The Mk V E-Meter as originally used in the CofS suffers from several major faults. The major trouble is that it is insufficiently sensitive to pick up reads on OT level 5 and above. At the higher levels one is often dealing with small reads sometimes through a fast moving needle.
The CofS attempted to produce a better meter in the Mk VI. While the quality of some parts was improved the essential circuit was very similar to the Mk V. The major change was in the meter movement. My own tests indicate that the Mk VI is actually less sensitive than the Mk V. The Mark VII is a much improved product that uses a superior quality of movement and picks up many more reads. Unfortunately the CofS appears to have an arbitrary in the design spec. The Mk VII movement, in common with earlier models does not come to rest after a read for several seconds. It actually swings 8-10 times before settling. This uncontrolled movement can obscure and confuse reads. I have had a meter movement produced that does not swing about after a read. An increased torque in the coil produces a faster needle response to changes in the pc. Using a device that I built a pc can be connected to 2 meters simultaneously. Comparisons using it show reads on the improved design not present on any CofS meter.
The original Mk 5 had a number of calibration presets and a more complex circuit for the TA/ PC bridge. I have simplified this circuit using a slightly different but much simpler relationship between TA and PC resistance.
Rather than having a scale going from 1 to 6.5 one has a scale from .941 to 6.5 .
The resistance for each TA position is defined by the formulae:
PC = 21250(TA - 0.941)/(6.5 - TA)
TA = PC/(PC + 21250)*5.559 + .941
This gives:
| TA | PC |
| 1 | 227 |
| 2 | 5k |
| 3 | 12.5k |
| 4 | 26k |
| 5 | 57.5k |
| 6 | 215k |
| 6.5 | infinity |
Electronically the PC is in series with a 21.25K resistor across a fixed voltage source.
This makes it possible to create a circuit that calibrates easily. The scale for the TA would be different for TA pots with different extents of rotation and its linearity would depend on the quality of the pot used. With a reasonable quality pot the accuracy is quite sufficient.
My meter development has gone through several stages.
I'll start giving the designs numbers.
The first was the Mk1. That
suffered from a few flaws including low sensitivity at high TA.
The Mk2 was a more complex design
using 6 op amps. It needed some difficult to obtain components.
The 3rd design - the Mk3 is
a very basic meter which I'm leaving up for reference.
The Mk4 is the latest design which
supercedes the earlier ones.