# Detecting coils inside steel cylinders

## Summary:

If I have a coil inside a piece of steel pipe, can I detect it from outside of the pipe using a tuned transmitter / receiver array?
Just some idle Sunday musings. I have a steel pipe that is 2" ID, 2.5" OD, 10' long. I want to place a coil inside the pipe midpoint along the pipe and detect it's presence with a transmitter / receiver array. The array can consist of two coils wrapped around the outside of the pipe and can be adjacent to the inner coil.

Can I detect the inner coil?

## Answers and Replies

anorlunda
Staff Emeritus
Is the inner coil just sitting there, or is it connected to anything?

Baluncore
I think you must say what you mean by a "coil".
Is it an LC circuit, or just a short coil of insulated wire with open ends?

DaveE
Gold Member
I would look at something like TDR. Maybe an ultrasonic wave launched from the end of the pipe. Or maybe something like tapping along the length and analyzing the vibrations (this is how I used to find studs underneath drywall).

Twigg
Gold Member
If the inner coil is open-ended, I think you are SOL for detection with an outside coil.

If the inner coil is connected to some circuit, the simplest method would only involve one outer coil. With one outer coil, a pipe, and one inner coil connected to some circuit, you have a stray-field transformer. You can measure the position of the inner coil (in one dimension only) by measuring the impedance across the outer coil. When the inner coil is infinitely far away, you would measure only the base reactance of the outer coil; when the inner coil is close, you would measure additional impedance from the inductively coupled inner circuit. I can't comment of the feasibility of this method because there are too many free parameters here.

DaveE
Gold Member
OK how about a single coil that you resonate with a capacitor (probably best at low frequency). Using a signal generator/analyzer you could look at the Q as the coil is scanned down the pipe. Like a metal detector, sort of.

Twigg
Is the inner coil just sitting there, or is it connected to anything?
The inner coil is not connected to anything except itself.

I think you must say what you mean by a "coil".
Is it an LC circuit, or just a short coil of insulated wire with open ends?
Coil can have as many turns as necessary and the ends of the wire connected, i.e. LC circuit.

Is the inner coil just sitting there, or is it connected to anything?
Just sitting there. Once installed there is no physical access. What I really want to know is if the coil loses integrity.

OK how about a single coil that you resonate with a capacitor (probably best at low frequency). Using a signal generator/analyzer you could look at the Q as the coil is scanned down the pipe. Like a metal detector, sort of.
I cannot connect any electronics to the inner coil. I was thinking of inducing a current in the coil and detecting the field with the second coil. But as this is inside a steel pipe, can I do that?

If the inner coil is open-ended, I think you are SOL for detection with an outside coil.

If the inner coil is connected to some circuit, the simplest method would only involve one outer coil. With one outer coil, a pipe, and one inner coil connected to some circuit, you have a stray-field transformer. You can measure the position of the inner coil (in one dimension only) by measuring the impedance across the outer coil. When the inner coil is infinitely far away, you would measure only the base reactance of the outer coil; when the inner coil is close, you would measure additional impedance from the inductively coupled inner circuit. I can't comment of the feasibility of this method because there are too many free parameters here.
I considered stray field transformer, but I don't have access to the inner coil. The position of the inner coil isn't important, it's integrity is. I need to know if the inner coil fails.

Twigg
Gold Member
For my earlier transformer suggestion, you only need access to the outer coil to measure its impedance. The inner coil would in theory add to the outer coil's impedance through the inductive coupling. However, if the inner coil is just shorted together I doubt the change in the outer coil's impedance will exceed your measurement uncertainty. Just a hunch, could be wrong. Also, if the pipe is a ferromagnetic steel (i.e. anything you'd usually encounter that isn't 300 series stainless), you're probably SOL because that will butcher any kind of inductive coupling with huge stray fields.

Baluncore
Also, if the pipe is a ferromagnetic steel (i.e. anything you'd usually encounter that isn't 300 series stainless), you're probably SOL because that will butcher any kind of inductive coupling with huge stray fields.
What does "SOL" mean ?

DaveE
Gold Member
I cannot connect any electronics to the inner coil. I was thinking of inducing a current in the coil and detecting the field with the second coil. But as this is inside a steel pipe, can I do that?
The idea is that the external excited coil will detect the metals in the environment, mostly by eddy currents or a change in the magnetic properties. Highly resonant circuits (high Q) can be quite sensitive to parameter changes. A change in the magnetic field distribution could change the inductance and shift the resonance. A change in losses from eddy currents could change the width of the resonance (Q). However, I'm not sure how well this will work in your case. You would have to experiment a bit to figure out feasibility and the best set up. You'll probably need someone to help with the electronics if you aren't already familiar with LC circuits.

Staff Emeritus
SOL = So Out of Luck.

hutchphd
Staff Emeritus
You have a coil with inductance L capcitatively coupled to the tube. Can't imagine that this will be very high Q, which means that it can't be sensitive to the exact configuration, which is what we want, isn't it?

Baluncore
Coil can have as many turns as necessary and the ends of the wire connected, i.e. LC circuit.
I think you have overgeneralised the problem statement, to the point of impossibility.
Why do you need to place a coil there in the first place ?
Why do you need to know where it is ?
What flows through the pipe ?
Why not put a permanent magnet there instead, then you can find it with a magnetic compass from outside the iron pipe.

DaveE
Gold Member
You have a coil with inductance L capcitatively coupled to the tube. Can't imagine that this will be very high Q, which means that it can't be sensitive to the exact configuration, which is what we want, isn't it?
No, I'd have a coil and a capacitor configured as a high Q tank. Then as the coil is brought up close to the tube I would evaluate how the inductance and/or losses vary along the pipe. Still, it may not be sensitive enough to detect extra metal bits inside the pipe. Similar to a metal detector. I'm not sure if it would work or not. You'd need some good instrumentation, something like a network analyzer or a lock-in amplifier; at least until you figured out the signature.

However, I still prefer the TDR idea. Like launch and EM-wave down the tube and look for reflections. Or the frequency domain version where you send CW uwaves into the pipe and look at the frequency response of the impedance. If the coil is big enough it may look like a stub, or such.

Or, probably my favorite: fasten a microphone to the end of the pipe, connected it to an oscilloscope. Then you hit the end of the pipe with tiny hammer (i.e. delta function excitation) and look for sound reflections from discontinuities. It's cheaper than waveguide uwave stuff and sound moves slower.

Then there's the idea of putting a long stick down the tube and measuring it when it hits something. I suspect that has already been ruled out though.

Summary:: If I have a coil inside a piece of steel pipe, can I detect it from outside of the pipe using a tuned transmitter / receiver array?
Can I detect the inner coil?
As far as I know, stud finder uses capacitive coupling, while metal detection uses inductive coupling. Therefore, it is possible to detect the coil in the pipe by placing another coil array at a very close distance outside, but this may not be an easy task. You may need to use quite complex electronic circuit to increase the sensitivity of the outer coil array.

Baluncore
The problem with a resonant circuit inside the pipe is that the skin effect on the outer wall of the pipe will prevent communication. You need very low frequencies to get through a thick metal screen. To have a sufficiently low frequency the coil will need a huge number of turns in parallel with a huge capacitor.

At the same time, if the coil was coaxial within the pipe, the inner wall of the pipe will have an induced counter-current that magnetically cancels the current in the coil.

hutchphd and alan123hk
Once installed there is no physical access. What I really want to know is if the coil loses integrity.
That coil... Is it your own making? Does it have some other function than being there?
More details about the actual goal would be useful.

If that coil is your own making then you might consider making some mechanical energy to penetrate the tube instead of magnetic field. With good orientation and an additional magnet you can set the tube vibrate from the outside with that ' tuned transmitter '. With carefully considering the orientation of the coil and with a small magnet you can have AC voltage there. Add a diode bridge, and you have AC with doubled frequency: if you transmit that outside (mechanically) it might be picked up.

the skin effect on the outer wall of the pipe will prevent communication. You need very low frequencies to get through a thick metal screen
Agree, the use of low frequencies is the key. However, low frequency means high inductance, but I think it may not be allowed to insert the magnetic core into the coil inside the pipe because I believe that for some purposes, something must pass through the pipe.

DaveE
Gold Member
Agree, the use of low frequencies is the key. However, low frequency means high inductance, but I think it may not be allowed to insert the magnetic core into the coil inside the pipe because I believe that for some purposes, something must pass through the pipe.
Yes, I think putting things in the pipe isn't allowed, or else it would be a tape measure you would put in there.

I was thinking you could have a coil with a core outside on a radial axis.

However, I just reread the OP and he said two coils wrapped around the outside as a transmitter and receiver. That seems hopeless to me given the close magnetic coupling to the pipe, in the transformer business we would call this a "shorted turn". Too bad the pipe isn't plastic, then it sounds similar to an LVDT.

I think you have overgeneralised the problem statement, to the point of impossibility.
Why do you need to place a coil there in the first place ?
Why do you need to know where it is ?
What flows through the pipe ?
Why not put a permanent magnet there instead, then you can find it with a magnetic compass from outside the iron pipe.
A little more detail of the problem.
The pipe has an inner pipe inside it that is carrying high pressure fluid. The high pressure fluid will corrode / erode the inner pipe and eventually break through to the outer pipe. What I want to do is wrap a coil around the inner pipe (ferrous steel) so that when breakthrough occurs the coil is cut and the system suffers loss of signal, which is detected from outside the outer pipe. The coil doesn't carry any information except that the coil has integrity. The coil failure is the signal.
Because of the nature of the system, I don't have any method of direct contact with the inner coil after initial assembly. An LP circuit could work and low frequency would be the only way through the skin effect. The capacitor is going to be an interesting challenge, as the pipe will be running at temperatures up to 250 deg f.

In such system the breakdown of the inner tube would not necessarily cause the coil to fail. I think you should go for detecting the pressure change between the tubes.
Maybe some airtight capsule with set (low) pressure inside. That would be safely crushed if the external pressure rises.
I would opt for some ultrasonic solution. Magnetic and steel is just not some simple thing.

DaveE