Multimeters

Over the Christmas holiday, I was out of town and picked up a three phase motor to use in my phase converter.  I wanted to check out the windings with a digital multimeter to make sure they weren’t open or shorted to the case.  I picked up a cheap $4 Harbor Freight meter to do so as I didn’t have one with me.  It showed continuity through the windings and that the windings weren’t shorted.  When I got back home, I rewired the motor for low voltage and did the same test again but with my Fluke meter.  I got resistance readings on the coils that were very different from what I’d gotten with the Harbor Freight meter so I decided to compare my meters to see how they all stacked up.

To compare them I’m going to measure the resistance of a resistor.  The resistor is a 8 ohm 1% tolerance resistor which means it should have a resistance of between 7.92 and 8.08 ohms.  My meters only have a tenth decimal place on the display so I’ll say that any reading between 7.9 and 8.1 ohms is good.  When measuring resistances this small you have to account for the resistance in the probes themselves.  To do this, I’ll touch the probes together to measure their resistance and then subtract that value from the measured resistance of the resistor.  I also used the meters to measure a 12V battery which sits on a charger in my garage.  The batteries in all the meters were checked to make sure they were good as well.

First up is the Harbor Freight $4 meter that started this all. When I attempted to measure the probe resistance, I got results all over the place.  With enough twisting of the ends of the probes it finally settled on a value of 3.1 ohms.

DMM1

Measuring the resistor which once again took repeated tries, it came up with 10.6.  Doing the math results in a resistance of 7.5 ohms.  On the 12V battery, the meter showed 13.05V which is a good value.

DMM2

The meter did not measure the resistance accurately and took a ridiculous amount of time to get a reading at all.  I’m not sure what’s wrong with it though I suspect some kind of connection issue between the probes and meter.  It’s $4 so I’m not terribly surprised.

 

Next, up is another Harbor Freight meter that I bought on sale a number of years ago because it had a clamp ammeter.  I’ve found that the clamp ammeter is accurate compared to an Amprobe clamp meter I recently fixed.  I first attempted to take measurements using the probes that came with the meter and found that this meter also had issues with them.  They erratically connected and usually wouldn’t connect at all when touching the probe tips together.  I believe I’ve used them in the past for measuring voltage though but now they’re giving me trouble.  I eventually chucked them and grabbed the pair off of the Craftsman meter and had better luck taking readings though I still occasionally had problems.  When everything was working the probes read 0.5 ohms of resistance.

DMM3

Measuring the resistor gave a value of 8.4 ohms.  Running the numbers shows 7.9 ohms.

DMM4

Though the stock probes gave me problems, with better probes the meter gave an acceptable measurement.  It seems I neglected to measure the battery using this meter when I did with all the others.  So, I ran out and measured the battery with this meter and it showed 13.15V.  To compare I measured the battery with the Fluke and Craftsman mentioned below and got 13.25V and 13.24V respectively.  So, it is a little low compared to those.  Honestly, this meter feels cheap.  The plastic seems thin and I fear if dropped, it would shatter.

Next up is a Sperry meter that I bought used somewhere I think.   I don’t use it much but pulled it out for my comparison.    It showed 0.3 ohms of resistance for its probes.

DMM5

It measured the resistor at 8.3 ohms which comes to 8.0 after subtracting the probes’ resistance.

DMM6

This meter has been well used by the previous owner(s) but still measured the resistance well.  On the battery it showed 12.99V which is a good value compared to the other meters that measured the battery at that time.  I don’t know how much this meter was when new, but a comparable model from Sperry is around $40 today.

Next up is my Fluke 87.  It showed 0.4 ohms of resistance for its probes.

DMM7

It also measured the resistor at 8.4 ohms which is 8.0 ohms after the math.  Measuring the battery with most of the other meters except for the Harbor Freight clamp meter showed 13.06V.

DMM8

I bought my Fluke 87 used and happily admit I’m biased towards it.  It’s accurate and much faster at taking readings than the other meters.  It also has a host of features including the ability to take relative measurements.  This means the meter can do the subtracting of the probes’ resistance itself.  You just hold the probes together, touch the “REL” button, and take a measurement of the resistor and it shows 8 ohms.  Easy.  As this is a fancy, professional meter which is made by arguably the top meter company it will set you back a good chuck of change.  A Fluke 87 will run you around $400 new currently and is several models beyond mine.

Up last is my little Craftsman that I found at an estate sale for $5.  It didn’t have probes but after making sure it worked with some other probes a trip to eBay solved that problem.    It showed the probes had a resistance of 0.4 ohms.

DMM9

For the resistor it found 8.4 ohms.  Doing the math give 8.o ohms for the resistor.  This meter showed 13.05V for the battery.

DMM10

This meter seems to work pretty well and a comparable model available today runs about $20.

I also have another larger Craftsman multimeter that is probably 20 years old that was one of my first meters.  It worked well but had developed issues with the dial even though I wouldn’t say I’ve used it very heavily.  I ended up taking it apart and cleaning the contacts on the dial throughly.  After that, it went back to working again.  It measured 0.3 ohms on the leads and 8.3 ohms for the resistor for a resultant resistance of 8.0.

In conclusions, I’d say all the meters did well except for the Harbor Freight models.  I’m sure some of the $4 work well but mine did not.  At $4 a copy you know there is no quality control going on.  A comic book costs more than this complicated electrical device.  The Harbor Freight clamp meter measured accurately once I replaced the probes but I occasionally still had to fiddle with the probes to get a good connection.  The Sperry, Craftsman, and Fluke meters all did well.  For most of the work I do, I don’t care if the meter is off by 0.1.  What frustrates me is if I have to spend time fiddling around with the meter before I can even take a reading.   If you’re a general hobbyist spend a little bit of money on a good meter and don’t buy the cheapest.

 

 

Alright so I’ve got one more meter.  This is my ultramodern RCA LV-10 Vacuum Tube Volt meter.  It never runs out of batteries because it runs off 120V AC.  You never misplace it or forget you’re carrying it either because it’s the size of a shoebox and feels like it weighs 10 lbs.  It also can zero out the probe resistance just like the Fluke.  It even goes above and beyond the Fluke because it allows you to set the infinity reading too!

As you can see it’s dead on 8.0 ohms (green bar).  Unfortunately, it was a little off measuring the battery voltage as it only measured 12V.  Guess I’ll have to fix that one day.

DMM11

 

 

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2 Responses to Multimeters

  1. plenzlerjm says:

    Hi,

    I have an old Craftsman band saw similar to the one you restored.

    I’d like to sell it to someone who would like to restore it.

    Any thoughts on where I could list it?

    All best, Joe

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