Tachometer Question

[James40]

Hi folks,

 

I bought a new Turnigy Tachometer from a  fleabay seller, it turned up after one day so no problems with delivery, however, when I turn it on the display reads and fluctuates between 3000 to 4000 rpm without it actually pionting at anything, just sat here on my lap in the lounge.

 

I don't have flourescent lighting in my lounge, just standard ceiling spot lights so I don't understand where it's getting this reading from ?   

Have I bought yet another duff piece of RC equipment or do all tacho's give a false reading until they are put in front of a turning prop ?

 

Thanks 

[Brian Parker]

It's the lounge light.
 It confirms that it's working OK (50Hz).

[001]

Hello James. Apart from the fluctuation, the Tachometer is probably OK. Ordinary house mains Tungsten light works at 50 cycles per second and will activate the tach. to show 3000 rpm.

If the item has a 3 blade prop setting, it should show 2000 rpm.

I suggest you point it closely at a light, it might settle down.

 

Also, if you have an electric drill you can test if the instrument is working, but not its accuracy, with a propeller, by fixing a bolt of the same size as the bore of a (balanced) propeller through it, tightening it with a nut, fastening it into the chuck and turning the drill on. Take a reading, in daylight as normal. BE VERY CAREFUL, especially if the drill does not have a speed control. Best to fix the drill in a Workmate or vice or get a helper and do it outdoors! (Usual disclaimer).

If the speed fluctuates erratically, on the first test with the light,  send the tacho back. If it is not accurate it is pointless having it.

Let us know the result.

Edited By Richard Bond on 27/01/2010 17:12:34

[James40]

Spot on advice as ever guys, thanks very much

 

You were absolutely right, on 2 blade prop setting it reads 3000, 3 blades is 2000 and 4 blades 1500 and it settles down nicely when I point it directly at the light, it must have been getting confused with the 6 spot lights.

 

Can't believe I didn't think of that, I know flourescent lights strobe at mains frequency but for some reason I didn't even think of a tungston filament doing it

[Sparks]

Although the mains frequency is 50Hz, a tungsten bulb will 'flicker' at 100Hz (brightest at each positive and negative going cycle and extnguished at each zero crossing). This is more pronounced with fluorescent lighting.

 

So, 100Hz = 6000cycles/min, which is blade passing frequency of a two blade prop @ 3000 rpm.

 

Another Richard!