Does anyone know where (in Adelaide) I might be able to buy precision-weighed weights, suitable for checking / calibrating a power meter? i.e. a 20 kg weight as seen in this video?
I don't fancy even getting a quote to import a 20 kg weight from an american online store! :)
Or alternatively, where I could go to get something that weighs 20 kg, precision weighed? I can use an ordinary dumb-bell if I can find somewhere to get it weighed to the nearest few grams.
pfft, only 20kg? I match your 20 and raise you ... oh buggerit, apparently good taste says I'm not allowed to say, but I still reckon I can match you (but not Sparticus, that man's a legend ... and I can't even match him on the road)
Precision? How many significant figures are you looking at?
Good old H2O is a well known substance and at 20degC 1cc weighs 1gm. Get your 20L container weighed on some digital scales, your water as close to 20DegC as you can and you are simply as accurate as you can be sure you have 20L minus the weight of the container...
If using water, avoid tap water, spring water and de-mineralised water. It is pure (distilled) water where 1cc weighs 1gm. Rossmg is correct about 20C being the temperature used in lab calibrations.
Purchase two 10 litre spring water packs, take them to your local post office and fine adjust to suit.
Have you found that there's enough clearance from the ground to hang two off of the crank arm?
Your clearance depends on where you park your bike. On a table/ledge/wall might work.
Your measurement of weight to 20.000kg +/- 1g is nonsensical unless you measure your crank length to the same length accuracy, eg 170.00mm +/- 0.01mm. 50g weight variation will give you +/- 0.25% error. You also (might) need a laser level for the crank/frame angles to obtain any accuracy. Don't breathe while doing the measurement.
If you doubt one post office scale, try 3 different post offices, take the mean value.
If you want to keep the weight for future testing, take a metal bucket to a steel merchant, garden centre or scrap yard, purchase the required amount of material and adjust with a stack of washers. Remember to weigh the chain/string too, and the plastic bag to keep heavy dust from settling in the bucket.
No need for the attitude
Jilden is corect.
Power = force x distance / time. It's a principle in science and engineering, there's no point in being super-precise about one measurement, if your other two are less precise.
Considering the precision (=significant figures) of the components:
The power output, will be say around 200 watts. To measure that using a 20 kg weight, that means letting the weight fall, at 1 meter per second .
The design of the measurement system, is constrained, by the least precise measurement.
Power - 3 significiant figures (200 watts)
Time - 1-2 significant figures (e.g 3.4 seconds) - human reaction time
Weight - a 20 kg weight accurate to 1 g, is 5 significant figures (20.000 kg)
Distance - 3 significant figures (280 mm).
So the least precise measurement will be the measurement of time .
This means that the power output will only be accurate to the nearest 10 watts, possibly less.
Also means, that there is no need to go to time and trouble buying an expensive reference weight from the National Standards Authority. The weight need be measured only to the nearest 100 grams.
Thus Jilden's idea of using scrap metal, is adequate; as is Helen's of using water.
A number of people here have, in a friendly way, gone to some time and effort to set down info, that can help with what you ask, and save you trouble of ordering expensive reference weights from overseas.
But is up to you.
Yes that's the idea of my original question, but I don't have a ledge handy. Thanks anyway! Further on in the discussions you'll see I've found a cheaper solution that's quite practical :)
Australia post scales are accurate to 1 gram, up to 20KG.