Hi Jeremy,
Nice thread on old stuf I hardly ever use. But now I got
encouraged to do some tests as well.
You certainly discovered that Appendix II of the O/S Manual of
the HP432A gives instructions to install long cable options. So HP
is very much aware of the total resistance values. Mine has Option
009 (10 feet cable) but I choose to disregard for the moment, I
have the standard cable. The calibration procedure 2 is quite
convenient making use of my HP34401 DVM. You need a long
stabilization time.? I was wondering: how about thermocouple
effects of the different connections in the chain from the
thermistors to the internal bridges.
For fun I measured the return loss of one of my HP8478B
(1833A13571) sensors. Quite good actually and certainly within
spec. See below.
Thanks for the nice work and observations. Best 73 de Harke
On 8-5-2025 22:14, jmr via groups.io
wrote:
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Hi Paul my name is Jeremy.
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From a metrology point of view, one key limitation of the
Wheatstone bridge system used in the 432A is that the thermistor
is at the end of a long and flexible sensor cable. This adds
resistance to the bridge as shown in blue in the diagram below.?
This introduces a tiny error in the power calculation unless the
resistance is known. On my 432A the internal mount resistance is
virtually spot on 200R within just a few milliohms. However,
when I include the resistance of the sensor cable this increases
to about 200.2R. This is a 0.1% error which is not that relevant
to most users of the 432A. However, when doing DC substitution
this resistance needs to be known and entered into the equation
for the power calculation. Otherwise, this contributes about a
0.1% error to the power calculation. You can see in my youtube
video I entered 200.2 ohms rather than 200 ohms. I think this
assumes the mount resistance is an accurate 200R. I check both
the mount and the mount plus cable resistance.
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The Tegam meter automatically measures the mount resistance
with a 4 wire system and so I think it claims an advantage here.
Unless the operator of the 432A measures this resistance with a
very accurate ohmmeter then this error can't be managed properly
when carrying out a DC substitution measurement. I use a
Keithley 2015 in 4 wire mode to measure this resistance using a
special set of 4 wire test leads. I don't think there is a sense
return wire in the 432A sensor lead and presumably the sensor
wire will be made of wire that is designed to be flexible and
reliable. So this probably means the wire has a bit more
resistance than you might otherwise expect. I think the ground
return wire/shield back up the cable will have very low
resistance and so this doesn't really contribute much in the way
of an error. I don't really see this need to measure the cable
resistance as a significant problem. It never seems to change
year on year and the error it introduces if ignored is tiny at
about 0.1% in my case.
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I think things might be slightly different for the sensor
cable used in the N432A and the Tegam meter. This cable may well
have an additional sense return wire in it that connects right
at the thermistor. So this may alter how this potential error is
managed. But I'm really just guessing. It does make sense to
have this extra sense wire with the Tegam system.
Regards
Jeremy
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