Digital ammeter "span" adjustment.

driver0606 said:
Thanks Brian,

I will try that and report back.

Do you think the "span" potentiometer may be faulty because it was turned throughout its range but the maximum change in reading was only 0.1 amps on a 5 amp reading?

John.
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The problem is if there is not an input, the adjuster will do nothing, at the moment I would say a corroded connection on the shunt, of connecting cable fault and you have intermittent fault on supply line.

Did you have multimeter set on millivolts ? remember 9 amp load is only 0.001 volt, 450 amp is 0.050 volt from memory on earlier post.

Brian
 
I used a load of about 15 amps and there was a 20 Mv reading.

Connecting 24V + to the "loop" feed wires gave the same 24V reading.

However, as I cannot access the shunt or wire a new one without major rewiring, I have only two options -

1) Accept the situation as is and create a chart showing actual and true readings.

2) Calibrate the ammeter using the span potentiometer. This is impossible with the ammeter I have as the range is only .1 amps in 5 amp. Do you think my existing (new) ammeter is faulty, please?

To simplify, what does "span" do? For example, an ammeter gives a reading of say, 5 amps at a particular input. If the "span" is turned well up or down, does the 5 amp reading go up or down significantly? Otherwise, what is the purpose of "span" please?

John.
 
driver0606 said:
To simplify, what does "span" do? For example, an ammeter gives a reading of say, 5 amps at a particular input. If the "span" is turned well up or down, does the 5 amp reading go up or down significantly? Otherwise, what is the purpose of "span" please?
Click to expand...

Some quick googling turns up results such as this
DC Current Meter | DC Voltage Meter | Laurel Electronics Inc
which to me suggests that both 'zero' and 'span' adjustments perhaps has to do with compensating for ambient temperature?
 
driver0606 said:
I used a load of about 15 amps and there was a 20 Mv reading.

Connecting 24V + to the "loop" feed wires gave the same 24V reading.

John.
Click to expand...

What that tells me is that the meter you have a a volt meter not at mV meter.

What you need is a millivolt meter that will display the number 15 when the mV meter is reading 20 mV.

The key is a meter that matches your shunt

I always use shunts that are calibrated to 100 Amp to 100mV so attaching a meter that reads 100mV displays 100 being Amps.

Measure the resistance of the shunt. This will help to specify then type of millivolt meter you need.
 
Rogershaw said:
Measure the resistance of the shunt. This will help to specify then type of millivolt meter you need.
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Agree in theory, but how could one accurately do this?
The resistance of a typical shunt would likely be in the region of 1 to 0.1 mOhm.
Seems to be difficult, especially without direct access to the shunt...
 
driver0606 said:
I used a load of about 15 amps and there was a 20 Mv reading.

Connecting 24V + to the "loop" feed wires gave the same 24V reading.

However, as I cannot access the shunt or wire a new one without major rewiring, I have only two options -

1) Accept the situation as is and create a chart showing actual and true readings.

2) Calibrate the ammeter using the span potentiometer. This is impossible with the ammeter I have as the range is only .1 amps in 5 amp. Do you think my existing (new) ammeter is faulty, please?

To simplify, what does "span" do? For example, an ammeter gives a reading of say, 5 amps at a particular input. If the "span" is turned well up or down, does the 5 amp reading go up or down significantly? Otherwise, what is the purpose of "span" please?

John.
Click to expand...

Okay we have a reading, now try turning something off, does the mv reading go down in a similar way, i.e. remove half the load, mv reading falls by half. You can try increasing load if possible and check new reading, if reading is roughly in ratio it would indicate shunt is reading.

The ammeter is probably the same display as the voltmeter, but has a high gain amp to raise the shunt mv to a voltage that will display on the voltmeter correctly, the span adjust is adjusting this value to match.

You said the shunt was 450 amp / 50 mv ?, this means your 15 amp load should be 1,66 mv, or 0,00166 volt, was the multimeter set on mv's ? if so what is the range ? your 20mv would be a 180 amp load if measured across your"loop wires" with your shunt.

This is the problem with shunts, you are working in such small voltage, a lot depends on the multimeter range and spec,

Brian
 
Yes, the reading goes up and down in at least approximately in proportion to the load.

All I can do is adjust "span" to try for a more reasonable reading (at present its about 10 to 20%.) It is not practical or necessary to be very accurate. This is for operation of the boat, rather than a scientific experiment.

BUT as the "span" adjustment range is only .1 amps on a 5 amp reading. Do you think my existing (new) ammeter is faulty, please?
 
driver0606 said:
Yes, the reading goes up and down in at least approximately in proportion to the load.

All I can do is adjust "span" to try for a more reasonable reading (at present its about 10 to 20%.) It is not practical or necessary to be very accurate. This is for operation of the boat, rather than a scientific experiment.

BUT as the "span" adjustment range is only .1 amps on a 5 amp reading. Do you think my existing (new) ammeter is faulty, please?
Click to expand...
I think it is more likely that you new meter does not match or is not calibrated for the existing shunt
 
VicS said:
I think it is more likely that you new meter does not match or is not calibrated for the existing shunt
Click to expand...

I agree and as when you put 24 Vdc on the "loop" wires and the reading was 24 I read this as a voltmeter not a millivolt meter that you need to measure the voltage across a shunt.
 
driver0606 said:
Yes, the reading goes up and down in at least approximately in proportion to the load.

All I can do is adjust "span" to try for a more reasonable reading (at present its about 10 to 20%.) It is not practical or necessary to be very accurate. This is for operation of the boat, rather than a scientific experiment.

BUT as the "span" adjustment range is only .1 amps on a 5 amp reading. Do you think my existing (new) ammeter is faulty, please?
Click to expand...

Looks like the shunt is working, now you have some values you can get back to the boat builder, point out the mv values to amps you get, and the problem with span adjust.

More likely they have supplied a voltmeter, not an ammeter.

Brian
 
The builder let me try another ammeter and it worked fine! It looked identical to the other. Thank you for all of your replies and I am sorry I cannot actually identify the problem.
 
Glad you sorted it.
4-20mA is a specific interface for measuring equipment. It is used with pressure sensors and the like. 4mA means 0% reading for whatever the sensor is and 20mA for 100%. It is not designed to be directly connected to a shunt for current measurement.
I guess some digital displays are dual-mode, with switch to select 4-20mA or voltage input - did yours not have this?
 
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