Re: Markings on Big shunt - what is R?

On Jun 2, 2:32 pm, Spehro Pefhany <speffS...@xxxxxxxxxxxxxxxxxxxxxxx>
wrote:
On Fri, 01 Jun 2007 19:28:57 -0700, the renowned bill.slo...@xxxxxxxx
wrote:





On Jun 2, 4:37 am, Spehro Pefhany <speffS...@xxxxxxxxxxxxxxxxxxxxxxx>
wrote:
On Sat, 02 Jun 2007 00:18:06 +0100, the renowned John Devereux

<jdREM...@xxxxxxxxxxxxxxxxxx> wrote:
bill.slo...@xxxxxxxx writes:

On Jun 1, 12:23 pm, John Devereux <jdREM...@xxxxxxxxxxxxxxxxxx> wrote:
bill.slo...@xxxxxxxx writes:
On Jun 1, 12:41 am, Hawker
<Hawker{removethispa...@xxxxxxxxxxxxxxxxxxxxxx> wrote:
I have two large current shunts a client gave me.
One is marked 25 amps 50mV SE company
The other is 20amps 50mV.

Am I to take it that they read 50mV at rated current?
Or in other words the 25A one is .002 ohms and 20amp one is .0025 ohms?
My VOM isn't very accurate down there.

If you want to measure that sort of resistance, you need a four-
terminal (Kelvin) measuring system.

Upmarket multimeters offer this facility - the Thurlby-Thandar 1906
(Farnell order code 724-026) offers four terminal resistance
measurement, but since the resoultion only goes down to 1 milliohm, it
wouldn't do you much good.

Thurlby-Thandar do offer a micro- and milli-ohmeter - the BS407
(Farnell order code 381-2364) which is somewhat more expensive, but
can resolve resistances up to 1.999 milliohm to one micro-ohm, and
19.99 milliohm to 10 uohm.

Top of the line Hewlett-Packard (now Agilent) and Datron multimeters
do better than the TTI 1906, but cost quite a bit more.

But for the OP to simply check his understanding of the markings, he
just needs to stick a few amps through it and measure the voltage
developed across the terminals of the shunt. I usually use a power
supply with an adjustable current limit.

He'd better measure the voltage twice, reversing the direction of the
current between readings - low level voltage measurements are
bedevilled by thermocouple voltages developed in the junctions between
dissimilar metals, and measuring with AC or at least reversing DC is
the standard way of getting rid of these offsets (or at least of
getting some idea how bad they are).

Would you not need quite a big temperature differential (between the
ends of the shunt), for that to be significant?

Several degrees C (more than 5 is my guess) at least just to tick the
LSD. Forget about it. You'd see it anyway when the power supply is
turned off because of the large thermal mass.

The last time I was using a really good DVM to measure low voltages, I
found draft shields were absolutely essential to keep the voltage
stable.

If you're down in low uV DC territory or below, for sure. But we can
get 30 micro-ohm resolution out of a 100uV resolution measurement @3A,
which requires no special care. That's a resolution of ~1.5% of the
expected value, which is fine for the intended purpose.

Do you have a reference on techniques for nanovolt DC measurements?
I'd be interested in that.

No. My impression is that any such reference would start off by
recommending that you immersed the active part of the experiment in
liquid helium and go on from there. Microvolt DC measurements are
already tricky enough.

The English national standards laboratory at Teddington does offer "A
guide to measuring resistance and impedance below 1MHz" ISBN 0 9044557
31.1

http://www.npl.co.uk/cgi-bin/guide_info.pl?guide=105

I've got a copy, but can't recommend it - it doesn't say anything
silly, but it doesn't help you understand what is going on.

You have to be very careful if you want to get anything useful out of
the uV resolution of a good DVM, particularly when measuring
resistances - which automatically involves dissipating some heat.

Heat in itself is not a problem. Nor even are thermal gradients. It
has to be an asymmetrical thermal gradient with dissimilar metals.

You can't dissipate heat without creating a thermal gradient. Ad hoc
connections are always asymmetric.

The fact that the manufacturers data sheets derate resistors linearly
against ambient temperature doesn't means that the temperature rise of
a resistor is a linear function of temperature - a low dissipations
the Maclaurin number is below 500 and you don't get any significant
convective cooling at all, so the the resistor is a lot warmer at low
power dissipations than you'd expect from linear extrapolation.

Maclaurin number? Do you mean the Reynolds number? or maybe the
Nusselt number?

Oops. Raleigh number - the Reynolds number applies to flow, the
Raleigh number applies to convection. Both show up in my Ph.D. thesis.
Why I keep on thinking the Rayeigh number is called the Maclaurin
number I'll never know. Check out

http://www.ldeo.columbia.edu/users/jcm/Topic3/Topic3.html

if you want a bit more detail.

Keep in mind that shunts typically dissipate a fair bit of heat at
full rated current. The OP's wee 25A one will dissipate in excess of
1W in normal use. Larger ones are in the hundreds of watts. The lack
of significant dissipation might affect the reading a bit.

Heat dissipation is proportional to the square of the current - 3A is
going to generate about 1.44% of the heat dissipated at 25A. 3mV isn't
too hard to measure, unless you expect the voltage to be accurate to a
couple of uV, but reversing the current is a useful check.

--
Bill Sloman, Nijmegen

.

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