Two Monitors on One Shunt?

[BFL13]

In another thread the set up is where two monitors share a ?? 500a shunt. Each monitor specifies 500a anyway.

On the side of a shunt you get the little screw terminals (I have only seen the Trimetric 500a one) for the wires that go to the monitor. These read tiny amounts of ? voltage across the shunt to provide the readouts. Whatever. So, I am wondering----

If you have two sets of these wires on the shunt's little side screws going to two monitors, will that mess up their readings by not having the right tiny amounts, because they are in parallel?

It seems the two shunts agree on their readings, but that still leaves the possibility they are both equally wrong?

[grizzzman]

BFL13 wrote:
The Trimetric wiring cable has some of its wires twisted together between display and shunt. Does it matter if they are not if you use your own wire?

Would it matter if the wiring for the second monitor (twisted) were lying near the other (twisted) wire run for part of its length?

The real issue is paralleling high power lines (12 or 120 volts)the twisting reduces the electromagnet forces.

[MEXICOWANDERER]

About 90% of the errors I see with correctly configured shunts is the deterioration of accuracy due to degradation of the terminations. Anyone who configures a shunt with crimped wires is screaming for problems down the road.

I solder the impedance pick up wires, polish the connectors then smear grease over top the connections.

The last 500-amp 75mv shunt problem I encountered rendered a 16 ampere error at some 400 amp load.

[DrewE]

The wire used makes little matter, and likewise its routing is not very important provided both wires are in close proximity to each other (so any inductive noise pickup is common mode--affecting both simultaneously--rather than differential mode). Twisting them together does help in this regard, but I would not think is essential. Having two sense wires near each other makes no matter at all; they are carrying what is essentially a DC signal, so there's no mutually inducted noise from one to the other to worry about.

The largest source of error in the wiring would likely be the DC resistance of the wires themselves, and that's utterly negligible for any somewhat reasonable installation as the current in the wires is minuscule.

Adding a baggage door hold-open latch to the side of an RV likewise does not add enough wind resistance to mess up the handling by making it pull to one side.

[MEXICOWANDERER]

Oh you underestimate how anal some are here...

Step-up to me horsee hypo needle...

Grab your ankl....

Whassa matter it's only a needle...

[BFL13]

The Trimetric wiring cable has some of its wires twisted together between display and shunt. Does it matter if they are not if you use your own wire?

Would it matter if the wiring for the second monitor (twisted) were lying near the other (twisted) wire run for part of its length?

[2112]

Your shunt is a 0.001 ohm resistor. Let's assume each meter has 100,000 ohms of input resistance.
Placing two meters in parallel across your shunt affectively changes the shunt resistance to 0.0009999998. 500A will measure as 499.99998A. The tolerance stack-up on the components will produce more error than the second meter.

[BFL13]

Not the same shunt but Mex said in his example:
•ONE millivolt = one ampere across the shunt

So I wondered if having two sets of wires would split or otherwise upset such small (millivolt)values and so make a big error at the monitors for AH count etc.

The replies say no it won't.

[ScottG]

DrewE wrote:

ScottG wrote:
It will work fine. The digital meter itself doesn't create any load that will effect the other meter.
In the old days of analog meters it would not work but the digital units have almost absurdly high impedance.

Even with a low input impedance, it would not affect the reading sufficiently to make any real difference. The source impedance of a 500A/100mV shunt is one-fivethousandth of an ohm (0.2 milliohms), as computed by a simple application of Ohm's law. It would have to be an extremely low input impedance meter input to see any loading effects of practical significance.

This is somewhere around the the same level of error as would be the loading effects from multiple voltmeters connected to the same 12V battery.

Oh you underestimate how anal some are here :B

Even if a meter dropped half the thickness of the needle some of these guys would be up in arms. 😛

[DrewE]

ScottG wrote:
It will work fine. The digital meter itself doesn't create any load that will effect the other meter.
In the old days of analog meters it would not work but the digital units have almost absurdly high impedance.

Even with a low input impedance, it would not affect the reading sufficiently to make any real difference. The source impedance of a 500A/100mV shunt is one-fivethousandth of an ohm (0.2 milliohms), as computed by a simple application of Ohm's law. It would have to be an extremely low input impedance meter input to see any loading effects of practical significance.

This is somewhere around the the same level of error as would be the loading effects from multiple voltmeters connected to the same 12V battery.

[grizzzman]

BFL13 wrote:
I don't know what an ADC is. The set-up in question is a Magnum inverter/charger with its monitor sharing a shunt with the Whizbang monitor going with a Midnite Kidd solar controller.

Does the whizbang still connect directly on the shunt? If you can connect the extra wires with out hitting any components it should work just fine.

[BFL13]

OK, thanks all.

[BB_TX]

If you had a very accurate measuring instrument down to several decimal points you could detect a slight difference simply due to the fact of paralleling another resistance. But that difference would be so small as to be inconsequential.

[MEXICOWANDERER]

Analog Meter NYET

Digital Meter DA

[BFL13]

I don't know what an ADC is. The set-up in question is a Magnum inverter/charger with its monitor sharing a shunt with the Whizbang monitor going with a Midnite Kidd solar controller.