How do you calibrate a hall probe?

[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?

 

[Gokul43201]

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.

 

[westerman16]

the lab is basically a school lab

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

 

[ZapperZ]

the lab is basically a school lab

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

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.

 

[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 got to 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.

pleasez help me :)

 

[Gokul43201]

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

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.

 

[westerman16]

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.

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??

 

[briton]

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??

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?

 

[westerman16]

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?

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

 

[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

 

[Gokul43201]

"i" is the applied longitudinal current.

 

[westerman16]

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

 

[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.

 

[ZapperZ]

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.

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.

 

[Gokul43201]

So unless there's a space large enough in between the wound coils to insert the probe, this may be a problem.

Zz.

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.

 

[ZapperZ]

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.

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.

 

[Gokul43201]

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 ?

 

[Chi Meson]

Is the physics for the magnetic field of a solenoid known at this level?
Zz.

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?

 

[ZapperZ]

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 ?

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.

 

[ZapperZ]

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?

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.

 

[Gokul43201]

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.

The few that I've come across are accurate to only about 1 mT, so they wouldn't work. But I'm not really aware of what exists now or what one might find in a high school lab, so I hand-waved over that "little detail".

So, I'll revert to the two solenoid suggestion, unless, as Chi suggests, the Hall bar at the end can be bent perpendicular to the stick.

 

[westerman16]

Thanks for all your help, i can do it now. I ran the calibration idea with my teacher and it was all good.

using a solanoid to calibarate the probe, using the equation, then plotting the points

cheers guys

 

[ipknightley]

Just for reference, approximately how strong would the magnetic flux density be for an average bar magnet?

 

[kiwi3007]

As a hint guys, try reserching Helmholtz coils and use the equation ((4/5)^3/2)(Uo n I/ R)
http://en.wikipedia.org/wiki/Helmholtz_coil

 

[Vanadium 50]

This is a 5 year old thread.

While it's not clear what is available to the OP, the way I would attack this is not to try and use a "known" magnetic field, but rather to use whatever field is at hand and an NMR probe. Those are absolutely calibrated.

 

[kiwi3007]

This is a 5 year old thread.

While it's not clear what is available to the OP, the way I would attack this is not to try and use a "known" magnetic field, but rather to use whatever field is at hand and an NMR probe. Those are absolutely calibrated.

Sorry to revive such an old thread, I didn't realize, but OCR A2 students are currently doing a piece of coursework which this threat is very relevant to. I figured I would share what I have found out with people, I found out that using the Helmholtz coils are better than using a solenoid.

Even if a Helmholtz coil is not avaliable, they are not to difficult to set up.

 

[Bob S]

Here is an entirely different way to calibrate the magnetic field in a Helmholtz coil.
This method uses an integrating opamp and a coil inserted in the magnetic field to integrate the field when the coil is flipped end-to-end. Use an integrating coil (flip-coil) which is based on Faraday’s law in integral form:

∫V·dt = -N·B·A volt-seconds

Here A is area of coil, N is number of turns, B is magnetic field, and ∫V·dt is the output. The coil is placed in the magnetic field B, and the coil is flipped (end-to-end), so the total volt-seconds is 2·N·B·A. Alternatively, for a Helmohtz coil, the current can be turned off or reversed, but this will not measure the Earth’s field contribution to B.

A circuit for measuring the volt-seconds is shown in the attachment. For the circuit shown, the opamp output voltage Vout is, using standard opamp equations and Faraday’s Law (above):

Vout = [1/(R1·C1)] ∫V·dt = - N·B·A/(R1·C1) volts (see attachment for R1 and C1)

Example:
Wind 500 turns of thin (30 Ga. Or thinner) enameled copper wire on a 3-cm diameter coil form. For a 100 Gauss field, NBA = 500 turns x 0.01 Tesla x 7.069 x 10^-4 m² = 3.53 x 10^-3 volt seconds.

If R1 = 10 k, and C = 0.1 uF as shown in the attachment (R1·C1 = 0.001 seconds),

and if the coil is flipped in a 100-Gauss field (Helmholtz coil field), the voltage output of the circuit is

Vout = 2 N·B·A/.001 = 7.06 volts, or to measure the field:

B = R1·C1·Vout/(2·N·A) Tesla

An operational amplifier with a low input offset voltage and a low input offset current should be used. The integrating capacitor should be a high quality, bipolar, low leakage capacitor. If necessary, the opamp leakage current can be nulled using a 10-meg resistor into the summing junction from a potentiometer -1 volt< V <+1 volt.

For general integrator construction, use a DIP package from Analog, National, or Linear with low voltage and cbias current offsets and low temperature drift characteristics.

This is an absolute magnetic field calibration, and depends only on the size of the integrating coil NA, and the R1 C1 product.

Bob S

[added] The resistance of the flip coil should be included in the value for R1. For the example I gave above, a 30-Ga wire coil would add about 15 ohms to R1. R1 is selected to equalize the effect of the input offset voltage and the input offset current in a typical opamp. Both can be zeroed out with a potentiometer.