I needed a way to measure the rpm on my Stirling and could not find anything on the internet, so I made my own. If there is an easier way, I am open to suggestions. In the meantime, here is my schematic and a short video. http://www.gtcom.net/~dbarnes/rpm_strobe.htm
DonnieB
Strobe to Measure RPM
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SScandizzo
- Posts: 66
- Joined: Tue May 16, 2006 5:06 pm
- Location: California
Actually, Donnie, there is a VERY easy way to measure RPM.
http://www.sherline.com/rpmgage.pdf
Basically, its a paper wheel with black and white divisions on it that takes advantage of the fact that fluorescent lights flicker at 60Hz. The page is pretty self-explanatory, and I can tell you from my own experimenting that it is reasonable accurate.
-Stefan
http://www.sherline.com/rpmgage.pdf
Basically, its a paper wheel with black and white divisions on it that takes advantage of the fact that fluorescent lights flicker at 60Hz. The page is pretty self-explanatory, and I can tell you from my own experimenting that it is reasonable accurate.
-Stefan
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valhalla418
- Posts: 3
- Joined: Mon Dec 11, 2006 8:59 am
I looked at the black and white rpm disc and think its a great way to measure rpm. My problem is that I live in the UK so my A/C is 50Hz, not 60Hz like in the US. So if I used this disc it would give me an incorrect reading.
I want to create a 50Hz, but I've looked at the 60Hz disc and is seems to me that it's incorrect. For example if you look at the 2400rpm ring (innermost) it has 3 white intervals that complete the circle. So each flash of the fluorescent would count a third of a revolution. So 60 flashes would give 20 revolutions (60 x one third). This is 20 revs per second. So 20revs-per-sec x 60secs = 1200rpm, not 2400rpm as the disc states.
Is the disc wrong? or have I worked this out incorrectly?
If I am correct then I believe the disc rpm values should be 1200, 900, 600, 450, 400, 360, 300, 240, 200, 150 (from innermost to outermost), and not the values printed on the disc.
Ben
I want to create a 50Hz, but I've looked at the 60Hz disc and is seems to me that it's incorrect. For example if you look at the 2400rpm ring (innermost) it has 3 white intervals that complete the circle. So each flash of the fluorescent would count a third of a revolution. So 60 flashes would give 20 revolutions (60 x one third). This is 20 revs per second. So 20revs-per-sec x 60secs = 1200rpm, not 2400rpm as the disc states.
Is the disc wrong? or have I worked this out incorrectly?
If I am correct then I believe the disc rpm values should be 1200, 900, 600, 450, 400, 360, 300, 240, 200, 150 (from innermost to outermost), and not the values printed on the disc.
Ben
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SScandizzo
- Posts: 66
- Joined: Tue May 16, 2006 5:06 pm
- Location: California
Good catch, Ben!
I thought this at first as well. Turns out that at 60Hz a fluorescent light flashes 120 times. I'm not sure of the electrical explanation, just that that's the way it actually works. Regardless, I built an electronic revolution counter using a tiny magnet as the trigger and a hall switch as the sensor, and I can confirm that the disk/light technique is accurate. Had I known that from the onset, I probably wouldn't have built the electronic one!
-Stefan
I thought this at first as well. Turns out that at 60Hz a fluorescent light flashes 120 times. I'm not sure of the electrical explanation, just that that's the way it actually works. Regardless, I built an electronic revolution counter using a tiny magnet as the trigger and a hall switch as the sensor, and I can confirm that the disk/light technique is accurate. Had I known that from the onset, I probably wouldn't have built the electronic one!
-Stefan
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valhalla418
- Posts: 3
- Joined: Mon Dec 11, 2006 8:59 am
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valhalla418
- Posts: 3
- Joined: Mon Dec 11, 2006 8:59 am
I've created a 50Hz timing disc to use with a fluorescent light in the UK:
http://www.globecc.co.uk/50HzRPMDisc.gif
Print it out and view rotations under fluorescent lighting.
http://www.globecc.co.uk/50HzRPMDisc.gif
Print it out and view rotations under fluorescent lighting.
Thanks Stefan, for the info. I'll give it a try.
The reason for the 120 flashes instead of the 60 is that ac current is a sine wave that swings from a positive voltage to a negative. So, in one cycle, the voltage rises from zero to 120 and back to zero then rises, in the negative direction, to -120 volts. So you have 2 voltage peaks in one cycle. The flourescent doesn't care if it is positive or negative... at least, that is the way I understand it. By the way, I have started on my second stirling. Hope it works. Thanks again.
Donnie
The reason for the 120 flashes instead of the 60 is that ac current is a sine wave that swings from a positive voltage to a negative. So, in one cycle, the voltage rises from zero to 120 and back to zero then rises, in the negative direction, to -120 volts. So you have 2 voltage peaks in one cycle. The flourescent doesn't care if it is positive or negative... at least, that is the way I understand it. By the way, I have started on my second stirling. Hope it works. Thanks again.
Donnie
