Damping Characteristics of Model 8 & 9 Avometers - UK Vintage Radio Repair and Restoration Discussion Forum

pmmunro · 13th Oct 2007, 11:27 pm

I have written some comments on Avometer pointer damping in reply to a question I was asked by Russel W_B which I think may be of general interest, hence this new thread.

Russell thought that the pointer damping on his Model 9 Mk II was less effective than on his Model 8 and had noticed that the Model 9 had a parallel combination of a thermistor and a 600 ohm resistor in series with the moving coil and the swamp.

He also noticed that there was some rust on components within his meter and wondered if this could be related to a tendency for the moving coil to stick at random points. (He had cleared this problem with the ingenious technique of applying a low frequency sine wave to cause the moving coil to oscillate continuously).

The Model 9 pointer moved slightly off zero when the meter was held in different orientations but never exceeded the 1% at 45 degrees specified.

Reply:

It would seem that the balance of your Model 9 is reasonable, so there should be no need to be concerned about that.

Concerning the damping, the moving coil assembly for the Model 8 Marks 1 & II has the same part number as the Model 9 Mk II and the resistance of the Universal shunt chain is 10K ohms in both cases. Likewise, the overall resistance of the moving coil plus swamp, and in the case of the Model 9 the temperature compensation components, is 3333 ohms, hence the current division of 37.5 microamps in the moving coil and 12.5 microamps in the universal shunt for full scale deflection at 50 microamps.

From this, I would conclude that the swamp resistances for the Model 8 Mk I & II and the Model 9 are different by the amount of the temperature compensation components at some standard temperature, such as 20 deg. C.

From these resistances, and because the same moving coil assembly is used, there should be no difference in damping due to the temperature compensation components.

If I can get the time in the next week or so, I'll try some experimentation with an 8 and 9 in series and see if there is any noticeable difference.

From the Mark III version, the Model 8 also had thermistor compensation, possibly because AVO were trying to increase exports sales after 1945, even producing special designs principally for export during the 1960s. I don't know exactly when suitable thyristors became available but probably not before the late 1950s. However, long before this time (in 1936), the Model 7 had temperature compensation in the form of a bi-metallic spiral which was used to drive a contact along a short length of resistance wire to limit the effect of temperature change. This feature was not used on other Avometers and when the Model 7 movement was redesigned to replace the horseshoe magnet with a block magnet around 1948, the temperature compensation reverted to being principally dependent on the swamp.

Damp is definitely a problem for Avometers. Despite other damage, I have not seen much trouble from rust on pivots. I don't know what alloy is used for the pivots and I've always assumed that they are hardened steel. Unless the alloy composition is one which is less vulnerable to rusting than mild steel, I would have expected that enough damp to rust other components would also affect the pivots. Even in panclimatic version, there are no obvious precautions to protect the pivots against damp. Perhaps contributors who have some knowledge of pivot metallurgy for clocks might be able to comment.

However, rust is not the only problem caused by damp. The moisture can dissolve contaminants within the meter which, alone, would not cause much harm, but in solution can either get where they can cause harm or form new compounds. Another possibility is that damp allows mould growth. There is obviously a problem when any such effects are visible but it would not take much to affect the pivots and it is quite possible that "exercising" the moving coil was sufficient to break down a slight film of contamination. If this is what happened it is also possible that the "fix" will only be temporary, but in any event, storing the meter where it will be dry can only help.

Damp definitely causes corrosion around and on hairsprings and this can cause solder joints to the hairspring collets to fail. Although I have never seen hairsprings so badly corroded that they cause false readings, this must happen in extreme cases. Steel springs can easily break due to the stress-raising effects of rust. I don't know if this also causes breakages in phosphor bronze springs.

PMM.

13th Oct 2007, 11:27 pm	#1
pmmunro Octode Join Date: Jul 2004 Location: Dundee, UK. Posts: 1,813	Damping Characteristics of Model 8 & 9 Avometers I have written some comments on Avometer pointer damping in reply to a question I was asked by Russel W_B which I think may be of general interest, hence this new thread. Russell thought that the pointer damping on his Model 9 Mk II was less effective than on his Model 8 and had noticed that the Model 9 had a parallel combination of a thermistor and a 600 ohm resistor in series with the moving coil and the swamp. He also noticed that there was some rust on components within his meter and wondered if this could be related to a tendency for the moving coil to stick at random points. (He had cleared this problem with the ingenious technique of applying a low frequency sine wave to cause the moving coil to oscillate continuously). The Model 9 pointer moved slightly off zero when the meter was held in different orientations but never exceeded the 1% at 45 degrees specified. Reply: It would seem that the balance of your Model 9 is reasonable, so there should be no need to be concerned about that. Concerning the damping, the moving coil assembly for the Model 8 Marks 1 & II has the same part number as the Model 9 Mk II and the resistance of the Universal shunt chain is 10K ohms in both cases. Likewise, the overall resistance of the moving coil plus swamp, and in the case of the Model 9 the temperature compensation components, is 3333 ohms, hence the current division of 37.5 microamps in the moving coil and 12.5 microamps in the universal shunt for full scale deflection at 50 microamps. From this, I would conclude that the swamp resistances for the Model 8 Mk I & II and the Model 9 are different by the amount of the temperature compensation components at some standard temperature, such as 20 deg. C. From these resistances, and because the same moving coil assembly is used, there should be no difference in damping due to the temperature compensation components. If I can get the time in the next week or so, I'll try some experimentation with an 8 and 9 in series and see if there is any noticeable difference. From the Mark III version, the Model 8 also had thermistor compensation, possibly because AVO were trying to increase exports sales after 1945, even producing special designs principally for export during the 1960s. I don't know exactly when suitable thyristors became available but probably not before the late 1950s. However, long before this time (in 1936), the Model 7 had temperature compensation in the form of a bi-metallic spiral which was used to drive a contact along a short length of resistance wire to limit the effect of temperature change. This feature was not used on other Avometers and when the Model 7 movement was redesigned to replace the horseshoe magnet with a block magnet around 1948, the temperature compensation reverted to being principally dependent on the swamp. Damp is definitely a problem for Avometers. Despite other damage, I have not seen much trouble from rust on pivots. I don't know what alloy is used for the pivots and I've always assumed that they are hardened steel. Unless the alloy composition is one which is less vulnerable to rusting than mild steel, I would have expected that enough damp to rust other components would also affect the pivots. Even in panclimatic version, there are no obvious precautions to protect the pivots against damp. Perhaps contributors who have some knowledge of pivot metallurgy for clocks might be able to comment. However, rust is not the only problem caused by damp. The moisture can dissolve contaminants within the meter which, alone, would not cause much harm, but in solution can either get where they can cause harm or form new compounds. Another possibility is that damp allows mould growth. There is obviously a problem when any such effects are visible but it would not take much to affect the pivots and it is quite possible that "exercising" the moving coil was sufficient to break down a slight film of contamination. If this is what happened it is also possible that the "fix" will only be temporary, but in any event, storing the meter where it will be dry can only help. Damp definitely causes corrosion around and on hairsprings and this can cause solder joints to the hairspring collets to fail. Although I have never seen hairsprings so badly corroded that they cause false readings, this must happen in extreme cases. Steel springs can easily break due to the stress-raising effects of rust. I don't know if this also causes breakages in phosphor bronze springs. PMM.