Quote:
Originally Posted by ericj
It all depends what you want to do with it. Like i said, I get to find out if the mastech kitchen sink meter is any good later today. I forget the model number.
I wanted inductance & capacitance & frequency counter, so, i went looking for it. I figure if it gives me the correct readings for known inductors and known clock sources and brand new 1% tolerance caps, that's Good Enough. for miliamps, I guess I'll have to compare it with a better meter.
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Well, after playing with the thing for a week, here's what i think.
The meter in question is a Mastech MS8226T, purchased on eBay for $59 + $15 shipping.
First, it turns out that i transposed some spec sheets when selecting my meter. It turns out that you can't get a relatively cheap chinese dmm with both a Mhz range frequency counter and an inductance meter. I have a 5mhz frequency counter instead of a 50khz frequency counter, so i have no inductance meter. That can be remidied, see the end of this post.
Build quality:
The probes are hefty and made of a sturdy but pliable and soft-to-the-touch plastic that has the right sort of texture and feel so that it will still be comfortable in your hand even if you start to sweat. I appreciate that. The probes say on them that they are "category III" and rated for 1kv. I will probably never come anywhere near 1kv, but they're probably at least good enough for the high voltage you might run into on a tube amp.
The unit itself is cased in a sturdy rubberized plastic that also has a pretty good feel to it. The battery door is hefty and held in by two rotary locks that have to be turned 90 degrees with a flat-head screwdriver before you can change the battery. This may seem annoying unless you've got tape holding the battery in on your other meter.
It has a fold-out stand on the back that will hold up the meter so you can read it while handling the probes. Works well enough, but you'll still need both hands to change probes without knocking it over.
It comes with a hard plastic carrying case that seems decent enough, but i doubt if i'll ever use it.
Functionality:
There are four probe positions. Common, ohms/volts, miliamps, and amps.
The temperature probe plugs into the ohms/volts position and the miliamps position. It seems reasonably accurate. I decided against holding the thermocouple against my Weller TC201 iron to see if that tip really is 700 degrees or not. If you do not attach the thermocouple probe, the meter displays it's internal temperature.
Someone actually wrote a linux application to continuously log the temperature readings from mastech dmms with serial ports. It's made for another model, but will probably work with this one.
Speaking of the serial port, I have not installed the Windows software yet. At some point i'll figure out if it will run under Wine, whether it's worth having at all, etc.
The serial port is optoisolated from the meter, so the computer you attach to it is in no danger whatsoever. The 'port' on the dmm is literally a rubber socket with an IR led/sensor in it, exactly like you would find on a learning remote control. The serial cable has another one just like it. This may mean that the serial cable might not work with some laptops that have extremely low voltage serial ports.
The ohms position on the rotary switch is also the 'diode test' position and the 'continuity' position for obvious reasons. You select other functions by pushing the 'select' button.
Auto-ranging is done in a matter of about 1.5 seconds. It's very convenient when testing a built circuit or when giving up on deciphering the faded color codes on an old metal film resistor.
Less convenient when trying to determine if an old potentiometer is linear, log, inverse log, or what. A manual range is easily selected by pushing the range button.
The 'diode test' function is as functional or lame as it ever was, depending on your own personal opinion of whether you should even have such a function.
The continuity function is pretty handy. All it does is beep if the resistance between the probes is less than 50 ohms.
There is a transistor HFE test function, but, there is no little socket to plug your transistor into. i don't recall the manual mentioning how to use it, I'll have to go back and look again. Probably means i get to build my own probe, or, just use the old centech.
The display is quite large and has a really decent green LED backlight. This is much better than the "backlight" on the $6 centechs that really just puts a haze of light at either end of the display. The whole display lights up, and you can in fact read it clearly in the dark.
Now that that is out of the way, accuracy:
For DC volts, and ohms, it's basically like any other budget DMM. It may be a little faster than the $6 centech, or i could be dreaming.
Accurate Enough. A disagreement of +/- 1mv is Close Enough, and i dare anybody to point out an audible reason why not.
AC volts and amps? I don't have anything to compare it with at the moment.
Capacitance? It's rated as having a range from some 100pf to 100uf and i more or less believe the rating, to some extent.
Really low numbers of picofarads, under 100pf, it does just say it's out of range.
Every 100uf cap I've given it has read as out of range, which is annoying, but honestly in more than 2 digits of microfarads, the more important reading is equivalent series resistance anyway. A 47uf cap reads as 47uf give or take the given tolerance range. An 82uf cap read fine as well. A "non-polar" 10uf electrolytic reads as 22uf while a 1% 10uf electrolytic reads as 10uf.
Between those extremes, it does in fact verify that a brand new 1% film cap is within 1% tolerance of it's given rating. The last few digits on the display, which are 10's of picofarads, will constantly fluctuate between a few different numbers, which ultimately you just have to average out.
This is still close enough for matching capacitors in crossovers and, for example, riaa equalizers.
At any rate, it's still far more accurate than reaching into a bag of 1% capacitors and figuring you're within 1%.
It was also pretty interesting to see how i could temporarily change the capacitance of a wrapped foil cap by squeezing it between my fingers.
The one really aincent capacitor in my posession - a 1uf oil & paper cap in my Precision Apparatus series 912 Dynamic Tube Tester, ca 1947 or so, tests out at 1.05uf.
Presumably, one could use this meter to determine if an aincent cap had gone out of tolerance, but this one hasn't. My bag of wrap&fill caps pulled from equipment proves that polypro wrap&fill caps are pretty durable, too.
A good ESR meter would be better for that job, as it would let you know if an electrolytic had a really high resistance, or had shorted out.
I haven't used the frequency counter yet. I'll compare it with an astable 555 some time soon. I do actually have one set up as a DC inverter on the workbench, so it's a matter of powering it up and attaching leads.
I will be augmenting my instrumentation by building this inductance meter:
http://earthground.8m.com/indcap.htm
And this ESR meter:
http://www.xmission.com/~alhaz/esrmeter.gif
Which is described half-way down this page, which is down at the moment:
http://www.glowingplate.com/welcome.shtml
fwiw, I also picked up a Craftsman 34-81079 DMM very cheaply on ebay, as a broken unit ("doesn't always turn on") from some pawn shop. So, while i have it open to clean the switch contacts and repair the broken solder joints, I'll let you know if it was made by Mastech or Fluke. Looks like a Mastech to me, just from the picture, but we'll see.
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