Buying a digital multimeter

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Larry Woods
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Buying a digital multimeter

Post by Larry Woods » Thu Jul 31, 2003 8:03 pm

I've been using an analog version for some time and want to buy a digital voltage meter ( multimeter). What features should you look for and are there any recommendations without spending a big dollar?<p>Thanks

rshayes
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Re: Buying a digital multimeter

Post by rshayes » Thu Jul 31, 2003 11:48 pm

You get what you pay for, so the trick is to only ask for what you need. Digital multimeters start at about $7 and go up to $400 to $500, or possibly several thousand (if you need to read microvolts).<p>Harbor Freight Tools (www.harborfreight.com) has a digital multimeter for $7. It has DC volt, current, and ohms ranges. AC volt ranges start at 200 volts, so it probably uses a simple diode rectifier for AC. Probably about the same capability as the old VOMs, but not much more. From here on , the price goes up.<p>The next step up will probably have an active rectifier for AC measurements. This means that AC voltage ranges will start at 200 millivolts and there are usually both DC and AC current ranges. This might cost from $20 to $50 depending on the source, mechanical construction, and brand. At this point you have the basic DVM.<p>Some meters have auto ranging. Sometimes it works, sometimes it doesn't (meter movements and nonlinear parts, for example). This is a matter of preferance, I prefer a meter that does what I tell it to.<p>True RMS AC measurements are a step up in cost. If the waveform is a sine wave, an average reading meter can be calibrated for RMS. If the waveform isn't a sine wave, you probably don't know what rhe RMS value should be anyway.<p>Capacitance ranges are another added feature. Probably of limited use, since many capacitors are used as power bypasses and filters and can't be measured easily as individual parts. The test leads would prevent accurate measurement of small capacitances.<p>Some add transistor beta ranges. Not very much use for troubleshooting, since the sockets are mainly designed for new transistors with full lead length. A diode test will find most catastrophic failures.<p>Some models have frequency ranges. Check the upper frequency, it is probably in the 100 KHz to 1 MHz range, and the sensitivity may be low. Possibly good for setting audio oscillators, but at low frequencies, such as speaker testing, the resolution is poor.<p>Most DVMs are 3 1/2 digit. Higher accuracy models (4 1/2 digit) are available. Normal troubleshooting does not require this much accuracy, but matching resistor pairs might.<p>A few of the more expensive models have temperature ranges. These usually use a thermocouple, which may be an accessory.<p>Some models have an RS-232 interface and software for use with a computer. This might be useful in special circumstances.<p>The portable models are more popular and usually less expensive. Bench models tend to have more of the bells and whistles, and may be more accurate, but they are considerably more expensive.<p>The cheaper meters come in from overseas. They are usable instruments, but don't expect technical support, repair services, or calibration services. You can get these with domestic models, but at a somewhat higher cost.

Mike
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Re: Buying a digital multimeter

Post by Mike » Fri Aug 01, 2003 5:09 am

I bought the one from Harbor Freight, and it works great. I only paid 3 dollars when it was on sale. I figured if it needs repair, it would be cheaper to buy a new one at that price. It tests ohms from 0 to 2000K, diodes, transistors, DC Amps up to 10A, and from 0 microamps to 200 milliamps also. AC: 250/ 750V
DC 0 microvolt - 1000V. Works great, just didn't like the cables, so I made my own with binding posts, wire and alligator clips.

bruinbear714
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Re: Buying a digital multimeter

Post by bruinbear714 » Fri Aug 01, 2003 9:00 am

If you are just using it for hobby use and whatnot, then a cheap $5 meter would do... but if you're are involved in the ee field, then I would recommend a mid-level Fluke meter. Not only are they much more accurate, they also are well designed, have a host of USEFUL features, and the batteries on them lasts forever.

Chris Foley
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Re: Buying a digital multimeter

Post by Chris Foley » Fri Aug 01, 2003 2:41 pm

I'd tend to agree with Lien -- you don't always get what you pay for, but you always pay for what you get. The "dime store" DMMs (I've had several, none still alive) will tend to have disappointingly inaccurate readings, especially on the AC voltage and current and on resistance. They also tend to spit up and die for no apparent reason (usually ESD causing the control logic to upset, causing overvoltage at the input).<p>For a hobbyist or someone new to the field, I'd recommend one of two used multimeters, both of which are available from multiple sources including Ebay at reasonable prices. The first is the Beckman Tech 300, which is not autoranging, but has all standard, off-the-shelf thru-hole components except the front switch. If you smoke it, you can fix it. If you're interested in an autoranging meter, try the Fluke 73/75/77 series. They're built like a tank (get one with the rubber shell), and the only things that ever seem to go on them are the fuse and the 1K ohm fusible resistor (which can still be obtained from Fluke service).<p>Design, hacking, and even getting things to work right is a craft as well as an art. The craft part, to a large extent, depends on the synergy between your energy and imagination, your intelligence and your tools. Properly chosen used instruments are a good way to start in the field.<p>[ August 01, 2003: Message edited by: Chris Foley ]</p>

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Re: Buying a digital multimeter

Post by josmith » Fri Aug 01, 2003 4:04 pm

I'm a great fan of the $3 HF meters. I've been using one on the job for years.<p>If you want to spend around $100 consider a Fluke 12 which has an autorange mode that measures ac dc or resistance depending on what you hook it to. Great for poking around on unknown circuits.

Dean Huster
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Re: Buying a digital multimeter

Post by Dean Huster » Fri Aug 01, 2003 4:55 pm

With DMMs, I will always recommend one brand over others, especially if a person is new to electronics and will have a tendency to commit a few "oops" a week as we all did, and still do to an extent. Fluke brand DMMs are quite literally "bullet-proof". They are probably the only brand that you can set to any range and zap the probes into a 120vac receptacle to have a blown fuse as the only major "failure". Many meters will eat themselves alive if you try that on their resistance or current ranges.<p>However, you will pay for that durability. But a Fluke will last you a very, very long time and come with a very nice, long warranty.<p>Note that virtually every DMM ever made will have a display that will outperform the A/D converter in the meter. A 3-1/2 digit display will display to a resolution of around 0.1%. Most DMMs with a 3-1/2 digit display have accuracies of ±1% or ±0.5% at best, meaning that the last digit of the display is really not an accurate digit at all. And note that nearly every meter's best accuracy is on the DCV range. A meter rated for ±0.5% on DCV may have an accuracy of ±1.5% on ACV and ±3% on OHMS, so the 3-1/3 digit display is really giving you a lot of not-so-accurate information!<p>Buying a DMM is like buying a car. Everyone has their own opinion of what's best and few folks are ever 100% right, except for their own uses of the meter, me included.<p>Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

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larussell
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Re: Buying a digital multimeter

Post by larussell » Mon Aug 04, 2003 10:11 am

If you want a good one (as opposed to a $5 one), definitely check out the resale market. Normally eBay is expensive, but there is so much surplus equipment out there (I think after all the .com failures) you can often be the only bidder on an item and get it really cheap). Also think about whether you want a benchtop or handheld. I have both; the benchtop with its vertical display is easier to read when you are working at a desk, but they are expensive unless you buy them used. Some of them even come with calibration tags on them - although probably out of date, it's still better than what you get at Radio Shack.

wd5gnr
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Re: Buying a digital multimeter

Post by wd5gnr » Mon Aug 04, 2003 12:29 pm

I have been happy with Elenco for "cheap" meters. They are inexpensive but work well and are pretty rugged. Read: http://www.wd5gnr.com/meter.htm

josmith
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Re: Buying a digital multimeter

Post by josmith » Mon Aug 04, 2003 3:12 pm

Rat shak is an example of cheap but not inexpensive. I've seen one of there meters die of an internal arc on 480vac,well within the rated maximum.

Will
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Re: Buying a digital multimeter

Post by Will » Mon Aug 04, 2003 10:23 pm

I understand that many mid-priced DMM's i.e. Flukes etc are fitted with optical or other RS 232 etc serial outputs which the manufacturers use to do original factory calibrations on each meter. If I were buying a new DMM I would get a Fluke with this facility so that I could connect it to my PC and play. I would always go to Fluke anyway - I bought my DMM from them ib 1978 - last year the Liquid Crystal display gave out (Became unreadable) and I got a new one from them for about thirty bucks with full installation instructions.
Dean is right about accuracies but most accuracies quoted by manufacturers are Two Standard Deviation accuracies i.e. What the manufacturers are actually saying to you is that, if they for instance quote 0.5% then they mean that 95% of all of their production is within 0.5%. This means of course that if you get one of the other five per cent then it will not be better than 0.5% It also means that 68.3% (One Standard Deviation) will be within 0.25%.
I have three DMM's which I use regularly - all 3.1/2 digit, a Fluke, A Ratshack $30 one and another (manufacturer forgotten) with AC current measuring tongs - and, when measuring low voltages, they always agree within one least siginificant digit. Perhaps I'm lucky ? If I am I never noticed it before.
BB

Dean Huster
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Re: Buying a digital multimeter

Post by Dean Huster » Mon Aug 11, 2003 5:58 am

I'll have to disagree with that "standard deviation" stuff, BB. If a manufacturer told their customers that 95% of their stuff was within calibration and the other 5% could be totally wacko, they'd be out of business within a year. Can you imagine Fluke selling one of their resistance standards and telling the customer, "You have a 5% chance of this $5000 standard being nothing but crap with an accuracy no better than you can get with a Simpson 260."<p>All electronic instrumention has specifications, some better than others. Analog meters are rated as a certain ±% of their full-scale range. Oscilloscopes are ±% of their reading. Frequency counters and DMMs throw in a 2-pronged accuracy specification: ±%, ± a certain number of digits on the display. So a typical decent DMM may be rated as ±0.1% of reading, ±1 digit. That ±1 digit doesn't figure in much unless you're measuring on the "low end" of the range, e.g., using the 2 volt range to measure 10mv on a 3-1/2 digit meter. Then the ±1 digit becomes a 10% liability!<p>The accuracy specifications are guaranteed for a certain ambient temperature range and certain line voltage (if line operated) range for a certain length of time (the calibratin interval). If you find that a Fluke, Tektronix, Agilent, Philips, or whatever instrument fails to make these specification upon delivery, you send that thing back to the manufacturer and they're obligated to make it right.<p>Dean
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

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Will
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Re: Buying a digital multimeter

Post by Will » Mon Aug 11, 2003 11:45 am

Healthy disagreement is Ok Dean - It's good for us all. What I said about two standard deviations is not just my opinion - it's fact - I have had this in writing from many instrumentation manufacturers over forty or so years. Quuite clearly if you are paying $5,000 or so for a piece of test equipment then it will be tested and calibrated before delivery and the test certificate which comes with it will cover all of that. It will still have a stated random uncertainty.
I'm sure no one believes that every $60.0 instrumenmt coming off a manufacturer product line is test calibrated - it would put them out of business. How they are checked is samples are taken from the production lines and tested sometimes one out of every 'n' products, sometimes random picking - Then the stanbdard deviation is calculated and they usually quote two standard deviations as the accuracy. They will rarely tell this to anyone they don't regard as 'needing to know' i.e. large and/or potential customers and people in certifying/licensing bodies. This not only applies to $60.00 instrument but to $1500 process control instruments. One manufacturer I know gives you the two standard deviation figure for $1500 but, if you want better they will give you a three standard deviation for $3500. This because, they will exhaustively test every one for which you are willing to pay $3,000. I used to be in the position where manufacturers regarded me as 'needing to know.
The rest of the stuff you mentioned is of course true - appropriate temperature ranges, drift and zero errors etc.
If you would like more info on this then get hold os ISA (Instrument Society of America) standard 67.04 part II which details instrument accuracy calculations for USA Nuclear Power Plants
BB

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Re: Buying a digital multimeter

Post by Crowbar » Mon Aug 11, 2003 1:21 pm

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by josmith:
Rat shak is an example of cheap but not inexpensive. I've seen one of there meters die of an internal arc on 480vac,well within the rated maximum.<hr></blockquote><p>That's precisely why a CAT III or IV rated meter should ALWAYS be used on high energy circuits.
Keep Prying...

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Edd
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Re: Buying a digital multimeter

Post by Edd » Mon Aug 11, 2003 3:21 pm

If you happen to end up with a mechanical design of unit, incorporating a large rotary selector. A switch that has to be rotated thru a myriad of functions to reach its power-off position each time, as well as its initial function. Being familiar with the mechanical function selector “innards”….thats not good news. Even the best silver or gold filled contact elements wear down and the pincer clamp elements loosen up with time and REPEATED (read as otherwise unnecessary rotations) uses. Some of the mechanical design on some units use molded plastic dimples activating metallic contact members and pressing into place…not good…the dimples erode in size. The latter being experienced in some Amprobe and Micronta units. Even two of my 14 different Flukes incorporate rotary switching between functions , albeit with auto-ranging.
To circumvent their untimely demise and not be up against an unavailable/complex/HARD to replace switching selector unit I have incorporated a separate Power On/Off switch into their circuitry. Its as easy as opening the batt + line and inserting the switch in line. I have been using a small spst slide switch placed in an area inside that space will permit. Usually I will mount with a slight spacing internally so that the recessed slide can be engaged with a fingertip or nail. This lets the flushness of the case not be interfered with if the back is utilized as well as any mistaken activation. One AC ammeter I use infrequently, for quick readings, received a momentary contact power sw….no more finding dead batteries on that unit after haven forgotten to turn off..<p>Will:
< I bought my DMM from them ib 1978 - last year the Liquid Crystal display gave out (Became unreadable) and I got a new one from them for about thirty bucks with full installation instructions.>
… I wonder if that was one of the few transmissive mode LCD display elements on some of their bench models…vice utilizing the common reflective LCD display elements………. and you merely had the internal illumination source (lamp) to fail. <p>
FIO to all interested parties: LCD displays in all modes of instrumentation/clocks/watches/etc.
In respect to all of the many, many types of instrumentation using LCD displays, I’ve certainly had enough of the common problem of reflective style LCD display propers digits dropping segment(s). This condition can usually be aggravated/temp confirmed by a slight pressure or a skewing pressure on the top/bottom edges of the LCD display to see if a segment changes…(comes or goes). My common failure analysis reveals a connectivity problem between the LCD elements “fingers” and the PCB blank “fingers” wherein their common interconnectant is a silicone rubber block with a Macro cluster of individually insulated graphite filaments equally interspersed that make interconnection from end to end when all is clamped together. (Sometimes the cheap plastic clamping construction eases off on the pressure between the PCB and the LCD element ..or even cracks….and that can also be the only problem…. Beckman/Sperry, particularly coming to mind.)<p>When instruments are stored in a HOT environment there is a tendency for some of the blocks composite siloxane elemental mix to ooze out the ends and contaminate the connection(s).
Repair is usually enacted by disassembly in a clean dust free environment…(but not quite into the laminar flow clean room category .hi hi)…and take a Sable bristled artist brush with the end freshly cut/squared off for optimum cleaning action. Then clean the LCD blanks chemically etched/ conductive coated “fingers with denatured alcohol, and then on to the interconnect block and then finally, to the instruments PCB propers “fingers”. On the PCB foil it will be permissible to slightly clean it a little bit more thoroughly with an eraser if it is tin plated and has an oxide buildup.(Considering that you didn’t luck up, finding utilization of some of Flukes,Tek, LeCroy’s instruments utilizing Gold plated contact fingers). On some units, I’ve even done a Brasso polish up on the instruments main PCB foil fingers …with a good final alcohol cleanup of ANY trace residue..
One very important caveat to the novice…on the conductive block, those individual blunt/sharp fibers need to never see a cleaning cloth/tissue/wipe, etc, as they will create fierce micro linters on their surface or themselves and/or the block…. and they don’t clean off easily ! The sable brush and the solvent present no problem. Also the conductive etched coating on the LCD blank is the most delicate of all, in respect to excessive abrasive action , but no problem experienced with the quick cleaning with the sable brush and denatured alcohol.
The procedure for inspection of the conductive block is to set up a lamp in front of you and position the block so that U can get the reflected light cast forward off the block. One in good shape gives a flat/matte surface light reflection. One that the Siloxane has done a job on will reflect a shiny glazed over surface. If the latter is found, use this procedure:
Boston and Acco make paper clamps .Use the Acco style with two clamp members that are intertensioned with an encircling spring steel element. This unit will afford U a good “Hand Vise” to precisely hold the conductive block during operations. Select a clamp of adequate width to hold the strip and clamp in the block so that one conductive end is exposed out of the clamp about 1/16 in. The conductive block will be aligned with the longest dimension from left to right in front of U and the short dimension (on an avg ~ 1/16 in).is. top to bottom. The resurfacing will be done with a virgin single edge razor blade.
Visualize a sheet of paper in front of you and the holding of a soft 2B lead pencil between only your index finger and thumb…to limit available pressure. Then you make a sweep from left to right across the paper to place a uniform marking across the page…too light of a pressure and a dim mark and too hard .. paper damage.
Impart this technique of estimation of the pressure as to what is to be done next. Take the blade and hold with index finger and thumb and align so that it will be on the an even 90deg plane to both the length and width planes of the conductive block. Establish this alignment at the extreme left of the block, and then lean the blade to the right just about 10 deg since U are going to be making a SINGLE surgical swipe from left to right. DO IT . Rotate the block and do the other conductive side. If you had the backlighting in alignment while you were doing this U should have seen the shiny surface go to the matte cleaned surface on the cleaning swipe. Clean the block of any residue produced with the denatured alcohol and Sable artist brush then reassemble your unit.<p>73's de Edd
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