Avometer Mod.47A DC current shunt oddity

[The Philpott]

I (still) haven't sussed out the exact thinking behind this DC current shunt, and wondered if anyone has seen one like it.

The incised lettering suggests it is an ACWEECO product; i've no reason to believe it's not...but either something's not right or i'm missing something.
I can't measure resistances this low with any accuracy or resolution, so i clamped it to an AVO 47A variant and ran current through it using a halogen lamp and a car battery. Starting off cautiously on the 12v range i got movement from the needle, changed to 1.2v range which gave around 22 on the 120 scale (ie 220mV.) This is interesting as the heftier shunts of this type for use with Model 40 series meters are designed for the 120mV range.

Having then used the 12A range in the normal way without an external shunt, i got a reading of 6.4A or thereabouts. This indicates to me that full scale USING the accessory shunt would be some 36A..so why does the lettering say
'12 + 12 AMP'..? I certainly wouldn't want to run more than 18A through the shunt for any length of time, encapsulated as it is..but i suppose a drifting reading would warn against overheating.

- Is the Avo giving an honest reading or is the use of rather high current pushed by a low voltage biasing my result? (An Avo 8 also registered 6.4 Amps in this test.)

- Am i using the shunt as it is supposed to be used, or does the Avo 40 series have a trick up it's sleeve that enables the shunt to correlate to the 0-120 or 0-480 scale?

- What's the significance of the marking '12 + 12 AMP'?

Thoughts welcome!
Thanks
Dave

[wave solder]

I would presume by the markings that if you fitted it across the terminals of an AVO 47A and set the meter to 12A DC then the FSD would then be 24A (12+12)....?

[The Philpott]

I did wonder if that was the case- but couldn't grasp how that scenario would give 24A.
I'm confident it can't do any harm with the 6.4A test current so i'll try and report back.

Dave

[Superscope]

That's a nice addition to an Avo collection Dave.

Never seen one of them before.

I would be inclined to agree with "Wave Solder" that it is
most likely suposed to be used on the 12" Amp Current Range.

If true, that should be easy enough to test with the Set Up you
have already, by measuring the Current of your lamp on the 12 Amp
Range with the Shunt, and hopefully you should see 3.2 Amps.

If not, then this Thread might be getting much longer
Always nice to have a new Avo mystery.

Let us know if you make any progress.


Edit: Posts Crossed
Ian

[Alistair D]

The 47A has a divide by 2 button. I wonder if that marking means the shunt can do 24A with the button pressed.

Al

[The Philpott]

Yes, that's it! Thanks wave-solder/Ian. I hadn't come across a shunt which is used on a current range before, hence the caution.

The battery's dropped a little so the test current is now a measured 6.2A using the meter alone. Then using the external shunt, i'm getting 5.8A on both the 'new' 24A range, and on the press-button 12A range. The cutout isn't achieving perfect contact while using the external shunt in this way, (which could explain the shortfall, it needs attention) but it's interesting this fault didn't show up when using the meter on it's own.

As the dissipation is shared across both internal/external shunts, i suppose the full range up to 24A is viable. So this accessory allows a Model 40/47A/48A to compete with a Model D in one respect at least!

(crossed with Al's response- 24A then becomes 12A range with DB2 button pressed)

Dave

[pmmunro]

On the question of the origin of this shunt, I would be inclined to think it was not made by ACWEECO as they would almost certainly have put their name on it. There were also the shunts made especially for the 48A which could of course have been used with the 47A.

It looks well made, and given that the 47A was almost, if not completely exclusivly, an Admiralty instrument, I would suggest it was probably made in a Royal Naval workshop to meet some reported need. It might even have been made on a ship, as any vessel of significant size would have had a well equiped machine shop and higly skilled people to use it.

The terminals look more like those for later Avometers than anything in use when the 47A was in production but they might also be commercial components or drawn from spares stock if the shunt was made after 1956.

I think I have seen a 48A with a RN calibration sticker frm the 1990s, so presumably some of these meters survived in Admiralty ownership much longer than might be thought.

PMM

[The Philpott]

Thanks Peter. A closer look at the terminals. The inner is swaged at the top to prevent the knob escaping. Not a type i recognise, and not quite as robust looking as any of the Avo ones i'm familiar with.

If inserted a particular way the shunt does manage to fit in a 40/48A test set box with the other two heavy duty shunts- either by accident or by design.

Dave

[The Philpott]

A further clue- i've just noticed that when fitted to the meter, the script is upside down.
Dave

[The Philpott]

- Seems a current of 6A generates a reading of around 5.7A. (once i had found a 40 series Avo that wasn't experiencing issues.) Close enough for an ad-hoc unit, might even be closer higher up the scale (not tested yet)
Dave

[Electronpusher0]

Since "AVO" is a registered trade mark the use of it incised into this item suggests either it was used with permission or possibly the shunt was a one off, perhaps made by a laboratory or educational establishment.

Peter

[The Philpott]

-i found the receipt for it the other day, it arrived here with a set of the standard shunts, transformers and multiplier that come with a 40 or 48A Test Set. No box or meter, just the bare bones. All in quite good condition, no clues to usage by smell or otherwise.
Dave