[C320-list] Tachometer

[Bruce Stanley]

Below is an article published by the C36IA.
I have not tested it yet, but on reading it, it would appear to be
excellent.

-+-+-+-+--+-+-+-+--+-+-+-+--+-+-+-+--+-+-+-+--+-+-+-+--+-+-+-+--+-+-+-+-

*Tachometer Calibration*

Fri, 06/20/2008 - 09:29 — admin

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."

-+-+-+-+--+-+-+-+--+-+-+-+--+-+-+-+--+-+-+-+--+-+-+-+--+-+-+-+--+-+-+-+-