This information is useful only if you want a very inexpensive, easy, and extremely accurate method of checking or calibrating a tachometer. The only equipment necessary is commercial shore power, a fluorescent light and some white tape. The fluorescent lamp is actually a gas-discharge lamp with the inside of glass bulb coated with a light-emitting phosphor. The gas is ionized by alternating current and emits pulses of energy. This occurs at the rate of 120 times a second. (60 positive and 60 negative) Because of our persistence of vision we dont see the flickers. We use this very accurate regulated commercial frequency to check our tachometers and also check the output of a genset or generator. A small fluorescent desk lamp may be used as space may be limited. First, gain access to the front of the engine. Place a piece of white tape on the face of the large pulley mounted on the crankshaft(usually the largest pulley). (My pulley is painted white and I could not see the tape but I could see the bolts that hold the pulley on.) Illuminate the front of the engine with the light and (watching where our fingers are) start the engine. Run the engine at 1800 rpm as shown on the tachometer. If the tach is accurate, four stationary or slow moving, white marks will appear on the face of the pulley where the tape was placed. Adjust the throttle until the marks stand still. The engine will be turning at exactly 1800 rpm. If there is a significant difference your tach might be adjusted by a small screw on the back of the tach. Next, if possible, run your throttle to 3600 rpm. At this speed there should be two tape marks visible on the pulley. Using the throttle to make these stationary and, if necessary readjust your tach. Dont use the boats generator to power the lamp because if the generator is off frequency the calibration will be in error. Now connect the lamp to the ships generator and see if the gensets frequency is exactly 60 Hz. The reference mark will be steady, not rotating. If it is rotating slowly dont worry, maintaining precisely 60 Hz is not critical for most uses of shipboard power.