Avo 8 - best practice ...

[Colin]

A couple of basic questions from someone fairly new to working with the Avo 8 (MkII);

1. Does the position it's used in affect the accuracy of the readings?

I've been using mine standing vertically on a shelf directly above the bench because it's easier to read from a sitting position and it's out of the way. However, I noticed a small c.2-5% error on the reading when compared with my DMM and other moving coil meters (in bench PSU's etc). (I know there is likely to be a difference in the readings of the DMM due to the lower impedance of the Avo).

When I use the Avo lying on its back on the bench it reads as it should and compares well with other methods of measurement including the DMM. Is the Avo movement meant to be used in this position?

2. What is the best setting of the range switches for storage and transportation?

I've been told that setting the range switches to 10A DC provides some damping for the movement. Also if it's left on one of the resistance range will there be any drain on the internal batteries?

I'm sure these are fairly basic questions to those of you more familiar with the Avo but as I'm beginning to develop a strange liking for using the thing I thought I'd better check them out.

Thanks
Colin

[AC/HL]

Hi Colin,
I'm not sure about the horizontal-vertical question. I would expect a meter of this quality to be balanced either way, at least when new.
Does the zero change?
As for transport, as I understand it switching the AC knob to DC & vice versa shorts out the movement for transit.
Cheers, Bill

[tvden]

Hi all well I have this exact model and am very happy with it if you look at the instructions on the back it states meter to be used face upwards for accurate readings, perhaps yours has the instruction plate missing ?? regards Den

[high_vacuum_house]

Hello there,
I always store my AVO with the DC volts set to 1000 volts so that there is no damage done if you forget to set it up before using it next.
regards,
Christopher Capener

[pmmunro]

As Bsaden says, the instructions do say that an Avometer should be used in the horizontal position. They are calibrated in this position and they should be balanced to within 1% when new or newly serviced. A significant overload will bend the pointer slightly (or greatly) and this will upset the balance. It's one point to check when buying any moving coil instrument. (Zero the pointer with the meter on its back, as for normal use. Hold the meter with the case vertical and so that the pointer should be horizontal and check that it lies on the zero mark. Still holding the case vertically, turn the case so the pointer should be vertical and again check the pointer lies at the zero mark).

If you've ever attempted to balance a moving coil movement with the length of pointer an Avometer has, you'll know that it takes much time and patience to get the balance better than this. You may have noticed, if you've ever been inside such a movement that there are sometimes paint on the shaft of the pointer - these are in fact fine balancing weights - the only trouble is that paint, or shellac, loses mass as it dries so that it has to be over applied by educated guesswork to get the correct balance. It can be quite frustrating the first time you experience leaving a perfectly balanced movement at night and finding it out of balance the next morning. A quicker method of fine balancing, to supplement the coiled wire weights, is to use a suitable hard wax.

If anyone is still with me, it may be of interest to know that a glass-fronted draughtproof box is needed to balance a movement. Even the convection currents generated by the heat of your hand will cause a strong enough air current to deflect an Avometer pointer significantly.

Another stipulation in the instructions, at least at one time, was that the correct test leads should be used as the resistance is significant. I'll have to admit I remain to be convinced on that.

[Duke_Nukem]

I always store my AVO with the DC volts set to 1000 volts so that there is no damage done if you forget to set it up before using it next

I always though that when not in use it was best to "short" out the meter ? I think this is what happens in the "off" position, or if there isn't one then switch to a high amps range ? This damps the meter movement when in transit.

TTFN,
Jon

[Lowemission]

Also if it's left on one of the resistance range will there be any drain on the internal batteries?

If the left-hand switch is left on the resistance setting then yes it will drain the internal batteries, i did that very thing when i used my AVO for the first time . Although you can (without draining the batteries) leave the right-hand switch on one of the resistance ranges as long as the left hand-hand switch is not set to resistance.

If i'd read the Instruction Manual first i would have seen a warning saying that the left-hand switch "must not be left at RESISTANCE".

[pmmunro]

From my reading of the circuit diagrams (you need the manual for that), the battery and cell in a Model 8 can only be in circuit if one of the resistance ranges is selected. There should not be any chance of discharging either the cell or battery unless resistance ranges are selected and the leads touch.

If the left hand (DC) switch is left on the RES position there will be spurious circuits through the meter, especially on the AC current ranges, which could lead to very heavy currents which would not be interrupted by the cut-out as this depends on movement of the moving coil. In all other circumstances, by design, the switches are "interlocked" electrically so that no current will flow unless one is in the "home" position.

In the Model 8 Marks V, VI and 7, the OFF switch position puts a short directly across the movement to give damping for transit. On earlier meters, the same effect can be had by setting the meter to 50 microamperes DC and linking the main terminals by a heavy-gauge wire. The universal shunt chain in a Model 8 is always in parallel with the movement giving some damping. If there is a break in the universal shunt the moving coil will characteristically be undamped, a point to watch for if buying a meter.

At the school I went to (early 1960s), we had a teacher of physics who often told us that leaving the school's only multimeter ( a Multiminor Mk I) on resistance would discharge the battery through HAND CAPACITANCE!!! We were too polite and deferrential to question this.