Matty said:
I'll have a look at the instrument tomorrow. It's a quality instrument,
and I used an identical one 40 years ago at University. There are only
two terminals. I do suspect self-heating in an internal resistor.
I am being criticized for blaming the meter. Do you have any references
for inaccuracy in measuring low resistance, i.e. around 0.2 ohm?
The boss has already suggested doing something like that, and is going
to bring his Avometer and other meters tomorrow. We have already put 30
amps through another solenoid, and it seems to work fine. We have a
large battery with a 100 amp meter and can change the current with a
compressible pile of carbon slabs.
All we want to do is to check whether all the solenoids are the same,
and whether they have shorted turns. Measuring the precise resistance is
not important. I was just wondering why it keeps changing.
In use, the solenoids get a variable high current (maybe 30 or more
amps) for a maximum of a minute or so.
According to tables I've just read, 100 feet of 14 AWG is about 0.25
ohms, which is in line with what I get.
Hi, Matty. The quality of the instrument doesn't matter if it's being
used to measure something it's not made to do. I'd be fairly confident
that if there are only two terminals, it's probably not made to
accurately measure less than an ohm. So don't worry about blaming the
instrument. From what you've said, that's the first place I'd look,
too.
Unless you have a resistance standard, you'll kind of have to wing it
on bouncing low ohm measurements. The first quick check is actually
the force current/measure voltage method. If you have a good handheld
DC voltmeter like the Fluke 77 (around 0.1% DC Volts accuracy), your
resistance measurement accuracy will be almost entirely dependent on
the accuracy of your ammeter. If you can crank 1 amp through your Rx,
another Fluke 77 measuring DC Amps will give you a resistance
measurement accuracy of 1.5%, which should be fine for what you're
doing. Inferring resistance by forcing a measured current and then
measuring voltage is theoretically sound as well as practical. Your
boss is making sense on this one. But in order to get more accuracy or
give you confidence in your measurements, Ohmite, Dale and others make
those aluminum-housed 50 watt 0.1% 100 milliohm Evanohm wirewound
resistors, which can be convenient, especially if you just want a quick
sanity check. If you thermal cycle the resistors in an oven several
times to stabilize them, measure resistance with a quality calibrated
instrument, and then make sure never to apply more than 1 amp of test
current to them (<20% of rated wattage), you can be fairly confident of
the results when you bounce instruments. I've got some myself in
decade values from .1 ohm to 100K ohm, and keep them in reserve for
when I need an ohms sanity check. All unofficial, of course.
High currents are more difficult to measure accurately, though. A
12VDC 1 amp unregulated wall wart, an LM317 with a heat sink, two
resistors and two 10uF caps will get you the regulated 10 volts. Most
of this stuff might even be in your junkbox or scrounge-able. Excess
currents will just cause Rx heating, and make your life more difficult.
You don't need and shouldn't use 30 or 100 amps to measure an 0.25 ohm
copper resistor.
Checking solenoids by checking resistance alone can be a bit of a
problem. The tightness of winding causes variations in wire length
which might account for your variations in resistance. Variations in
drawn wire diameter can make resistance values easily vary by 10% or
more. I think resistance _and_ inductance measurements might be a
better way of getting where you want. A better wound coil would have a
somewhat lower resistance, but a somewhat higher inductance. A coil
with marginally smaller wire diameter would have higher resistance, but
almost equal inductance. Missing or shorted windings would be more
noticeable with both measurements. If you've got one of those handheld
meters that measure inductance, that might help a lot.
Another practical, valid method you might use if you're serious is
putting a standard voltage across the solenoid, and measuring pull
force. I'm not sure if that's practical, though, for a 4-off check.
Good luck. Feel free to post again to let us know how you're doing.
Chris