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[Phys-L] Re: Lab DMM Recommendations?



Two comments...

* * * Concerning students buying meters

We do not have a lab fee. Being a small university, and competing for
students not only with other universities, but even with other
departments within the university, we haven't wanted a science major to
cost more than any other major. The department to which we lose the
most physics students is math. A lot of students find math easier than
physics and change for that reason. We don't want to give them the
additional reason that math is less expensive. If we would require lab
fees for expendable supplies, and even small non-expendable supplies, we
might have to charge a couple hundred bucks a year for physics majors,
and even more for chemistry majors. So far our administration has not
forced us to do that and we see that as a good thing.

Would we get into the business of selling meters so that students would
all have the same meter? If so, do we want that administrative hassle?
Or would the bookstore handle that? Our bookstore is already somewhat
mad at science because they get stuck with so many unsold expensive
science books that have to be returned, and sometimes cannot be
returned. That is, we are having trouble getting students to buy the
textbook (let alone a multimeter). Some try the course without a book,
but many try to share a book. I wonder how many students would try to
share a multimeter. And what would happen if one breaks or someone
comes to lab without a meter. Would we rent them a meter or make them
take a zero? The "cost" to us might be higher in the end.

I do believe students would be more careful with their own things. I
also acknowledge that students don't seem to mind buying a calculator.
But at least calculators are viewed as having a wide-spread utility,
i.e. in many courses.

* * * Concerning buying a 200mA meter and modifying it for 2A

Been there; done that. The typical multimeter has a 200mV voltmeter at
its heart. This can become a 2A ammeter with the addition of a
0.100-ohm shunt resistor. To maintain a 1% accuracy we need at least a
1% resistor, and we need to use it properly. Mouser Electronics does
list some 1% shunt resistors for $3.40 which would serve this purpose.
To use them properly one ought to attach a pair of two-level (stackable)
banana jack/plugs so the shunt resistor can plug into the jacks where
the test leads normally go, and then the test leads can plug into that.
That adds another couple more bucks, but still doesn't solve the problem
that the typical test leads won't plug into it because the banana jacks
on test leads have to be shrouded so the user cannot get shocked from
them. We also need to address the issue of making the shunt resistor
durable enough to be plugged in and out without breaking it. We also
need to store (and not lose) the shunt resistors.

Educational? Extremely. Practical? Not very. The cost would soon
exceed the cost of buying the more expensive meter.

When I want to discuss turning one meter into a different meter or
different range, we have an antique (but quite excellent and quite
accurate) Siemens & Halske 45mV analog meter that resides in a
felt-lined suitcase with precision shunt resistors and precision voltage
dividers to allow DC current measurements from 150 mA full scale up to
150 A full scale, and voltage measurements from 45 mV full scale to 750
volts full scale.

It is a very beautiful instrument.

With my supervision, I make the students use it for one experiment so
they can actually read a mirrored analog scale, and also deal with
conversion resistors.


Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu