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I teach introductory physics labs, and inevitably when we do our labs on introductory circuits or mapping electrostatic potentials, there are one or more multimeters whose batteries die or fuses blow. This is always something of a hassle, since you have to find a screwdriver, undo the screws, open the case, swap out what actually needs to be swapped out, and refasten the screws. It would be simpler if the batteries and fuses were in a separate compartment that could be opened easily, the kind that slides open/closed and latches shut when in place.

Why are multimeters designed this way? I can think of a few possibilities:

  1. There is some reason, related to how a multimeter works, that means that the batteries/fuses need to be very securely inside the case, and which a separate compartment with an access door would not satisfy. But I have no idea what this reason would be.

  2. It would cost more to design & manufacture multimeters with this extra compartment. Perhaps, but multimeters are already pricey enough that I have a hard time believing that it would make a significant difference in the purchase price.

  3. It's always been done this way. What's more, people who use multimeters are already the types of folks who are used to opening up the case of a malfunctioning device with a screwdriver, and in fact find some pride & enjoyment in doing so. So there is an active incentive for manufacturers to design multimeters this way. This requires a bit more armchair psychoanalysis of multimeter users than I'm comfortable with, though.

Does one of the above reasons explain it sufficiently? Or are there other factors that I'm not thinking of?

Michael Seifert
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  • I don't know if this question is quite appropriate for this Stack, but I figured I'd give it a shot here. If there are other locations where it'd be more suitable, I'm all ears. – Michael Seifert Jan 21 '21 at 16:53
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    multimeters are used by not just scientists and engineers, but also in places where you throw the tools in a box, throw the box in the back of the truck, and if it breaks, that's life. (I do not advocate this philosophy). Anyway you get the rubber bumpers and molding, and maybe the secure battery compartment is part of that. – Pete W Jan 21 '21 at 16:59
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    Students are not the typical user. A typical user blows the fuse a few times, then will be more careful. I haven't blown my DMM fuse in 30 years. – Mattman944 Jan 21 '21 at 16:59
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    They're also used outside sometimes in the rain, and it also helps prevent water ingress. – John D Jan 21 '21 at 17:00
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    Cost. That's usually the answer to why people make apparently strange design choices. – Neil_UK Jan 21 '21 at 17:01
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    Think about **SAFETY**, while you're measuring mains voltage, the battery door pops out for some reason. The batteries will then be **at mains level voltage** and you would not want to touch them. – Bimpelrekkie Jan 21 '21 at 17:02
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    What multi-meters are you using that don't have a separate screw compartment for the battery? The Agilent meters and Flukes I have seen all have them. But even these require removal of the case for the fuse (but that is probably for high voltage safety reasons). So I am pretty sure the issue is cost. By the way, what is your definition of an expensive meter? 100+? Or $500+? Because $100 is cheap. – DKNguyen Jan 21 '21 at 17:17
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    keep the screwdriver, fuses and batteries together – jsotola Jan 21 '21 at 17:28
  • @jsotola I have a multimeter at work with two fuses I found in a random box for it taped to it because if they get separated, never again shall they cross. – DKNguyen Jan 21 '21 at 17:31
  • With the nicer meters it's also worth noting they have electronic circuit protection that protects the meter without blowing a permanent fuse. In my labs, the instructors used it to motivate us to be careful about the connections we made. Each lab group was allowed 3 replacement fuses per term, and you couldn't get the replacement fuse until the instructor was sure you understood what went wrong. Keep in mind that this kind of error is common, so it's natural to be a bit complacent about it. Keep also in mind for cost or safety reasons, it may be critical to not make that mistake even once – K H Jan 22 '21 at 04:19
  • Do everything you can to trick them into caring even with simple safe circuits. The difference between an electrician and a power linesman is that a power linesman only gets to make one mistake. – K H Jan 22 '21 at 04:20
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    If you think multimeter batteries are difficult to replace, try replacing the clock battery in a laptop. – Dmitry Grigoryev Jan 22 '21 at 09:57
  • @Bimpelrekkie, HOW can a typical 1½ or 9V battery survive mains voltage? I would expect it to blow up or something! Is there no electrical separation between the DISPLAY and the METERING? (...I can make this a separate question, but don't think it would survive for long on its own...) – noughtnaut Jan 22 '21 at 15:04
  • @Noughtnaut The battery is floating and the double insulation the DMM case, display, controls I presume. – DKNguyen Jan 22 '21 at 16:01
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    *HOW can a typical 1½ or 9V battery survive mains voltage?* The same way as that a bird can sit on a high voltage powerline and survive. The mains voltage will not be **across** the battery. The complete circuit (including batteries) will be at a high voltage **relative to ground**. As the multimeter has no connection to ground (same as the bird), it does not notice the high voltage. But you are standing on a floor so you do have a (weak) connection to ground, weak but enough to get a shock if you would touch the multimeter's circuit. The batteries are simply part of that circuit. – Bimpelrekkie Jan 22 '21 at 16:09
  • Regarding your problem: buy some 9V lithium batteries. They will last much longer. – jpa Jan 22 '21 at 16:23
  • @Bimpelrekkie, bird on a wire - all you needed to say was painting that picture. Thanks! May your weekend be as pleasant as you are. :-) – noughtnaut Jan 22 '21 at 22:23

5 Answers5

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The internals of a well designed handheld multimeter are designed to contain any explosions that may occur from failing parts.

Imagine the following situation: you're holding a meter in your hand, carefully and following all proper safety procedures measuring what you think is a 120 VAC line... and then it turns out that a fault in a transformer somehow managed to put the 13.8 kV distribution voltage on the line. The multimeter is not surviving that; its fuse blows, but not before a MOV catches fire and a few capacitors explode when their electrolyte flash-boils. If the multimeter was designed like any average consumer electronic device, the force from that explosion and the flames from the MOV would shoot out the sides of the meter, likely severely injuring your hand. So all but the cheapest multimeters have their internals tightly sealed, with tall blast walls separating the inside from the outside.

As a consequence of this, anything internal to the meter must also be sealed inside. Have you noticed how the battery compartment has tall walls, instead of just being a flat lid you put over the batteries? This is part of the multimeter's safety design--and sure, a situation like the one described above is very unlikely, but it's still possible, and I'm sure there are more likely scenarios that could accidentally put much higher voltage on the meter than it's meant to handle.

Likewise, that battery compartment needs to be firmly attached to the meter, and that means no plastic clips that will break off after a few uses--it must use screws to secure it in place.

Hearth
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    "measuring [a line] you think is 120VAC... and then it turns out [to have a] 13.8kV distribution voltage on the line." This is a bad day. A very bad day. This is a day where you thank everyone who put effort into making their products safe and trying their best to permit this day to end with a cold beer and a good story. – Cort Ammon Jan 22 '21 at 02:17
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    @CortAmmon Fortunately, it's not a very likely thing to happen. But the possibility is why test equipment is designed to survive the kinds of conditions it is! You don't generally see the same level of protection on bench multimeters, simply because you're not likely to be using them on distribution equipment (they generally have a lower category rating, too), and the operator is physically more distant from the device so an explosion would singe eyebrows at the worst, not cause you to lose fingers! – Hearth Jan 22 '21 at 02:44
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    I think it would be more common in an introductory physics lab to have someone try to measure the internal resistance of a power outlet. Fun to watch when you stand 2 meters apart :-) –  Jan 22 '21 at 04:52
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    In addition, consider the case of measuring a fairly standard supply - say, 208-480V, but your meter has a tiny bit of swarf in it that shorts the meter leads. You stick the probes on the main incoming breaker just off a few hundred kVA of transformer, and the swarf immediately vaporises and becomes an arcing fault. In the few milliseconds before the breaker or fuse clears the fault or your leads vaporise, as much as 10kA could be flowing, mostly limited by the meter leads. – SomeoneSomewhereSupportsMonica Jan 22 '21 at 09:45
  • @SomeoneSomewhereSupportsMonica I'll admit, finding 13.8 kV on a 120 V line is a pretty unlikely scenario compared to other possibilities that will also result in meter explosions! I don't know why that's the first thing that came to mind. – Hearth Jan 22 '21 at 15:18
  • @Hearth Its definitely a good mental image. Crisp and clear. Kind of like the mental image of [breaking](https://www.youtube.com/watch?v=MAlyEMQxTN0) the circuit on one of those high voltage lines! – Cort Ammon Jan 22 '21 at 16:40
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    @CortAmmon High-power switchgear is *terrifying*. Faulty HV switchgear once landed one of my father's coworkers in the hospital for months just from the arc flash alone; the guy was lucky to survive. – Hearth Jan 22 '21 at 16:53
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It's a safety thing:

A multimeter needs to be safe to connect to the mains while holding in your hand, and there are stringent tests for this. One of the hardest to pass is that it must not spark to the hand of the operator, when a several-kV spike occurs on the mains.

To achieve this the meter has special baffles around all openings - compare the screw gallery on yours to that of a non-mains product. You'll also see them around the opening line of the case. These make the path longer and prevent a breakdown from occurring.

Now the 9 volt battery is part of the meter circuit, probably connected to the 0V rail. Although it would be possible to pass the DC through a special 2 kV isolated DC-DC converter, this would be expensive and unnecessary. But this means that the battery terminals are just as live as the rest of the meter, and need to be fully protected against a several-kV impulse.

The screw-down battery cover is the only way to be sure the meter is closed and safe. A clip-on cover is too easy to remove.

The same applies for the fuse, of course, as it's connected to the circuit, though it is not expected to be replaced so often.

This is also the reason you don't see USB ports for data or charging, on multimeters. When they do add serial ports, they're elaborate optically isolated ones.

Digikey has a nice summary of the multimeter categories. Your fairly average Cat iii 600 V meter needs to withstand an impulse of 6000 V, this can jump quite a long way.

I learned most of this by watching the difficult construction of the Mooshimeter. This blog post describes how the housing had to be modified to pass the CAT-III tests. The meter is wrapped in foil and a high AC voltage is applied between the test leads and the foil.
mooshimeter test

There was also breakdown through the SD card slot, which had to be closed to be safe.

tomnexus
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  • What about the metal screw in the battery cover? – user253751 Jan 21 '21 at 20:00
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    @user253751 isn't that isolated by a plastic receptacle? – Sredni Vashtar Jan 21 '21 at 23:43
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    @SredniVashtar I just opened up my (CAT III) meter and the metal bolt (not a screw) screws into a metal nut that is fully encased in the plastic of the front-side of the housing. No electrical connection to to innards of the device. – Tonny Jan 22 '21 at 10:15
  • "To achieve this the meter has special baffles around all openings" is that that bulky yellow plastic bit I took off mine ages ago because I didn't like it? I should put that back on if so... – Adam Barnes Jan 22 '21 at 13:12
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    @AdamBarnes The baffles are internal, built into the plastic case. If it's removeable, the rubber is almost certainly not part of the protection. – tomnexus Jan 22 '21 at 14:45
  • @AdamBarnes The bulky yellow (or blue, or orange, depending on manufacturer) rubber bit is meant for drop protection. It might not survive a fall without it, but it's not necessary for safety. The baffles are internal to the meter; you won't see them unless you open it up. – Hearth Jun 13 '23 at 15:12
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I would say that safety is the main objective.

Imagine that some irksome person had dropped the meter and the battery bounced out or, maybe some idiot decided to steal the battery and, then I went and checked that the AC wall socket wasn't live that I wanted to add a spur to.

I would measure 0 volts and I'd believe that I'd disabled the right isolator in the distribution board hence, my AC circuit is fine to work on.

But, without a battery, my meter would still measure 0 volts and I would be in for a nasty shock.

However, if working on AC and relying on your meter for confirmation that a circuit isn't live, you should always check it can register a live voltage before confirming a different circuit isn't actually live. I would always do this as a matter of course but not everyone would maybe.

Footnote - my meter which I bought in 1990 is still working and using the original battery so I don't know what your students are up to if you have to keep changing batteries!

Andy aka
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    Original battery from 1990 :O Isn't a bit of exaggeration ? – Eugene Sh. Jan 21 '21 at 17:14
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    No exaggeration at all. It's bloody 30 years old and still working!! Every time I use it I check on the battery though. Two days ago I used to fit batteries to my oil tank RF sender and it was still running good. – Andy aka Jan 21 '21 at 17:18
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    Hard to believe.. but well, I don't have a good reason not to :) – Eugene Sh. Jan 21 '21 at 17:20
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    I've never taken it out of its yellow rubberized surround. Bought it from Tandy (a la Radio shack device) and it may well have been 1989. I expect I'll eventually curse when I try to replace the battery and can't find how to do it. – Andy aka Jan 21 '21 at 17:20
  • The students have a nasty habit of not turning off the equipment when they leave, which chews up the battery a bit faster than one might expect. Some of our multi-meters have an auto-off feature, but not all of them. – Michael Seifert Jan 21 '21 at 17:50
  • A cheap meter might use a high current and a cheap battery is garbage that fails soon. – Audioguru Jan 21 '21 at 18:09
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    What DMM reads zero volts when it has no battery? – tomnexus Jan 21 '21 at 18:35
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    Not all multimeters are digital. – Andy aka Jan 21 '21 at 18:42
  • @EugeneSh. The 9V battery in my CAT III meter is its second battery and at least 25 years old now. I bought the meter in 1984 and used it intensively (several times a week) in the 80's and 90's. I replaced the original battery in 1995 or 1996 (expiration date on the battery says 1998) and it still works. This millennium I only use the meter 3 or 4 times a year though. – Tonny Jan 22 '21 at 10:23
  • @tomnexus My AVO model 8 for one, still better then my fluke when trying to null something, and the 20k ohms/volt means that you do not usually get silly spurious readings from long unconnected wires. – Dan Mills Jan 22 '21 at 16:00
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    But the AVO will read even with no battery... I know there are FET-input analog meters that would incorrectly read zero if their battery were flat but these would be rare, and would have a battery check / Ohm range to check. – tomnexus Jan 22 '21 at 16:39
  • @Andyaka "But, without a battery, my meter would still measure 0 volts and I would be in for a nasty shock." Aren't you supposed to test your multimeter on a known powered socket before you trust the reading? – stickynotememo Jun 15 '23 at 06:45
  • I don't expect that it will. – Andy aka Jun 15 '23 at 07:39
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Cheap flimsy $5 multimeters have those.

Most professional multimeters have a hatch with a screw, or the whole case needs to be opened. This is because they have had a waterproof seal so water does not creep in. A sliding door for a battery compartment would not be quite as waterproof.

It's also about safety. It is made to be so difficult that you must know what you are doing. If it would be simple, there is a chance that for example the leads are still connected to mains voltage when you open the battery hatch and are able to touch circuits live with mains voltage.

Regarding safety, the multimeters have to be rated for certain level of safety standards depending on what class/category of device it is, i.e. basically what are the voltage levels it is intended to measure.

Justme
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Most multimeters are rated (or claim to be rated) to be safe if connected to 600 volts or more (many cheap meters claim category II, 600 volts.)

Consider what happens if you connect a meter set to the 10 ampere scale to a 1000V power line.

The fuse will blow - violently.

You want something more than a clipped in plastic cover between you and the bits of glass or molten metal.

The housings are (usually) thicker than the typical clip in covers you see on other devices. Screwed solidly together, the housing provides at least some protection when things go bang.

Outside of that, imagine the dinky little plastic cover falling off and the battery falling out while you are measuring a high voltage. There's potential (hehe) there to touch the battery and get zapped - hard.

JRE
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