Confirming RPM?

Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
Wasn't sure where this would fit so just stuck it in OT.

I have a brain freeze. I am using my scope. trying to confirm the RPM on the optical tachometer I built. I am measuring the output of my optical sensor.

I am getting about 11ms from falling edge to falling edge (I interrupt on external edge from my sensor)


What would my rpm be?
 

GopherT

Joined Nov 23, 2012
8,012
Wasn't sure where this would fit so just stuck it in OT.

I have a brain freeze. I am using my scope. trying to confirm the RPM on the optical tachometer I built. I am measuring the output of my optical sensor.

I am getting about 11ms from falling edge to falling edge (I interrupt on external edge from my sensor)


What would my rpm be?
60,000 mSec / Y mSec = 5455 rpm.

60000 / 11 = 5455
 

Papabravo

Joined Feb 24, 2006
15,019
5400 rpm sounds more like it for a pc fan?
Are you sure you're reading the scope correctly? If the scope was set on 50 msec/div and you had 11 divisions from edge to edge that would give you 110 rpm. You might also want to check that your scope is not in uncalibrated mode.
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
Oops wasn't multiplying by 60! Made some previous changes to test something and forgot to put it back. :(.

It is a little closer now but still off. I need to check my clock. Also I think I need to place a better target on the fan, I am getting inconstant readings on both the scope and my display. But that is for another night. ;)
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
Are you sure you're reading the scope correctly? If the scope was set on 50 msec/div and you had 11 divisions from edge to edge that would give you 110 rpm. You might also want to check that your scope is not in uncalibrated mode.

It is a digital scope. I am using the cursors to measure from falling edge to falling edge. It gives you a nice read out. I lot easier to use than trying to read the scale on that small screen. ;)
 

Papabravo

Joined Feb 24, 2006
15,019
It is a digital scope. I am using the cursors to measure from falling edge to falling edge. It gives you a nice read out. I lot easier to use than trying to read the scale on that small screen. ;)
It's just that you seemed surprised by the result, and I wasn't sure what kind it was. I've made the mistake I described on an analog scope (TEK 2236) more than once.
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
It's just that you seemed surprised by the result, and I wasn't sure what kind it was. I've made the mistake I described on an analog scope (TEK 2236) more than once.

I was surprised by the huge difference in what I was seeing on my project then I realized it was not displaying what I thought it was displaying.
 
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