measuring inductance with a meat-and-potatoes DMM

[Mark Hammer]

Flipping through the catalog for Active Electronics the other day, I noticed that they carry a DMM made by MTP for $70 that includes inductance measurement in its toolbelt of features.


Are there reasons why I might want to avoid this, or is this a sound purchase until it breaks?

[RedHouse]

Originally Posted by Mark Hammer

Are there reasons why I might want to avoid this, or is this a sound purchase until it breaks?

Hey Mark, try a quick search. There has been a lot of threads here this past year on why a standard DMM won't do in this case.

IIRC it's to do with the freq at which the measurement takes place, and someting about the series resistance.

[belwar]

Inductance (L) is frequency dependant. I.e. it changes based on the signal that is fed to the item you are measuring. When a device that measures inductance is turned on, it will send a signal at a specific frequency (Hertz) through the pickup and then the inductance at that specific frequency will be reported back. Knowing the inductance at one specific frequency doesnt tell you very much.. You can really only use that number to check consistance between several pickups. My fear with an item like this is that you dont know what frequency the L is being measured at. It could be 20hz, or it could be 200khz.

I bought an Extech LCR meter thinking I would learn alot about my pickups. I really havent. It measures at 120hz or 1khz which isnt usefull enough to get any good information.. The human ear can hear frequencies between approx 20hz and 20,000hz (20khz) so this meter covers only a small range of that frequency. Honestly I regret buying the darn thing now.

The best device you could get would be one that would give you a bunch of different Inductance readings between 20 and 20000 hz. Then you could plot them on a graph and get some idea of whats going on. I believe you need an occiloscope for that though - Im not an electronics guy so im not sure.

I recently bought an Audio Oscilator that can be manually sweeped from 20-20000hz which I use for determining resonance peak, and my next task is to see if I can use it to determine L.

b.

[Possum]

On the contrary, the Extech should be in every pickup maker's tool box. Its the only thing that spots coil shorts for instance. Even without a higher test frequency its super useful for my work. A frequency sweep is useful but its not so great for giving digital numbers when tiny changes happen in your pickup design. If you are only testing one type of pickup and nothing else, well it might not be so useful, but for pickup makers who make a wide variety of designs as well as proprietary work, I'd be lost without mine, I use it every day. The more you use it the more you'll be using it over time. My first LCR meter was a DMM was a Meterman that had inductance capabilities, but over time it was frustrating to use because it couldn't be set on either test frequency and would switch from one to the other if it was hitting near 4 henries, so alot of information I got wasn't real accurate. The Extech was like opening a magic door for me. I did alot of experiments just to learn about coils, what different types do compared to other types, what different winds do. I did the frequency sweep thing too, but never found them to be all that useful in the long run, plus how you set up your driver coil etc. is real hard to make perfectly the same each time, so you get confliciting results sometimes. I tried Joe's resonant peak setup but found it horribly unreadable most of the time, it just didn't work very good for hand wound coils, because hand winding tends to have real slow peaks, vs. machine wound, the DMM just wouldn't see these slow peaks very well, so it would look like there was a never ending peak.... If anything I would like an LCR meter with a 10khz test signal, at least the ACR would probably be accurate enough for my purposes. They are pricey though but its on my list of tools coming soon.....

[RedHouse]

Originally Posted by belwar

...an Audio Oscilator that can be manually sweeped from 20-20000hz which I use for determining resonance peak, and my next task is to see if I can use it to determine L.

b.

That's the approach I'm taking also. It's way more usefull to plot a curve than have a basic two number reading. Then you can actually see what changes, when making changes, rather than just know that it changed and verify that with one or two readings.

[-Elepro-]

isn't it most simple with white noise and A-B spectrum analyzer?

[salvarsan]

Originally Posted by -Elepro-

isn't it most simple with white noise and A-B spectrum analyzer?

Yes. It is expensive, too.

-drh

[salvarsan]

Originally Posted by belwar

I bought an Extech LCR meter thinking I would learn alot about my pickups. I really havent. It measures at 120hz or 1khz which isnt usefull enough to get any good information.

An LCR meter won't tell you shit about the pickup sound
and anyone here who led to you believe otherwise
should be spanked and send to bed without cookies.

An LCR meter is for quality control on a batch of pickups
made from the same component stock, nothing more.


It will also help you quickly identify defects like
winding cut-throughs and shorts to magnet poles
even when the DC resistance is only 150 ohms low
and still in spec.

-drh

[-Elepro-]

Originally Posted by salvarsan

Yes. It is expensive, too.

-drh

you can with a pc.... try this ..... (last version doesn't work well with my pc...)

[David Schwab]

Originally Posted by -Elepro-

you can with a pc.... try this ..... (last version doesn't work well with my pc...)

Too bad there isn't a Mac version (or even Java). It looks interesting.

[Mike Sulzer]

Originally Posted by belwar

Inductance (L) is frequency dependant.

Inductance (L), Capacitance (C), and resistance (R) are three concepts that describe the behavior of circuits that can be represented by lumped elements.

L does not vary with frequency. However, any real "component" needs to be described by all three concepts. Some useful real components can be described predominantly by one of the three over some frequency range.

Guitar pickups are complicated little beasts. At low frequencies they can be described very well by an inductance in series with a resistance. A good L meter measures both (the low frequency measurement). At higher frequencies, pickups have a complicated impedance. An L meter gives you some information at the higher frequencies, but not really enough to tell you everything about the components necessary to describe a pickup.

[tedmich]

Originally Posted by David Schwab

Too bad there isn't a Mac version (or even Java). It looks interesting.

Mac the Scope may suit you, although its not cheap (officially...)

[voodoochild]

Excuse me for my ignorance but it seems to me that once you understand how everything works together (materials, tension, TPL, etc) you can wind pickups until they sound like you want them to. In regards to testing them why not just read it on a meter for resistance and check the output with a tuning fork. The whole meter with inductance etc, opened a can of worms for me and I ended up ditching it. If you have a short in the coil it won't have enough output or it will read totally off on the resistance. All can be done with a $2 tuning fork and a $20 multimeter IMO. On the other hand I could totally see the use of an inductance meter if you were trying to copy another pickup, active pickups, or rewinding. None of which I do so it pretty much deems itself useless for me.

[David Schwab]

Originally Posted by tedmich

Mac the Scope may suit you, although its not cheap (officially...)

Yeah, I haven't been able to find it, unofficially that is.

[Dmac252]

Certainly off topic here, but I visited your site. I hope not being presumptuous but where do you het your SGD covers made ?

[David Schwab]

Originally Posted by Dmac252

Certainly off topic here, but I visited your site. I hope not being presumptuous but where do you het your SGD covers made ?

I buy my covers from GuitarJonesUSA. Then I have my logo laser etched on them.

[Possum]

Wrong. I have had pickups with coil shorts, or overheated solder joints holding the cable on, they read the right DCR and output was normal. Who checks output anyway if you're shipping boxes of pickups? The ONLY thing that would show a coil short was the LCR meter. The pickup sounded darker than it should have. I don't listen to pickups before they go out, you can't really tell much unless you actually solder them into a real guitar and play it. I discovered the ability of the LCR meter to spot shorts in humbuckers when doing prototype testing. I had a bridge pickup that was sounding nice, good output, but suddenly it was darker than usual, the LCR meter didn't show alot of change in inductance but the ACR had nearly doubled! It wasn't even a coil short, the lead cable had been soldered too many time and the insulation on the inner wire was breaking down (cloth covered stuff). There was NO difference in DCR or anything else, but ACR caught it. I put a new cable on and problem was solved. The same thing for an actual coil short, nothing else will catch it. An LCR meter is a tool and you have to learn how to use it to appreciate it. As I mentioned my first meter that could read inductance wasn't all that useful and was a really limited tool. the Extech is a great tool to learn and use, you'll notice in photos of Bill Lawrence in his ads guess what tool he is using to check his pickup? I keep waiting for one of you guys to buy a meter with 10khz test signal to see how useful they are and what a good brand is Anyway, I'd be lost without my Extech, I've actually had it surgically implanted into my right arm

[voodoochild]

Originally Posted by Possum

Wrong. I have had pickups with coil shorts, or overheated solder joints holding the cable on, they read the right DCR and output was normal. Who checks output anyway if you're shipping boxes of pickups? The ONLY thing that would show a coil short was the LCR meter. The pickup sounded darker than it should have. I don't listen to pickups before they go out, you can't really tell much unless you actually solder them into a real guitar and play it. I discovered the ability of the LCR meter to spot shorts in humbuckers when doing prototype testing. I had a bridge pickup that was sounding nice, good output, but suddenly it was darker than usual, the LCR meter didn't show alot of change in inductance but the ACR had nearly doubled! It wasn't even a coil short, the lead cable had been soldered too many time and the insulation on the inner wire was breaking down (cloth covered stuff). There was NO difference in DCR or anything else, but ACR caught it. I put a new cable on and problem was solved. The same thing for an actual coil short, nothing else will catch it. An LCR meter is a tool and you have to learn how to use it to appreciate it. As I mentioned my first meter that could read inductance wasn't all that useful and was a really limited tool. the Extech is a great tool to learn and use, you'll notice in photos of Bill Lawrence in his ads guess what tool he is using to check his pickup? I keep waiting for one of you guys to buy a meter with 10khz test signal to see how useful they are and what a good brand is Anyway, I'd be lost without my Extech, I've actually had it surgically implanted into my right arm

Thanks for the insight on the Extech Possum. Like I said, please excuse my ignorance. I am in a different situation I guess. I just make pickups for guitars I build. So I can listen to every one I wind. I use a tuning fork before I go to all the trouble of installing it to make sure it has the right output and it has worked for me. Maybe I will give the meter a try again.

[Possum]

No problem, I'm making pickups full time now and have always been somewhat of a technical guy, wanting to understand coils as much as I can with my poor mathematical abilities, so the meter helps. Here's one I've been looking at on Ebay, yeah its Chinese but that doesn't necesserily mean they are bad, maybe:
LCR RCL LC meter TH2821A inductance capacitance, 10kHz - eBay (item 320361219247 end time Jul-16-09 04:35:58 PDT)

[soundmasterg]

Originally Posted by voodoochild

Excuse me for my ignorance but it seems to me that once you understand how everything works together (materials, tension, TPL, etc) you can wind pickups until they sound like you want them to. In regards to testing them why not just read it on a meter for resistance and check the output with a tuning fork. The whole meter with inductance etc, opened a can of worms for me and I ended up ditching it. If you have a short in the coil it won't have enough output or it will read totally off on the resistance. All can be done with a $2 tuning fork and a $20 multimeter IMO. On the other hand I could totally see the use of an inductance meter if you were trying to copy another pickup, active pickups, or rewinding. None of which I do so it pretty much deems itself useless for me.

If you have a short in the pickup, it may not show up on a DC resistance check, but it will show up with an AC resistance check. The Extech checks the ACR in addition to inductance, and capacitance and Q also. These other things that it does are useful to a pickup maker.....its not the holy grail piece of test equipment, but it is useful. Big companies such as Rickenbacker use this same Extech meter to check their pickups.

Greg

[Joe Gwinn]

Originally Posted by Mike Sulzer

Inductance (L), Capacitance (C), and resistance (R) are three concepts that describe the behavior of circuits that can be represented by lumped elements.

L does not vary with frequency. However, any real "component" needs to be described by all three concepts. Some useful real components can be described predominantly by one of the three over some frequency range.

Actually, L does vary with frequency in practical iron-cored coils, and the effect is not so easy to model using discrete R and C elements. Naturally, this effect is caused by eddy currents.

[Mike Sulzer]

Originally Posted by Joe Gwinn

Actually, L does vary with frequency in practical iron-cored coils, and the effect is not so easy to model using discrete R and C elements. Naturally, this effect is caused by eddy currents.

The primary effect of eddy currents in pickup cores is resistive; that is, it increases the loss. As we have discussed, pickup cores act as a loosely coupled secondary, and a good model is an inductor (the so-called leakage inductance) in series with a resistor aross the main pickup inductance. This is a pretty simple model, and it works OK.

[StarryNight]

Originally Posted by -Elepro-

you can with a pc.... try this ..... (last version doesn't work well with my pc...)

this looks really cool. As I've never worked with an oscilloscope before how would one go about capturing and reading an image from a pickup's signal? I have a feeling the answer is going to be - learn how to use an oscilloscope.

[tedmich]

Originally Posted by David Schwab

Yeah, I haven't been able to find it, unofficially that is.

some more potential Mac scope programs to keep an eye out for:
Audio Xplorer (AudioXplorer - Sound analyzer for Mac OS X) does realtime only (?)

Inspector XL, from RNDigital, previously Elemental Audio Systems

Macintosh Music Software: Oscilloscopes (Shareware Music Machine)

Software

MacCRO X 0.1.3 Pre1

Mac Audio Function Generator Software Macintosh

Scope - Oscilloscope Software (Macintosh and Windows) - Research - ADInstruments

[Joe Gwinn]

Originally Posted by Mike Sulzer

The primary effect of eddy currents in pickup cores is resistive; that is, it increases the loss. As we have discussed, pickup cores act as a loosely coupled secondary, and a good model is an inductor (the so-called leakage inductance) in series with a resistor across the main pickup inductance. This is a pretty simple model, and it works OK.

The resistance effects are there for sure, but the inductance does vary because the eddy currents buck part of the magnetic field from the coil, reducing the field energy per amp of coil current. By definition, inductance is field energy divided by coil current.

You can demonstrate this effect with an air coil, a piece of metal, and an Extech. The inductance of the coil does change when the piece of metal is brought near. So does the AC resistance.

[Mike Sulzer]

In the case of a very low loss system, I would believe the Extech reading. With significant losses, no. There is both loss due to the series resistance of the coil and parallel loss due to the mutual impedance effects. You cannot model both correctly with a single amplitude and phase reading.

Your main point above was the frequency dependence of the inductance. One of the simplest case one can think of is a lossless, poorly coupled transformer with a shorted secondary. A leakage inductance appears in parallel with the primary inductance, resulting in a lower overall inductance, but it is not frequency dependent.

Originally Posted by Joe Gwinn

The resistance effects are there for sure, but the inductance does vary because the eddy currents buck part of the magnetic field from the coil, reducing the field energy per amp of coil current. By definition, inductance is field energy divided by coil current.

You can demonstrate this effect with an air coil, a piece of metal, and an Extech. The inductance of the coil does change when the piece of metal is brought near. So does the AC resistance.

[Joe Gwinn]

Originally Posted by Mike Sulzer

In the case of a very low loss system, I would believe the Extech reading. With significant losses, no. There is both loss due to the series resistance of the coil and parallel loss due to the mutual impedance effects. You cannot model both correctly with a single amplitude and phase reading.

The Extech is used by pickup makers precisely because it does handle high loss (low Q) inductors. The Extech works down to a Q of one half, if I recall.

Many years ago we were looking for a meter that would work with pickups. Most handheld LCR or DMM meters expect fairly pure inductors, and only the instruments costing thousands worked. I forget who discovered the Extech, but the specs looked good so I bought one, and verified it against a maxwell wein impedance bridge: Joe Gwinn's website homepage. So far, only the Extech and perhaps one other $200 handheld (made by the same outfit in Taiwan) work on pickups.

Your main point above was the frequency dependence of the inductance. One of the simplest case one can think of is a lossless, poorly coupled transformer with a shorted secondary. A leakage inductance appears in parallel with the primary inductance, resulting in a lower overall inductance, but it is not frequency dependent.

Umm. Just do the test. Then we'll talk.

[jmaf]

Hi, while I don't work with pickups I thought the read here was fascinating. First time I read that L changed with f I thought "oh no, he mixed it up". When Joe Gwinn explained it made perfect sense. So in theory it does not, but in practice L does change with frequency due to imperfections in the core(saturation, which reduces the magnetic capacity of the core), which is news to me.

For my work with output transformers and chokes, what induction meter would you guys recommend? I'd like to measure 50/60 hz chokes, 50/60hz transformers, OT's from 20hz to 20khz. I've owned a cheap DMM before that had L measurement but I blew it up and am now looking for a replacement. Any pointers greatly appreciated. Thanks.

[Mike Sulzer]

Originally Posted by Joe Gwinn

The Extech is used by pickup makers precisely because it does handle high loss (low Q) inductors. The Extech works down to a Q of one half, if I recall.

That is not the point.

A pickup, in particular a humbucker with steel cores, has both series and parallel loss.

A measurement of complex impedance gives an amplitude and phase. Given that the loss is series, one can determine both the inductance and the resistance. Same for parallel. But not if you have both (unless they are both small, and so do not matter). If you have both you can measure at a sequence of frequencies and construct a model that matches the measurements. Two might do it in certain cases, but not in general, especially if you have no control over the two frequencies. If you disagree with this please show me how to get simultaneous series and parallel loss with the Extech. Maybe I am missing something.

What measurement do you want me to make?

[Joe Gwinn]

Originally Posted by jmaf

For my work with output transformers and chokes, what induction meter would you guys recommend? I'd like to measure 50/60 hz chokes, 50/60hz transformers, OT's from 20hz to 20khz. I've owned a cheap DMM before that had L measurement but I blew it up and am now looking for a replacement. Any pointers greatly appreciated. Thanks.

For a handheld instrument costing less than $200 and measuring R, C, and L at 120 Hz and 1000 Hz, it's hard to beat the Extech LCR Meter 380193:

Extech Instruments: Specialty Meters: Specialty Meters: 380193

The next step up in capability costs thousands.

[Joe Gwinn]

Originally Posted by Mike Sulzer

A measurement of complex impedance gives an amplitude and phase. Given that the loss is series, one can determine both the inductance and the resistance. Same for parallel. But not if you have both (unless they are both small, and so do not matter). If you have both you can measure at a sequence of frequencies and construct a model that matches the measurements. Two might do it in certain cases, but not in general, especially if you have no control over the two frequencies. If you disagree with this please show me how to get simultaneous series and parallel loss with the Extech. Maybe I am missing something.

Several things have become tangled together. Let's tease them apart:

1. I agree that in practical inductors, there can be both series and parallel resistance components, due to different physical effects, and that a two-frequency measurement of complex impedance cannot tease these apart except in unusual circumstances. However, in practical inductors, the parallel resistance is usually very large, and may be ignored, and the parallel capacitance (the self-capacitance) has no significant effect at 1000 Hz, given that self resonance is around 10 KHz. One measures self-capacitance most accurately by finding the self-resonant frequency with various external capacitors, and solves for the extra capacitance needed to make it all work out. (This method is very old, and I first saw it in Terman.)

2. Bringing a mass of non-magnetic electrical conductor, like brass, copper, or aluminum, near to a coil will reduce its inductance due to eddy currents counteracting coil currents.

A magnetic conductor such as steel will have permeability increase and eddy decrease in opposition, and things quickly become complex. Laminating the iron greatly reduces the eddy currents, causing the permeability effect to predominate, up to some frequency determined by the thickness of the laminations.

For a datapoint, variable-inductance RF coils can be ordered with a variety of core materials, including ferrite, powdered iron, or nonferrous (brass or aluminum). The ferrous cores increase the inductance, while the non-ferrous cores decrease the inductance.

Aside from eddy currents, the inherent permeability of most core materials varies with frequency, sometimes in complex ways.

3. Because eddy currents vary with frequency, so will the inductance of a coil with nearby metal, even if the geometry of coil and metal does not change.

What measurement do you want me to make?

Take an air core coil, hook it to an Extech LCR meter set to 1000 Hz, and bring a sheet or rod of brass or aluminum nearby or into the core hole. Observe how inductance and AC resistance react to the metal being brought near.

[jmaf]

Originally Posted by Joe Gwinn

For a handheld instrument costing less than $200 and measuring R, C, and L at 120 Hz and 1000 Hz, it's hard to beat the Extech LCR Meter 380193:

Extech Instruments: Specialty Meters: Specialty Meters: 380193

The next step up in capability costs thousands.

Thanks Joe. That is exactly what I was looking for.

[Mike Sulzer]

Originally Posted by Joe Gwinn

Several things have become tangled together. Let's tease them apart:

1. However, in practical inductors, the parallel resistance is usually very large, and may be ignored...

The humbucker pickup coil, with its steel cores, is an exception. Its impedance can not be modeled with just an L, C and series R. It is necessary to include the parallel loss from eddy currents induced in the cores, and furthermore it is necessary to account for the imperfect flux coupling to the cores with usual inductor in series with the parallel loss. Although an Extech is a useful tool for a pickup maker, it cannot do what I want.

Originally Posted by Joe Gwinn

2. Bringing a mass of non-magnetic electrical conductor, like brass, copper, or aluminum, near to a coil will reduce its inductance due to eddy currents counteracting coil currents.

But can the Extech measure this accurately? Not if the parallel loss caused by the eddy currents is significant and there is significant series loss. So if you see the inductance and ac resistance change, you cannot be sure either change is correct because you might simultaneous significant series and parallel loss.

Originally Posted by Joe Gwinn

Aside from eddy currents, the inherent permeability of most core materials varies with frequency, sometimes in complex ways.

Yes, and this might matter in a humbucker coil. I was recently reading a paper showing that steel (1018, I believe) shows a loss of permeability starting at about 1 KHz. The permeability is also complex; the induced magnetization lags the applied field.

Originally Posted by Joe Gwinn

3. Because eddy currents vary with frequency, so will the inductance of a coil with nearby metal, even if the geometry of coil and metal does not change.

But which is cause and which is effect? In the pickup model, the eddy currents decrease at high frequencies because the impedance of the inductor (in series with the loss resistor, included to account for the flux leakage) increases. It is not so simple in general.

Originally Posted by Joe Gwinn

Take an air core coil, hook it to an Extech LCR meter set to 1000 Hz, and bring a sheet or rod of brass or aluminum nearby or into the core hole. Observe how inductance and AC resistance react to the metal being brought near.

I do not own an Extech, but it does seem like a useful tool. It would be interesting to compare its measurements with the system I have.

[Joe Gwinn]

Originally Posted by Mike Sulzer

The humbucker pickup coil, with its steel cores, is an exception. Its impedance can not be modeled with just an L, C and series R. It is necessary to include the parallel loss from eddy currents induced in the cores, and furthermore it is necessary to account for the imperfect flux coupling to the cores with usual inductor in series with the parallel loss. Although an Extech is a useful tool for a pickup maker, it cannot do what I want.

But can the Extech measure this accurately? Not if the parallel loss caused by the eddy currents is significant and there is significant series loss. So if you see the inductance and ac resistance change, you cannot be sure either change is correct because you might simultaneous significant series and parallel loss.

Yes, and this might matter in a humbucker coil. I was recently reading a paper showing that steel (1018, I believe) shows a loss of permeability starting at about 1 KHz. The permeability is also complex; the induced magnetization lags the applied field.

But which is cause and which is effect? In the pickup model, the eddy currents decrease at high frequencies because the impedance of the inductor (in series with the loss resistor, included to account for the flux leakage) increases. It is not so simple in general.

This all reminds me of my first attempt to design a one-transistor amplifier in the early 1970s, when the ink on my EE degree was still damp. Despite taking all the usual and necessary courses, I couldn't design that damn amplifier. For one thing, my textbooks wanted ten or twenty device parameters, but the datasheet for the ten-cent jellybean transistors everybody used specified only two or three parameters, and yet life went on and amplifiers were built. Without angst. Very perplexing. And embarrassing. So I quietly scuttled over and looked at the books the electronics technicians were using, and borrowed a copy. Then it became clear. All but a very few of those parameters were for minor effects, and could be ignored in safety. It also became clear that my professor had never built anything, and didn't know big from small.

I do not own an Extech, but it does seem like a useful tool. It would be interesting to compare its measurements with the system I have.

I would recommend that you buy one. Failing that, the maxwell wein impedance bridge (look here) will duplicate the Extech results to within 0.1% (or the accuracy that you can measure R and C values, whichever is worse). The Extech is a lot faster, but operates at only two frequencies. The bridge is slow, but will work at any frequency, although the answers get strange as resonance is approached.

[Mike Sulzer]

For a humbucker coil, these effects are large, not small. That is why I have taken the trouble to put together a system that collects enough information so that one can model them. The evidence is in the earlier thread.

Originally Posted by Joe Gwinn

This all reminds me of my first attempt to design a one-transistor amplifier in the early 1970s, when the ink on my EE degree was still damp. Despite taking all the usual and necessary courses, I couldn't design that damn amplifier. For one thing, my textbooks wanted ten or twenty device parameters, but the datasheet for the ten-cent jellybean transistors everybody used specified only two or three parameters, and yet life went on and amplifiers were built. Without angst. Very perplexing. And embarrassing. So I quietly scuttled over and looked at the books the electronics technicians were using, and borrowed a copy. Then it became clear. All but a very few of those parameters were for minor effects, and could be ignored in safety. It also became clear that my professor had never built anything, and didn't know big from small.

I would recommend that you buy one. Failing that, the maxwell wein impedance bridge (look here) will duplicate the Extech results to within 0.1% (or the accuracy that you can measure R and C values, whichever is worse). The Extech is a lot faster, but operates at only two frequencies. The bridge is slow, but will work at any frequency, although the answers get strange as resonance is approached.