Review: What On Earth Is An Electromagnetic Radiation Tester And Why Would I Need One?

One of the joys of an itinerant existence comes in periodically being reunited with the fruits of various orders that were sent to hackerspaces or friends somewhere along the way. These anonymous parcels from afar hold an assortment of wonders, with the added element of anticipation that comes from forgetting exactly what had been ordered.

So it is with today’s subject, a Mustool MT525 electromagnetic radiation tester. At a cost not far above £10 ($13.70), this was an impulse purchase driven by curiosity; these devices claim to measure both magnetic and electric fields, but what do they really measure? My interest in these matters lies in the direction of radio, but I have never examined such an instrument. Time to subject it to the Hackaday treatment.

Who Knew Everything Was So Harmful!

Out of the box comes a smart handheld unit with three buttons and an LCD display, and an instruction leaflet. On the rear of the case is a battery compartment, this takes three AAA cells. It claims to measure electric fields from 1 V/m to 1999 V/m, and magnetic fields from 0.01 μT to 99.99 μT, with a bandwidth from 5 Hz to 3500 MHz. Opening the instruction leaflet it becomes clear that this is intended for use in monitoring electromagnetic fields for the purpose of human safety, with a list of conditions from leukemia to blindness that they can cause.

Open it up, and here's what we find.
Open it up, and here’s what we find.

Powering it up gives a zero reading for both measurements when held in free space in this very average house, increasing markedly when held near various electronic appliances. Particularly concerning is a beeper that sounds when either level is considered harmful, which it does in proximity to a horrifying number of items including the laptop keyboard and trackpad this is being written with. As expected the door seal of a running microwave oven generates particularly high readings, as does a Baofeng handheld transceiver at full power in the 430 MHz band. Sadly my CRT TV is in storage at the moment, but I would expect that would also generate high readings.

The sensors for magnetic and electric fields.
The sensors for magnetic and electric fields.

The alarm threshold values for that beeper are 40 V/m and 0.4 μT respectively, and given that it sounds the alarm on such mundane things as my keyboard I have to question what that means. Reading around the subject of typical safe exposure levels it becomes obvious that the alarm is set far too low, and I have to wonder whether it might cause unnecessary worry in a person who had bought this unit because of a personal concern for electromagnetic field levels. Fortunately the beep can be disabled with a long press of a button.

So having described what it does, what’s inside the case? On the back are four small screws, and with those undone it splits into two to reveal a circuit board. At the business end of the unit are the sensors, a metal plate for electric fields and a wire-ended inductor for magnetic ones, and their outputs are each fed to their own TI TLC27M2 dual op-amp. These in turn feed a Weltrend WT56F216 microcontroller which boasts an 8052 core and analogue inputs, that drives the LCD via a Holtek HT1621 LCD controller. It appears well-engineered, and the sensors follow what I would expect for a device measuring these properties.

Does It Have A Use?

The Mustool MT525 electromagnetic radiation tester then. It appears to be a well-engineered instrument that does what it claims, and measures the presence and strength of oscillating electric and magnetic fields. But is it an instrument that’s any use, should I trust it, and does it have a space on my bench? I am curious about its calibration for a start, it’s hardly as though I’d expect any traceable calibration certificate from such a cheap item, but are the V/m and μT figures even in the right ballpark? I am also concerned about the way it and devices like it are sold for health purposes, it worries me that there might be people driven to unnecessary worry by its “Harmful” beeper when in reality they have no need. For myself, I can report that it’s a handy way to check for a field at close quarters, and it works at common amateur radio frequencies, though I can’t really claim it has much use as a bench instrument. There is one thing at which it’s quite handy though, it detects live mains cables buried in walls more reliably than the traditional metal-detector type of wire finder, perhaps that’s where I will use it. For yourselves though, I bought one so you don’t have to.

73 thoughts on “Review: What On Earth Is An Electromagnetic Radiation Tester And Why Would I Need One?

  1. I’d put this in the same bin for brand-new garbage, that already contains the cheap PH meters, $20 oscilloscopes and $1 LiIon chargers.

    Also usable for smart home devices that come with horrendous security holes, never get updates, and stop working when the backyard manufacturer goes out of business.

    1. Yup. More pricey magnetic field sensors seem to at least have a ferrite rod. Used them for checking whether a scanning electron microscope would be happy – they’re particularly sensitive to unwanted magnetic fields (and weighty people jumping up and down next to them even on strong concrete foundations).

      Rather wondering how it measures the electric field – assume it can’t go to DC as it’s not a field mill (just looked at spec – DC not mentioned).

      One for the tin foil hat wearers.

  2. Ooh an accurate wiring detector for £10, many projects that a home DIY person could use that for. The oddly mounted resistor causes concern, but that might be the calibration part.

    1. I’ve already used one to find the source of interferences on wifi.

      Note that these exact same are also sold as ghost detectors. Obviously at a much higher price than when sold as electromagnetic noise detector or similar.

  3. The H field sensor needs to be shielded to discriminate between H and E fields. The arrangement shown sums the two.

    I use a scope with DIY H and E field probes to chase down EMI sources. Very handy.

    Dave on EEVblog did a video on making them.

  4. “There is one thing at which it’s quite handy though, it detects live mains cables buried in walls more reliably than the traditional metal-detector type of wire finder, perhaps that’s where I will use it.”

    I think most already have something like this. Mine looks like a fat marker with a small nib at the end and goes…*BEEP*

      1. I’ve been an electrician for 15 years and the only people I’ve ever heard call it a chicken stick are people who aren’t electricians or old electricians who are unpleasant to work with. And even they usually have one of their own they use.

    1. I used one this week to diagnose the electric starter on my snow blower in order to isolate whether the switch or the starter motor was at fault. No disassembly: just hold the sensor next to the power wires with and without the switch being depressed. Handy and fast.

  5. If you want to be accurate, is there any reason they made it a large box rather than having a small probe or protrusion with the sensing elements in it?
    It seems like this design choice would make it less useful as a wiring detector.

  6. Can’t you use it to scan for ghosts and other paranormal phenomenons? It seems it would be perfect for the job, as long as it blinks, beeps, and shows some numbers, you’d amortise your money from some suckers in minutes.

      1. Yes Egon was a bad a$$ but do you really want to try and quantify relative field strength by how fast a gizmo is spinning and how bright it’s lights are. I mean lcd’s were created for a reason. Yes it looks cool on camera, but is it practical.

  7. Having a field measuring device could be useful, though this seems a bit too crude, and I at least would want some guarantee of any specified accuracy.

    Though, it is fun how many people misunderstand why and how radiation is dangerous.
    A lot of people hear the word “radiation” and think of ionizing nuclear related radiation with severe fallout, even though this is a silly conclusion.

    I can remember back to when some research group “found” indications that a mobile phone had potential for inducing cancer and how the researcher recommended that one doesn’t have the phone close to one’s own body.

    Anyone with actual physics knowledge could fairly easily point out the exaggeration and why it is bogus. (Phone’s do not emit ionizing radiation, though the body will still absorb some of the RF and the result is a bit of heat, though rubbing one’s hands easily generates tens of times more heat on one’s palms, not to mention one’s muscles. Should one stop rubbing hands or going to the gym just because the little bit of added heat might give one cancer?)

    Though, it isn’t uncommon for people to fear or love/idolize what they have a rough understanding of, and usually put of studying the thing deeper in fear of changing their opinion on the matter, though some study deeper together with confirmation bias to strengthen their argument/opinion.

    1. An area that has attracted more than its fair share of bogus myths. Back in a previous reincarnation I was the EMC/EMI/Electromagnetics/SI guru for an international computer company. We had a particular Board of Education for a major US city that had happily bought a lot of our monitors and PCs and then hired a health specialist who convinced them they might be killing the little darlings by exposing them to EM radiation. Spent pointless hours debating with their “health expert” on the subject. His take on EM fields was that if he could detect an EM field then it was hazardous and had to be eliminated from a product. The magnitude of the field was immaterial to him, so arguing was pointless.How he expected electronic devices to operate without accelerating electrons and therefore inevitably generating EM fields I have no idea.
      On which subject, has anyone heard anything recently on the Karolinska Institute and their decades long quest to prove that cellphone radiation is a significant health risk?

      1. Of course, there is a very obvious source of EM radiation in our environment that can cause cancer. And that’s why you should use sun-screen.
        Strangely, people who worry about “EM hazards” don’t seem to be bothered by the sun, despite it being an unshielded fusion reactor.

        1. It’s “natural”, like the radioactive sands at Guarapari beach, used for medicinal mud baths.

          Technically the entire Brazilian coast from Rio-de-janeiro to Bahia is full of the same Monzanite sand, which contains uranium and thorium, yet people freak out about a bit of tritium water at Fukushima.

  8. 5Hz to 3500MHz my foot, the cited input op-amps choke around 1 MHz. As for the “design” of the so-called sensors….The fact that you can get people to buy something as fake as this just confirms that there really is one born every minute.

    1. Yes, going to go out on a limb here and say that the claimed frequency range might be *just* a little optimistic. The opamps aren’t a problem if they’re acting to amplify the detected RF envelope, but I highly doubt that the response is flat over the claimed range.

      I’m thinking this device just possibly might fall into the category of “expensive devices which are sold to unsuspecting people scared of their own shadows”. There’s a whole subculture of folks who suspect “electromagnetic fields” of causing all sorts of symptoms, and who wear things like shielded underwear. If there’s a profit to be made, devices like this will fill the “need”.

    2. “Based on recent election data roughly 50% are or will be suckers.”

      AND the IQ bell curve, although a large portion of those on the right hand side of it don’t take full advantage of their gift because they never acquired the learned skill of critical thought and analysis.

      I believe that so many claim to find science-based courses in school more difficult and avoid them because, unlike their other courses which require only rote memorization, science courses require that skill. If one is never exposed to that requirement, that skill is never learned and what one effectively ends up with is our national idiocracy.

  9. 40 volts per meter alarm threshold? Better not go outside. Plain old sunlight E field is 600 volts per meter.
    0.4 uT alarm threshold? Even staying inside won’t save you. Earth’s field is >30 uT. Sure, it’s DC — As long as you don’t move in it. Or breathe. Or let your heart beat.

  10. This Thing can detect a magnetic field. It has an inductor as sensor and the display says µTesla. That is all legit.

    It can be used to detect if a wire is in your wall before you put a nail in it.

    It can not detect electromagnetic radiation.

    (Besides you would never find harmful levels of magnetic fields anywhere).

    1. Not to defend the accuracy, efficacy or larceny of this device, but:

      An inductor cannot detect a static magnetic field. However, it may produce a voltage signal in the presence of a time-varying magnetic field, a distinguishing feature of electromagnetic radiation.

      The metallic plate on the front of the device will detect time-varying electric potential relative to the “ground” of the device (typically the person holding it). In other words it is an antenna to detect AC electric fields, also a characteristic of electromagnetic radiation.

      Can you explain the basis of your assertion that this device “can not detect electromagnetic radiation.”?

        1. If it is detectable at any distance from the wire, outside the encosure, etc., it is radiating. You might be firmly in the near field, the coupling to 377-ohm free space might be inefficient, but it’s still radiating. If it’s an AC field, and you can detect it without actually touching the conductor, it’s radiating and propagating.

          Keep it in a coax, put it in a Faraday cage etc., then yes, it won’t be radiating, but then there is nothing to detect.

          1. No. There’s a specific meaning to “radiation”.

            Take the classic rope experiment, where you tie a rope from one end to a pole and leave it slack on the ground. When you lift and lower the free end in your hand, that is like a time-varying field. When you flick the rope up and down so waves start to propagate along its length, that is radiation.

            In electromagnetism, radiation generally requires a self-sustaining oscillation through both the electric and magnetic fields, whereas a local field can be entirely dominated by electric charge or magnetic flux, which may change to make it a time-varying field, and it doesn’t mean it will necessarily radiate anything to any significant degree.

            Radiation propagates through space disconnected from its original source. A field does not, although it is often said that a field “radiates” when the speaker means that it extends through space.

            These sensors are able to pick up the local electric or magnetic field, very close up to the source where it is reasonably strong, but they don’t have effective antennas for actual electromagnetic radiation. It’s more likely that they simply reject and reflect it, because the detector is not impedance matched to the propagating medium.

          2. Hey Dude,
            Only because the coupling may be inefficient, a time varying electric field always creates a magnetic field and vice versa. And these fields propagate with c, like paul says. The fields can be dominated with the electric or magnetic component, but as soon it is time varying, you inevitably have both components. The ratio of them is a question of the impedance of the medium.

  11. At one point I was considering getting an EMF meter to measure emissions from the high-power radio/TV emitters on San Francisco’s Sutro Tower (26 megawatts). Having a site survey done by a PE costs $1500, the cheapest meter that is not a piece of junk sold to believers in the paranormal would be the Agilent V3500 ($2000) or the Wandel & Goltermann EMR-300 ($6000)

  12. “Particularly concerning is a beeper that sounds when either level is considered harmful, which it does in proximity to a horrifying number of items including the laptop keyboard and trackpad this is being written with.”

    I am sad that the author didn’t have two of these, in order to test whether or not proximity to a Mustool MT525 would set off the alarm.

    “My cancer-wave tester causes cancer!”

  13. A question that is somewhat on topic: How much $$$ do i need to spend to check that the Wifi uh i mean the radiation from my microwave oven does not exceed safe exposure levels? I would not trust a 10$ device for this, but i can’t (and won’t spend) thousands of $ either. As of today my microwave oven is safe because i didn’t mess with it (and i assume it is well built by default), but let’s assume it breaks and i want/need to fix it, how much $$$ to make sure it still works “savely” afterwards? I am not scared about some tens or hundreds (no idea, never checked the actual values) of Milliwatts from my router, but i am seriously scared of hundreds of Watts(!) of high frequency stuff.

    1. “…but i am seriously scared of hundreds of Watts(!) of high frequency stuff.”

      Why? No, seriously, consider what you’re scared of.

      Take a walk outside or go to the beach on a sunny day. You’ll be absorbing a kilowatt per square meter (or 100 mW/ of power from the sun. That will be absorbed in the first half-millimeter of skin thickness. That’s a power density of 2 watts per cubic cm.

      Note: At the very same time, you, being a warm body, are *radiating* about half that amount of power away to the environment.

      Now, compare with a microwave oven: disable the interlocks, turn it on, and stand a meter away with the door open. You’ll be assaulted by a whopping HUNDRED WATTS PER SQUARE METER of radiation. Or about a tenth of what you get from a sunbeam.

      Further, the microwave energy is absorbed far less in your body meat than the shorter wavelengths of sunlight: you’ll be diluting that power over 50 mm of depth instead of 0.5 mm — a hundred times further diluted. So instead of 2 watts / cubic cm you get from sunlight you’ll a thousandth as much from your seriously compromised microwave oven (0.002 watts / cubic cm). All the while, you yourself are still radiating away hundreds of watts per square meter.

      So why don’t you promptly freeze to death, radiating away so much power? Because your walls, floor, ceiling, and everything else in your environment are ALSO radiating that much power, so you are pretty close to equilibrium with it all.

      Exercise for the skeptic: how much radiated power do you get in your face when you open your conventional baking oven?

      1. Paul,

        to paraphrase you: No, seriously, consider what you’re not sufficiently scared of.

        The issue is not total power, there you’re right: sunlight or even took temperature thermal radiation can reach similar or higher total body exposures than a consumer microwave oven will produce.

        However, we’re not talking about isotropic, homogeneous irradiation here! Microwave ovens create complex radiation patterns with loads of standing waves. Place a dozen shot glasses with a few ml of water each in a microwave and observe. There’s a reason every microwave oven either has a rotating platter or a rotating internal reflector.

        You will get a significant fraction of the power of a microwave oven concentrated in a smallish number of several-cm3-sized hot spots. That’s orders of magnitude higher power densities than you’ll get by insolation. Think roughly focusing the sun on your skin with a magnifying glass.

        Next, your estimates of the penetration depth are off. The dielectric constant of tissue varies from a few to a few tens. That means a significant fraction of the energy can be deposited in the first cm or so. That’s why you have to heat large items slooowly in the microwave. You don’t evenly heat to a depth of 5cm or more, it takes time for the deposited energy to dissipate and the food to thermalise.

        Microwave oven accidents are not the horror story material some make it out to be (and afaik the hard-boiled eyeball stories are incorrect). But they most certainly are a thing! Play around with bare magnetrons all you like, but educate yourself and beware.

        Check out this Wikipedia article, it lays out out nicely:

        1. “complex radiation patterns with loads of standing waves” happen inside a closed microwave oven, and ONLY because the radiation is confined within a small cavity with negligible material to absorb it: the patterns are caused by multiple reflections reinforced by multiple reflections. Add a significant absorber or permit the waves to escape to free space and the standing waves simply don’t exist.

          Of course “You don’t evenly heat to a depth of 5cm” — that’s how absorbers work: the power deposition decreases exponentially with depth, with the characteristic material-dependent parameter “skin depth”. For meat at microwave frequencies, that’s 1.5-2 cm. By 3 skin depths down (~5cm) 95% of the power has been absorbed.

          1. Accidents with consumer microwave ovens occur when the interlock or the choke flange fail.
            In our target audience they also could occur when someone decides to rip a microwave oven apart and play with the magnetron in an unsafe way.

            All situations where either you can gain access to standing wave patterns in the oven’s cavity or are likely to see inhomogeneous patterns due to short distance.

            Further, you correctly point out that the absorption depth will be a lot less than the 50mm you assumed in your original message, and that it will not be homogeneous over that volume but show exponential decay.

            Anyway this can easily lead to scenarios where the situation is NOT, to quote you, “instead of 2 watts / cubic cm you get from sunlight you’ll a thousandth as much from your seriously compromised microwave oven (0.002 watts / cubic cm)”.

            The gut feeling for power densities involved in tinkering with consumer microwave ovens should not be “a thousandth of solar radiation”, but rather “playing with a bunch of Fresnel lenses summing up to half a square meter on a sunny day outside, with your eyes closed”. It will probably be okay, but it’s also not hard to get burnt.

            Please don’t tell someone who’s expressing a healthy respect for several-100W microwave emitters due to a lack of expertise they shouldn’t worry, nothing bad could possibly happen. Tell them to educate themselves and help them doing it.

            Microwaves are not the stuff off “fried eyeballs” horror stories some make them out to be. One can tinker with them safely, YouTube is full of fun crazy examples.
            However, accidents do happen, the medical literature is full of them. Like with so many fun things you do have to know what you’re doing if you want to ensure not getting burnt.

        2. When you play around with that stuff, there is another strong hazard: The high voltage. 2000V with short term at least 2kW from the transformer can very easily kill you. Just be very careful.

    2. Whether it’s the microwave or your wifi, if you’re concerned about the level of EM radiating from it the solution is to put it in a metal box and ground the box. Meshes and perforated metals sheets (like in the window of your microwave) work as well if you’re concerned about specific frequency. On the other hand why trust math just go full metal for safety, because if it’s for safety you’re a monster to say no.

          1. Ah, yes, the Nader who should have been indicted for crimes against humanity for fomenting anti-microwave oven action, slowing its adoption, and thereby indirectly causing human disease, suffering and uncountable deaths.

    3. First off the device is junk. Anything that cheap and from China isn’t worth squat.

      As for your microwave, just keep a one meter distance while cooking your hot pocket(which is probably worse for your health than eating cardboard) and you’re good to go.

      Now if you want to experience some weird sensations, work in a microwave relay site loaded with Light house tubes and Klystrons. It had to be one of the most uncomfortable rooms I ever worked in. In fact none of the techs would ever spend time in there unless they had to.

      Also there are high end systems used by the military that you do not want to be in front of when they are emitting but 99.9% of folks will never encounter them.

  14. Interesting, it cannot be calibrated since it has no x,y,z, sensor and also nothing that classifies as a real antenna. For exposure measurements you also need something to convert the measured field to rms power. The fact that there are two sensors, one for E and one for h field is nice for direct coupling to cables etc, makes it a glorified cable detector.

  15. I have an older analog one put away somewhere. It has two sensitivity modes, one on internal pickup, extra sensitive on external pickup, which has decent thin coax to it. Picked it up cheap at an estate sale. The microwave I had at the time, was either very good, or the meter wasn’t, though it did have fine woven mesh and double gated seals around the door. The only place it picked anything up was by the grille on the back side. It did however seem to pick up the IF on radio receivers. Also the remote pickup is directional and I was able to locate a light switch that was arcing with it and change it out. It also could detect bluetooth devices. I messed around with it for a bit and got bored, but got the impression that it might be useful for finding bugs with.

    I have since seen the same shell, if not the same internals sold as “ghost hunter” equipment, without the external sensor. Don’t know though if it was a knockoff of the original, or they went downmarket when the tooling was paid off, or sold it off.

  16. Or.. might it just be as simple as an wrongfully labeled/translated product description. Don’t have the accurate numbers available and too late/lazy to find them. But it might proof a usefull tool to identity hazards for EPA’s (ESD Protected Areas) so one can determine a specific tool/product can be used or not in such an area.

  17. @Jenny List said: “It appears to be a well-engineered instrument that does what it claims, and measures the presence and strength of oscillating electric and magnetic fields.”

    Howdy. Why do the electric and magnetic fields need to be oscillating? Put a magnet in front of it and see if it can detect a static magnetic field. Rub a baloon on your head and see if it can detect an electrostatic field. I looked at the picture of part of the PCB and I don’t see a diode detector; maybe there’s one underneath the board. Can you provide a reverse-engineered schematic of the front end? If not, can you send it to Big Clive? Thanks.

  18. I’ve occasionally wanted a device like this, more from curiosity than need. What I have instead is a device the designer calls an Elektrosluch, which is an analog EM field to audio converter. They market them to musicians and audio designers for €100, but the design is open source, with schematics and board files on their github,; I built mine on stripboard, mounted in a neon plastic case that originally held a dollar-store first aid kit. The circuit is basically two copies of an inductor, fed into an op-amp — the gain is roughly 1.3k, enough to run earphones — and the inductors are positioned for a stereo output.

    Of course,the Elektrosluch is even less informative with respect to power levels than the Mustool — you can tell whether one field is stronger than another by its volume, and you could probably plug it into a sound level meter if you wanted numbers from that, but it’s inherently not very precise. Nonetheless, it does provide a quick probe. Lots of devices are quite interesting listening; I’m particularly fond of the chordlike sound of a string of WS2812s.

    1. Cool project, despite that it looks a bit pricey at first glance. An interface to a nanoSA or something similar would be nice to broaden its use cases. That would combine the intuitive with the more technial approach in exploring the EM world.

  19. The keywords usually used with this stuff definitely, absolutely include “Radiation”, no doubt to bamboozle some first time buyer who is worried about radiation levels. Little do they know at this point that EM radiation and ionizing radiation are not one and the same, especially in terms of the Internet shopping business. I’m sure you’ll find masses of incorrectly panic-purchased EM meters in households in Japan, gathering dust in a drawer, while in European and English-speaking countries, people worried about cellphone towers and other sources of scary “radiation” likely are also a target for these well-chosen product keywords.

  20. Built a version of this a while back using EPE plans and a sensor removed from a dead VCR capstan motor.
    It certainly worked though never quite determined why the accuracy was terrible.
    Later found out that it was missing a key component that degaussed the ferrite rod(s) or on the original a sanded down iron nail bent into a U shape and attached to each side of the sensor with Epoxy.

    On the flp side its all going in the paper I’m submitting to arXiv as a simple way to verify that my material is indeed superconductive and at >270K to boot. Watch this space!!

  21. I work installing car audio systems, and I think a device like this could be very useful in that line of work. I currently use an old walkman cassette player to detect electromagnetic radiation that could interfere with the fidelity of audio signals, video signals, or radio signals. If you press play on it without a tape in the player and move it close to wiring in the vehicle you can hear the electromagnetic interference through the earphones. I use it when deciding the path I am going to take running a signal cable. I also use it in a circumstance where I have excess antenna cable that I cannot shorten and I must decided what to do with the the excess.

    For only about $13 dollars I am tempted to buy one and see if it would work well for this application. If that doesn’t work out, my girlfriend can use it to make sure our home is free of gosts. It remindeds me of the depiction of a similar device from her favorite TV show Supernatural.

    1. Think of how much EM will come off an electric car with those orange cables about! I think most of your interference problems could be ground loops. I find full transformer isolation works when in a 12 volt system just like it does on stage. Heck I even had to go that way with 3 transformers on my portable lithium-guitar-processor-amp. Whine from the switcher in the processor was a problem on this self contained situation.

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