# How do you calibrate a hall probe?

1. Apr 6, 2005

### westerman16

For a planning exercise i have to calibrate a hall probe to measure the magnetic flux density mid-way between opposite poles of two permanent bar magnets.

anyone know how to calibrate a hall probe so that i can do this experiment?

2. Apr 6, 2005

### Gokul43201

Staff Emeritus
This can vary depending on your experimental set-up.

The basic idea is to measure hall voltages in known fields. Depending on what your lab works with you could do this in your magnetometer, susceptometer, electromagnet or other such.

3. Apr 7, 2005

### westerman16

the lab is basically a school lab

i think i have to use helmz colt coils to calibrate to probe but im not sure how you do this. does anyone know what to do

4. Apr 7, 2005

### ZapperZ

Staff Emeritus
So you don't have a "standard magnet" around for such calibrations?

If you use a helmholtz coil, you need to know the details (and the physics) of such a geometry, such as what B-field corresponds to what current along the axis of the coil. In fact, your coil would have to be verified first with a Hall probe (hopefully a calibrated one).

Zz.

5. Apr 7, 2005

### westerman16

i guess i havnt put this very well. iv got a planning exercise to complete init it says.

1 calibrate an uncalibrated hall probe to measure magnetic flux density

2 use the calibrated hall probe to investigate how the magnetic flux density mid-way between opposite poles of two permanent bar magnets varies with the separation of the bar magnets.

and basically these are the things you gotta consider
-how the hall probe will be calibrated, including details of the additional equipment required

-how the magnets would be mounted for the investigation

-the procedure to be followed for the investigation

-any particular features of your design that may ensure the accuracy of your experiments

so anyone can help with any of this, especially the calibration of the probe?

basically i have to find the magnetic flux density between two bar magnets, and the magnets have to be mounted on something to hold them in position.

plzz help me :)

6. Apr 8, 2005

### Gokul43201

Staff Emeritus
This is key to the calibration. You need a calibrated adjustable magnetic field. There are several kinds of "equipment" that can do this very accurately...none of which are likely to be found in the average high school lab.

For lower precision, you can get by with a regular solenoid (or toroidal) electromagnet. You could even (if you're lucky) do a one-point calibration on this magnet to roughly within the error in the known value of the local terrestrial field strength (~0.4 Gauss ?) using something as simple as a compass.

7. Apr 9, 2005

### westerman16

ok i understand what your saying

so if i have a solenoid electromagnet, to calibrate the probe do i simply place it in the magnetic field?

surely i have to take some sort of readings so i can calibrate it to measure the magnetic flux density between two bar magnets??

8. Apr 10, 2005

### briton

yeah I'm doing this too (for OCR).

I think you measure the pd at the known magnetic field strength, the pd is "Hall pd". and you can use $$B \ = \ \frac{B_0}{V_0} V$$ where Vo and Bo are what you've just measured.

is this correct?

9. Apr 10, 2005

### westerman16

iv seen that equation befor so i think your on the right lines, ill have a look into it and speak to my teacher tomora too

10. Apr 12, 2005

### 98chilljb

hey dudes, like we're doing the same thing too. thanks for your help we didn't have a clue man. we found these equations, may help you some more:

V=AKiB
where V is the amplified hall voltage
A is the area
K is a constant
B is the flux density perpendicular to the magnetic field
still don't know what i is

11. Apr 12, 2005

### Gokul43201

Staff Emeritus
"i" is the applied longitudinal current.

12. Apr 12, 2005

### westerman16

im still really unsure if theres some super clever physics people around, hint hint, to come and help us

13. Apr 13, 2005

### Chi Meson

All those guys are indeed super clever, but it seems you are looking for a more "high school" way of doing this:

Use the solenoid while recording the current through it and the reading from the hall probe ("V"). Calculate the magnetic field as a function of the current through the solenoid. Take at least five data points (more is better) and plot B vs. "V" . make a best fit line from this graph.

voila, you have a calibration. Whatever the "V" reading is on the hall probe, find what B it corresponds to on the graph.

14. Apr 13, 2005

### ZapperZ

Staff Emeritus
Is the physics for the magnetic field of a solenoid known at this level?

I also see a possible problem. If you have a tightly-wound solenoid (which is what should be used), then the hall prove needs to be perpendicular to the B-field for it to correctly sample what the calculation is predicting. Most simple hall probe that I've seen are mounted at the end of a long stick and parallel to it. So this means that the probe will be positioned almost parallel to the B-field, which will cause it to not sample the true max. value of the field. So unless there's a space large enough in between the wound coils to insert the probe, this may be a problem.

Zz.

15. Apr 13, 2005

### Gokul43201

Staff Emeritus
I too was concerned about this. They may have to make do with the field at the end (not the center) of the solenoid...or perhaps jury rig a pair of identical solenoids and hold them end to end with a little gap inbetween to stick in the probe.

16. Apr 13, 2005

### ZapperZ

Staff Emeritus
Just don't forget to check the polarity of the the two solenoids, that's all I will advice! :)

I think this is why I didn't follow up on my initial comments. After I discover that this is high school level project, I became unsure of how much knowledge is "known" that can be used as a calibration device. If the students know about solenoids and know about the field profile in it as a function of location and current, then sure, this would be possible. If not, then do you just do this for them? Or is this part of the exercise?

... or I can just loan them a standard magnet and get it over with. :)

Zz.

17. Apr 13, 2005

### Gokul43201

Staff Emeritus
I have an idea !

Since it's only the normal component of the B-field that matters (if the probe is sensitive in the < 1 G regime), a reasonable calibration can be achieved against the normal component of the terrestrial field, by tilting the plane of the hall bar and measuring ambient signals as a function of $sin \theta$.

Thoughts, Zz ?

18. Apr 13, 2005

### Chi Meson

Only the "infinite" solenoid formula: B = (Mu) (N/L) I where N/L is the number of turns per unit length. It's a half decent approximation of the field and good to two sigs at the very center.

And we have a Hall probe that is bent 90 degrees at the end. I'm certain this affects the reading, but as you noted, how else are you supposed to get it in the solenoid. This probe is an old "Thornton" . I haven't seen this brand in a while. I just looked into the Pasco catalog, and those probes seem to be encased in a plastic tube. Hmm... Is there a reason why you shouldn't bend them?

19. Apr 13, 2005

### ZapperZ

Staff Emeritus
That's an ingenious idea. But what do you use as the value of the maximum B field? Someone must have measured the magnetic field at that location/area/city already. I'm also afraid that, considering this is a high school instrument, it may not have that sensitivity - or maybe it had, at one time, but not anymore.

Zz.

20. Apr 13, 2005

### ZapperZ

Staff Emeritus
Yeah, that expression should be sufficient for a "long" solenoid. As for the probes, there ARE those that you shouldn't bend, but I'm not sure if the Pasco ones fall into this category. I know that the one I have should not be bent (or even pressed). However, I think the standard lab equipment ones should be sufficiently robust (but this is purely a guess).

Zz.