Tachometer Calibration

Below is an article published by the C36IA.

Remember that a meter is available from the C320IA Toolbox or you can use a fluorescent lamp as described below.

Also, for our Yanmar engines, you may want to make the adjustments at 2400 and 3000 rpms.

by Bob McCullough, Tech Editor Emeritus

One of the problems we often encounter is a tachometer that is not accurate. Our Universal engines, depending on the model, are designed to run between

2800 and 3200 RPM. If the tachometer is inaccurate we don't know if we are exceeding our rated RPM. There is a simple way of checking engine speed and

calibrating the tachometer. This method was also written by Chuck Husick in the March/April issue of Ocean Navigator, entitled: "Tachometer Calibration." Below is a copy of the calibration instructions. Chuck Husick's instructions are very easy to follow. Who knows, we might find out that we have been adjusting our engine speed to a way-out tach.

"There is no guarantee that a boat's tachometer accurately portrays engine speed. Those most likely to be accurate are digital units driven from pulse

generators which sense crank shaft rotation. Many tachometers are actually frequency meters, measuring the frequency of the alternating current

developed in the belt-driven engine alternator or the frequency of a tachometer pulse generator on the engine. Some older designs are driven from

a rotating cable similar to an automobile speedometer cable. Checking the accuracy of a tachometer usually requires use of another tachometer which is

known to be accurate; this tach can then be used as a measuring standard.

There is a way to check the accuracy of a tachometer using the highly accurate and stable frequency of AC power supplied from any source of

commercial shore power. The concept is to use the AC line frequency as the measuring standard. The only tools needed for this calibration check are a

piece of tape and a fluorescent lamp which operates from commercial AC line power.

A fluorescent lamp is actually a gas-discharge lamp with the interior of the glass envelope coated with a light-emitting phosphor. When the gas within

the lamp is ionized by alternating current it emits pulses of energy. One pulse occurs for each of the voltage excursions of the AC waveform. For the

6O-Hz power common in North America, there will be 120 such flashes per second, 60 positive and 60 negative. The pulses of energy created within the

lamp excite the phosphor coating, which in turn emits visible light. Because the energy driving the lamp is not continuous, the light emitted is not

continuous. The fluorescent lamp emits 120 pulses of light per second, but the human eye's persistence of vision makes us think the light is always on.

We can use the pulsing light output of the fluorescent light as a very accurate measuring tool with which to check the calibration of the engine tachometer.

First, obtain access to the front of the engine. Place one piece of white tape on the face of the large pulley mounted on the engine's crankshaft

(usually this is the largest pulley in sight). Illuminate the front of the engine with light from the fluorescent lamp. Run the engine at 1,800 rpm, as

shown on the tachometer. If the tachometer is accurate, four stationery, or very slowly moving, white marks will appear on the face of the pulley where

the tape was placed. If the tachometer is inaccurate, the tape marks may be rotating in either direction. Adjust the throttle until the four tape marks

appear to stand still. Note the tachometer reading. If the difference between the reading and 1,800 rpm is at all significant, look for a small

adjustment screw on the back or within the body of the tachometer. Turning this screw slightly should make the indicator needle move to exactly 1,800

rpm. Increase the engine speed to 2400 rpm. At this speed, only three tape marks should be visible on the crankshaft pulley. Repeat the check of the

tachometer reading and, if necessary, readjust the tach.

The basis of this stroboscopic speed calibration is quite simple. At 1,800 rpm, the engine is turning at 30 revolutions per second. The lamp is

flashing at 120 flashes per second, or four flashes per engine revolution. Therefore, if the engine is turning at exactly 30 revolutions per second the

tape mark will appear four times, with each apparent tape position 1/120 of a second or 1/4 revolution apart. When the engine runs at 2,400 rpm there

will be only three light flashes per engine revolution. If the boat is in a country where the standard AC power frequency is 50 Hz, the check speeds

would have to be 1,500 and 2,250 rpm since the light would flash 100 times per second.

Don't use the boat's generator to power the lamp when doing this calibration test. If the genset is off frequency, the calibration will also be

inaccurate. It is possible, however, to make use of the engine to check the frequency of the genset. After adjusting the engine speed so that the

reference mark on the crankshaft pulley is exactly stationary, connect the fluorescent light to the output of the genset. If the frequency of the

genset is precisely 60 Hz, the reference mark will still be steady, not rotating. If the reference mark is rotating slowly don't worry, maintaining

precisely 60 Hz is not critical for most uses of shipboard AC power."