because of bad measuring protocols.
To change the subject a little
I have not seen seen us provide guidence on the setting up of a pool with a bulkhead. The distance should be measured before the meet begins and it must be measured at the end of each day.
If someone is setting up a 50 M pool that has a bulkhead and two standard 8mm touch pads (pads at each end), the pool is being measured with a laser that has a 3mm + or - . What should be the distance from the edge of the pool to the beginning of the bulkhead?
I would suggest the distance should be 50.19M. This allows for both pad (0.16M) plus the measurement error. just a thought
michael