BASIC CALIBRATION
This calibration is necessary when any component affecting the
electric or magnetic circuits of the impulse tachometer is altered in any
way.
The figures quoted in the following instructions will only apply to
instruments which have a range of 0-8,000 rpm. For instruments having
alternative ranges, eg. 0-6,000 rpm or 0-10,000 rpm the values of the
calibration points on the dial should be noted and the speed of the test
apparatus calculated by using the following ratios:-
Test Apparatus
Tachometer
4
cylinder engine
1
2
6
cylinder engine
3
4
8 cylinder engine
1
1
Calibration of the impulse tachometer must be carried out with the
movement mounted in the instrument case. It is therefore necessary to use a
dummy case with a section cut from the side to allow the bridge spanner
SR/D.360 to be fitted to the anchor plate.
4
CYLINDER
4
STROKE
Connect the impulse tachometer to the ignition pulse simulator and
speedometer test apparatus as previously described in the Testing Procedure
on Page
4.
Switch on the supply to these units and run the speedometer test
apparatus at 1,500 rpm. Using the potentiometer "L" (See Fig.l. on Page S.5)
set the impulse tachometer to read 3,000 rpm and wait for two minutes to
allow the equipment to stabilise. Reduce the r.p.m. of the speedometer test
apparatus
to 875 rpm and check that the impulse tachometer reads opposite the low
calibration mark 1,750 rpm. If the impulse tachometer does not read 1,750 rpm
fit the bridge spanner SR/D.360 to the hairspring anchor plate and rotate
the plate in the desired direction until the pointer is in line with the
calibration
mark. Care should be take to avoid causing an electrical short circuit during
this adjustment.
After the impulse tachometer has been checked at 1,750 rpm the
speedometer test apparatus should be set at
3,475
rpm and the impulse
tachometer should now read 6,950 rpm. If any adjustment is necessary at this
point it can be made
by adjusting the potentiometer "L" (See Fig 1 on Page S.5.) in the
required direction, but, any adjustment of the potentiometer will make
it necessary to re-check the calibration at the 1,750 rpm position. This
cross checking
should be carried out until accurate calibration is obtained at both positions.
Finally, set the speedometer test apparatus to read 2,300 rpm and check
the mid-calibration point on the impulse tachometer. Switch off the supply
and remove the iron core, former and pulse lead.
The tachometer is now calibrated for
4
cylinder
4
stroke application.
S.8
