Re: Markings on Big shunt - what is R?
- From: John Devereux <jdREMOVE@xxxxxxxxxxxxxxxxxx>
- Date: Sat, 02 Jun 2007 00:18:06 +0100
bill.sloman@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?
--
John Devereux
.
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