Test meters are probably our most useful tool in diagnosing problems with well logging electronics. Multipurpose meters are called "multimeters" for obvious reasons. Multimeters come in two basic flavors: the analog meter with a conventional electromechanical meter movement (the old kind with a pointer), and the more modern digital multimeter (DMM). All multimeters will measure voltage (dc and ac volts), resistance (ohms), and current (milliamps and sometimes amps), with continuity/diode check, frequency (hertz), capacitance (microfarad), inductance (millihenry/microhenry), temperature measurement, and other features available on some models.Analog vs. Digital
Blaster's Multimeters are specialty meters used to check electrical blasting circuits. They differ from conventional multimeters in that they have special current limiting provisions to prevent accidental firing of the detonators or blasting caps under test. Blaster's Multimeters and related test instruments are used in the well logging industry to check the continuity of electric detonator circuits commonly used in
well shooting and perforating.
In antiquity (the late fifties when your writer started playing with electronics), all multimeters were analog meters. In those days, the best voltmeter available was a "VTVM" or vacuum tube voltmeter (early semiconductors lacked the high impedance of vacuum tubes, but more recent semiconductors like the field effect transistor (FET) have solved that problem), and a multimeter was called a "VOM" for volt-ohm-milliameter. One of the best analog meters ever made is still manufactured by Simpson; we have one, but it died many years ago. Check out the Simpson 260 analog multimeter; it is a classic in continuous production since before WW II. See the Simpson Profile for a brief history of the company. Triplett is another famous old company still making analog multimeters. The Triplett Model 630 analog multimeter is also a classic in continuous production for over a half century. We have a Triplett 630, Type 4 in pristine working condition. See the Triplett History for an interesting synopsis of events at that company covering about a century.Cheapie (Off Brand) DMMs
Some of us old timers had difficulty adjusting to digital multimeters. We would say things like "I don't trust the display" or "I can see trends on an analog meter more easily." But modern DMMs are far more accurate and precise than the very best analog multimeters (except on those rare occasions when they are not and only an analog meter will get the job done). Some DMMs are now offered with one or another kind of bar graph near the digital display that emulates the action of an analog meter for trend identification. Fears about DMM accuracy are not entirely unfounded, and it is a good rule-of-thumb not to trust the last digit of any DMM. Despite what objections we may have previously voiced, even the most stubborn of ancient technicians now rely almost exclusively on DMMs (and your author even finds the bar graph feature increasingly unnecessary). Though analog multimeters have an advantage or two, there is not much reason to invest in one at this late date.
There are scores of outfits offering inexpensive DMMs. Most are manufactured in one of a handful of Pacific Rim plants and are house labeled for the particular vendor. Many use the very dependable ICL7106 multimeter chip. But the quality of the associated parts, and especially the electromechanical elements (switches for example), is usually inferior in these cheapie meters. Some of these DMMs are acceptable, but many are not. We had an inexpensive DMM marked "Kelvin" brand that was fairly accurate, but it would drain its battery in a very few hours; it now resides battery-less in our junk box. We also had a Radio Shack (Micronta) 22-174 that was so inaccurate right out of the box as to be unusable. Further, the case and ruggedization features are typically inferior to more respected brand name DMMs.Good DMMs
If your DMM needs are not particularly demanding, you might be satisfied with one of the cheapie DMMs. Take a look at the Wavetek / Meterman XL line for meters that may be a little better than most of the off brand crowd. One very competent technician we know has an APPA DMM and swears by it, and others report good results with a number of different cheapie DMMs. But be prepared for a much shorter life expectancy than that of the better DMMs; we have also had bad luck with short lived liquid crystal display (LCD) elements in off brand meters. All things considered, do not waste money on one of these off brand DMMs when for a few dollars more you can buy a bottom-of-the-line quality meter. And for that matter, it is better to save one's pennies and go ahead and get a good middle-of-the-line quality made DMM...you will be glad you did (very nice DMMs can be had in the $100.00-200.00 price range). Many vendors offer special deals from time to time on very good meters with steeply discounted prices.
At one time or another, we have had a DMM from just about every manufacturer, many of which are no longer in business. A few years ago, a Beckman DMM was considered one of the best choices for logging electronics service work (the HD series had a DC impedance of 20 megohms and was good to 1500 volts). But the Beckman meter line was absorbed by Wavetek, and has now all but disappeared. To Autorange or Not?
Fluke was our new favorite, but a recent development has shaken our faith in Fluke's excellence. We initially liked our new (July, 2001) Fluke 177 with .09% basic DC accuracy and a capacitance scale, which meter replaces a Fluke 21 destroyed with a huge accidental overvoltage. But we soon discovered "sleep mode" does not function properly in the Fluke 170 series DMMs (reportedly in all of the 17x models). There is a sleep mode software bug Fluke will hopefully correct in future production, though what they intend to do about the units already in circulation is not clear. Further, as a result of an inexperienced design team, the Fluke 170 series cannot be brought out of sleep mode with the main function dial switch, though this is standard on all other Fluke DMM products. Needless to say, we are sadly disappointed in Fluke, and we cannot recommend the 170 series meters until these problems are resolved.
Many Fluke meters now come with a limited lifetime warranty (the LCD element is warranted for 10 years free, and thereafter for a fee based on acquisition cost, reportedly due to concerns over LCD availability). Many considered the Fluke 8060 series to be the ultimate hand held DMM. The 8060 line has been recently replaced with the 180 series. These 4-1/2 digit, 50,000 count DMMs are very nice if you can justify the cost. A word of caution...it is not true that Fluke DMMs are sold under off brand labels at huge discounts. If Fluke made the meter, it will say so at least on the back of the meter and usually on the front also. Fluke acquired the TX1 and TX3 DMM designs from Tektronix when Tek decided to quit the DMM market (now offered as the 183 and 185, only available in Europe).
If you need an inexpensive inductance meter, the Wavetek / Meterman 27XT is pretty dern decent for not much more than $100.00; it includes capacitance as well as the other normal DMM parameters, but it is not autoranging. The newest production XT DMMs are a wonderful "put your eyes out" red-orange, where earlier models were a peculiar dark blue color with yellow trim. But be cautioned that Wavetek XT meters seem to be somewhere between the real cheapies and the Fluke grade DMMs. Interestingly, the Wavetek DMM line was acquired by Fluke recently to be their low end offering; the Meterman name was adopted because the Wavetek name belongs to another company (presumably Wavetek will eventually be dropped from the Meterman trademark).
Many technicians hate autoranging DMMs. There are usually two main complaints: (1) autoranging meters are slow, and (2) the values shown on autoranging meters can be confusing. Both of these complaints have a good deal of validity when it comes to older DMMs, and even with some current models (especially in the cheapie class). However, modern quality autorangers like those made by Fluke are fast enough, and easy to read. More importantly, it is much harder to destroy an autoranging meter since you cannot easily inadvertently apply hundreds of volts when you are manually set to a millivolt scale. In any event, current designs almost always provide for manual override of scale selection on autoranging DMMs.Bench Meters
Like hand held meters, there are excellent, and not so excellent bench meters. We like Fluke and Agilent Technologies (formerly Hewlett Packard) for bench DMMs. We have a Fluke 45 bench meter, but we are barely smart enough to turn the complicated thing on. At around $1,000,00, the Agilent 34401A is a dream machine (some discounting can be had). Before investing in a bench meter, seriously consider if you can get by with a top end hand held meter, like perhaps a member of the Fluke 180 series. This latter option is almost certainly a better choice than an off brand bench meter.Tips
No matter how painful, we recommend a careful reading of the manual that comes with a DMM. While a complete DMM tutorial is beyond the scope of this effort, here are two links to tutorials we stumbled across on the web: Mechatronic's Multimeter Tutorial (part of a university website) and MultimeterWarehouse.com's How To Use A Multimeter (this particular page is at several websites with slight variations). As mentioned above, put little trust in the last digit of any DMM. Also, AC readings are optimized for a narrow frequency range, hence even "true RMS" meters can give misleading readings. If you want really flexible test leads, forget about the vinyl leads shipped with most meters. Get silicone insulated test leads; we like the ones manufactured by Meuller. If you have an old analog meter like the Triplett 630, the 30 volt NEDA 210 battery will set you back about ten bucks; if you do not really need the 100,000 ohm scale, just omit said 30 volt battery since all the other scales will work just fine without it.Where To Buy
A good resource is Beta Lambda Instruments, a company started by a couple of long time Tek Field Engineers (25 and 26 year veterans). They offer extensive before and after sales support. Also consider MetersandInstruments.com. They will meet or beat any price on the web; we have purchased two DMMs from them and they have been a pleasure to deal with. Many vendors run specials with very steep discounting from time to time, so keep your eyes peeled for the very best deals.
Talk to our friend Bruce Divine at Test Equipment Plus (TEP) in Tucson, Arizona, for an honest and knowledgeable source of used test equipment. Contact him at (520) 575-6967, fax (520) 575-6936. Avoid dealing with AST Global Electronics; we once purchased what was supposed to be a verified accuracy bench meter from them that was unusable (and they gave us grief about returning it).
Disclaimer and Credits: The foregoing material constitutes the opinions of AnaLog Services, Inc. personnel. Many technicians report excellent results with the cheapie DMMs discussed above, but we have had too many bad experiences to advise anyone doing serious electronics work to rely on them.
Thanks to the many subscribers of the Chip Directory mailing list who responded to our inquiry about meter recommendations. The Chip Directory and the mailing list associated with it are wonderful resources.