DS21 & DS21M / DS19A & DS19MA Repair Manual — Page 25

Tightening the union does not increase sealing efficiency. TACHOMETERS (REVOLUTION INDICATORS) Certain checks and adjustments cannot be effectively carried out with- out the use of a tachometer. To ensure accuracy when making these checks a precision instrument must be used. At 600 rpm particularly, it must be accurate to within +20 rpm. Electric tachometers The following instruments have been tested by us with satisfactory results ;