Tachometer Calibration

[GordMay]

Tachometer Calibration ~ by oscar @ ‘Renegade Cruisers’
http://www.renegade-cruisers.org/

Too easy not to do.....compliments of the Catalina 36 Owners Site....(modified)

One of the problems we encounter is a tachometer that is not accurate. Most modern light diesel engines, depending on the model, are designed to run between 2600 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.

“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 crankshaft 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 60-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. If the speed range of the engine permits, increase the engine speed to 3,600 rpm. At this speed, only two 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 3,600 rpm there will be only two 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 3,000 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."
_________________
Oscar, Vice Commodore Commodes-BMYC
Catalina 42 #76, Lady Kay
Chesapeake, SE Florida

Originally posted at “Pauline's Pub” on Renegade’s Forum @:
http://www.renegade-cruisers.org/bb/viewtopic.php?t=928

[Greg B]

I got one of those direct read Dial Tach's from Mcmaster Carr for 40 bucks. My engine tachs are electric and run from the Alternator. I just get up to operating speed and touch the rubber end on the dial tach to the engine crankshaft to get an accurate read. Then I just tweak the adjusting screw on the underside of the dash mounted electric tach to match the direct read.

Zoom Zoom easy

Greg

[GordMay]

Greg:
You're making it too easy.

Gord

[Alan Wheeler]

Yeah Greg, sheeesh, if you are going to make a job tooo easy, what are you going to do with the rest of the day. and plus, you can't tell the wife what a terrible job it all was and get sympathy.
Hey, just one comment to do with the original statement. Most deisels, at least all I know of, have a govenor. This will stop a deisel from over reving, so one's shouldn't be worried about their engine exceeding the rated max RPM. Infact, the engine SHOULD reach it's rated max RPM and stop right there. If it can exceed that rating, then the pump should be looked at and adjusted to ensure it can't.
Of course, having an accurate Tacho is essential to knowing if the engine IS reaching the Rated RPM or not. The flouro idea is one I have never thought of and is a brilliant idea. Although I used to d a similar thing with a Strobe light, whilest testing large arrays of speakers at live concerts. You could set the strob so as you captured the cones of all the transducers moving slowly in time and hopefully unison. If a cable was wired incorrectly, then one or more of those drivers would move the opposite direction. But thats off topic.

[irwinsailor]

Hey Greg! It sure must be nice! I want and need one.

[irwinsailor]

Greg they are $59.95 now. And they say infation is under control!