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Old 17th Mar 2016, 10:57 pm   #1
Phil G4SPZ
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Default Was there an Avo "Model S"?

I had a visitor to the museum today who was adamant that Avo produced a special version multimeter for the Royal Navy, called the Model S, with a sensitivity different from the standard Avometer of the time.

This visitor claimed to be an ex-services radar technician. Based on his vague responses to my suggestion that this meter might be the Admiralty Pattern 47A, and taken in context with some of his other reminiscences, I am left wondering... or could he have been thinking of the Avo Electronic Testmeter?

Can anyone confirm, or otherwise, the existence of the elusive "Model S"?

Phil
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Old 18th Mar 2016, 1:38 pm   #2
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Default Re: Was there an Avo "Model S"?

Phil,

I have no recollection or records of a "Model S" Avometer but there are a number of possibilities, most of which will be familiar to you. The "S" suffix was also used to designate a panclimatic version of standard Avometers but these all had the same ranges and sensitivity as the basic models.

Model 9SX - subject of a thread a few years ago. A military development of the Model 8 and forerunner of the commercial Model 9 Mark II which adopted internation symbols for the engraved markings. The 9SX had the same sensitivity as other Models 8 & 9 so appears to be disqualified on that feature.

Universal Testmeter No. 107 - This is a military special with a NATO Type number. I posted a request for information about this model under the thread "Avometer Special - Universal Testmeter No. 107" in 2014. This is also Model 8 derived but has a sensitivity of 10,000 ohms per volt. In default of any other information I had thought that it might be to an Air Ministry Specification because it has the rotary sensitvity switch arrangement as found on the AM Type 'D', but it could also have been used by the Royal Navy or even been to an Admiralty specification.

I also have a Model 8 variant made for the Swedish Armed Forces which has more direct current ranges than standard but the normal sensitivity of 20,000 ohms per volt.

These are all in standard Avometer format, but as you state, there was the Electronic Avometer and there was also the CT38 (ammunition box Avometer) also known as the Electronic Multimeter. Later there was the CT471A and the DA116 was bought in significant numers by the Admiralty, but all these had an entirely different format.

AVO certainly were not averse to making "specials" and the introductory paragraphs of the early Model 8 user manuals positively encourage suggestions from customers so it is certainly possible that a modified type was made for the Admiralty and well documented that many Air Ministry specifications were issued for Avometers.

It might be worth contacting the Radar and Radio museum at HMS Collingwood http://www.rnmuseumradarandcommunications2006.org.uk/ who may be able to help.

The Air Ministry used several models of Avometer each of which they assigned a "Type" letter. The last of these on which I have information is Type V which was a panclimatic version of the High Resistance Avometer (AP 1186E Vol. 1, Sect. 9 issued with A.L. No. 77, September 1948). This would suggest that if there was a "Type S" it would have been from the second World War era.

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Old 18th Mar 2016, 8:53 pm   #3
turretslug
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Default Re: Was there an Avo "Model S"?

Could "Model S" have been memory-slip for "TS No.1" with a 1-3-10 multiplier sequence, as opposed to 1-2.5-10? That might have involved a slight correction factor of service manual quotes in high resistance circuits under some circumstances, hence the "sensitivity different".
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Old 18th Mar 2016, 11:01 pm   #4
Phil G4SPZ
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Default Re: Was there an Avo "Model S"?

Thanks Peter for the comprehensive and useful information, and also to Turretslug.

The gentleman concerned was of similar vintage to myself, if not younger, so I felt he wasn't mis-remembering something from just after WWII. Perhaps the "Avometer Special Universal Testmeter No 107" is just what he meant - the "S" standing for "Special" - because he recounted a tale of using a standard model of Avo and failing to obtain the expected results, until a senior rating pointed out that he was using the wrong instrument and that "a Model S was specified". As they didn't possess a Model S or know its sensitivity, the outcome (in military parlance) was SNAFU.

In high-resistance circuits, doubling (or halving) the load presented by the voltmeter could indeed cause the referred-to discrepancy in readings.

However... just thinking aloud now, would fitting a "K=1 / K=2" switch to the Model 8 have automatically resulted in an instrument with switchable sensitivities of 10k OPV or 20k OPV? From memory, the K-switch on the AM Model D worked in the opposite sense to the /2 button on the Model 40, in the sense that in the K=2 position the pointer deflection for a given applied voltage was halved, and the current for FSD drawn from the circuit under test was doubled, but the sensitivity effectively remains the same. In the Model 40, pressing the /2 button doubles the pointer deflection for a given applied voltage and the same current drawn from the circuit under test, thus doubling the sensitivity as the voltage actually indicated at FSD has effectively been halved.

Fascinating! So I think that the "Model 107" and "Model S" are one and the same, as pictured in Peter's 2014 posting.
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