Inverse square might be a good starting point, depending on the material the magnet is mounted in.

So...double the distance = 1/4 the field strength. I'll go play with the numbers on the vendor sites. Most of the sensors seem to require a very low magnetic field to quit conducting. That's probably the best part for a design engineer. Small cheap magnet activates sensor at less than 1/2 inch and rapidly seems to disappear as it moves away.

I thought I defined the material. The plastic and water seem magnetically transparent to me.
How many plastics inhibit magnetic fields and what engineer would design a water flow wheel with a sort of plastic that is known to interfere with the magnet which is inside it?

Then again, who would mix talc into the plastic to be sure a fan blade would disintegrate in time to sell a car owner a new fan? (General Motors)

 

Hi

I have an Gauss meter app on my iPhone. When sampling is turned on..it reads 0.451 Gauss.
If I measure a small magnet...it reads about 2.7 gauss. Don't know how accurate it is but maybe it can be used to measure the field?

I just had a friend show up with an iPhone 4 and used an app. It showed 16,350 gauss @ 1 inch and 19,050 gauss at 0.6 inches. I believe this is a very wrong measurement.

 

I just had a friend show up with an iPhone 4 and used an app. It showed 16,350 gauss @ 1 inch and 19,050 gauss at 0.6 inches. I believe this is a very wrong measurement.

Your probably right and not surprising....some apps are good, others not so good. I think that's true for just about any iPhone app.

BTW, I have an IPhone 6 with an app named "Gauss Meter".

 

There are digital hall sensors (on or off, depending on the field strength... that also have a certain minimum hysteresis), and then there are ratiometric (also called linear) hall sensors. The latter have an output voltage that is proportional (though I'm not sure if it's directly proportional) output in relation to the magnetic field density.
If I remember correctly, magnetic field density is proportional to the cube of the distance related to the source.
But don't take my word for it, right now... I'm going to check my sources tomorrow.

 

And btw, they're fairly cheap and can be found in the package size that you want. Just do a search in digikey using the "ratiometric hall sensor" keywords...

 

AK and Max got me straightened out on what kind I need. The sensor will find the side of a bar magnet at 0.6 inches and switch its output from open collector to grounded. When the magnet moves to 1 inch, the sensor will go open collector. Why would I need a "ratiometric" sensor?

 

Hello,

Ever thought of hall sensors used on fan boards?
Here are some pictures of the boards taken from some computer fans:



Bertus

 

Good idea. I can pillage a fan for a Hall sensor and measure the magnet with that. If it has a part number, I might derive what I need to buy.

 

AK and Max got me straightened out on what kind I need. The sensor will find the side of a bar magnet at 0.6 inches and switch its output from open collector to grounded. When the magnet moves to 1 inch, the sensor will go open collector. Why would I need a "ratiometric" sensor?

Because if you're unable to find the proper digital sensor-magnet combination with the adequate sensitivity-field strength ratio to do what you want, then you could use the output of a ratiometric sensor coupled to a comparator that would allow you to calibrate your circuit so that it would behave exactly how you want it to.

 

Because if you're unable to find the proper digital sensor-magnet combination with the adequate sensitivity-field strength ratio to do what you want, then you could use the output of a ratiometric sensor coupled to a comparator that would allow you to calibrate your circuit so that it would behave exactly how you want it to.

Read post #3. I am not designing the circuit, I am trying to buy a replacement part.

 

Read post #3. I am not designing the circuit, I am trying to buy a replacement part.

... post #3 is probably the only post I didn't read....