What is the fan speed?

Mates,
Are fans generating one or two, or maybe even three pulses per revolution on the "third wire"?
The reason I'm asking is that when measuring the rpm on one of my fans (80 mm) I got around 6000 rpm at 12V. This sounds a wee bit too high, so the question is if it should be divided by two or three to get the correct value?

Another question on the subject: Assuming that I always keep the same ground, will the speed sensor always deliver TTL levels?

Cheers,
Rob

Why not just plug it into a computer and compare the results you get and what the computer reports?

It depends on the fan. I think most of the time it's 1 pulse per wire, but it can be multiple. When you set up MBM I know it has several dividers for fan sensors so they read correctly.

6K RPM may not be that unusual unless you know the fan is supposed to be running slower. Heatsink fans tend to spin faster (mine is close to 5K), while case fans slower (I've got one that spins only 3.5K)

JigPu

But if there could be one pulse per rev or multiple pulses, wouldn't that mean that pretty much all software (apart from MBM) could present highly inaccurate results? (>50% wrong)
In that case I wouldnt' be able to check the speed of my fan by connecting it to my computer, as Mr. Radar suggests.

I will try to check the speed using a phototransistor as well, since with that I know exactly how many pulses I get per rev (one per blade).