TM 9-1829A, April 1944
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2021-07-07
c.
Check against master head:
Check the charged unit on calibrating machine with master head reading 10
miles per hour and adjust hair spring regulator arm on field plate so that unit being tested reads 18 to 20 miles
per hour. Refer to chapter 13 for further instructions.
d.
Check magnet:
Run the master head up to 30 miles per hour and the unit being checked should read not less
than 45 miles per hour. If this minimum reading cannot be obtained, it is usually caused by the magnet not
being fully charged. This can be caused by the electrical equipment not functioning properly, or the inability
of the magnet to retain a charge. Check that proper speed cup assembly is used as too strong a hair spring will
show low, and too weak a hair spring a high speed reading. Test battery for specific gravity and individual
cell voltage. If 12 volts were used the first time, try 24 volts. If magnet cannot be charged to show minimum
of 50 percent overcharge, the magnet and first gear assembly must be removed from the speedometer and
replaced with a new one from stock.
e.
Demagnetize:
When the minimum of 45 miles per hour or a greater reading is obtained with the master head
at 30 miles per hour, the unit should be demagnetized down to 30 to 32 miles per hour with magnetizer (T-
189785). Use the same 12-volt direct current source, obtaining the effect of an alternating field by running
the speedometer on the tester at a master head speed of 30 miles per hour. Demagnetize by holding one pole
of the magnetizer near the field plate and press button, immediately releasing it. When this is done it may be
well to hold one finger between the field plate and the pole presented to it, thus preventing the magnetizer
from being pulled into contact with the field plate, completely discharging the magnet. When unit is
demagnetized below master head speed, it is necessary to start over again by first charging the magnet and
then proceeding as outlined above.
f.
Complete the calibration:
When accuracy is obtained at 30 to 32 miles per hour the unit should be checked
at 10 miles per hour. Adjustment, up or down, at this point should be made only with the hair spring regulator
arm to obtain reading of 9 to 11 miles per hour. Make final check for accuracy at the following speeds:
Master head (mph)
Calibration limits (mph)
Adjust with
10
9-11
Hair spring regulator arm
30
30-32
Hair spring regulator arm and demagnetizing
60
60-63
When checking calibration and pointer bank, it is permissible to tap the instrument lightly.
g.
Tachometer calibration:
Follow the same method as outlined above except use the following chart, this
chart being based on a 1:1 ratio tachometer. If ratio of unit being checked is 2:1, multiply revolutions per
minute limits shown by 2; if 3:1, multiply by 3, etc. Refer to chapter 13 for special information regarding the
calibrating machine when used to check tachometers.
Master head (rpm)
Calibration limits (rpm)
Adjust with
166
160-175
Hair spring regulator arm
500
500-525
Hair spring regulator arm and demagnetizing
100
1000-1050
1500
1500-1550
2000
2000-2050
3000
3000-3050