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Go Back   SailNet Community > On Board > Gear & Maintenance > diesel rpm
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Topic Review (Newest First)
10-30-2007 11:13 PM
Gene T Very cool!
10-30-2007 10:57 PM
voyagersail
Easy...

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.
10-30-2007 02:04 AM
groundhog I just used one of the handhelds on my diesel this weekend.
Not sure if this is right, but I put a little piece of the reflective tape (comes with the handheld) on the flywheel.

If I had the monitor too close, I got some funny high readings. I think that is because I was getting too much reflection from the non-tape metal area of the flywheel. So I backed off about to 6-8 inches off and got good readings.

The weekend before I used an oscilloscope and a piezo thingy. You flick the piezo thing with your finger and it makes a blip on the scope. I put a tape flap on the flywheel and let it hit the piezo thing every time it went around.

Also measured the drive shaft RPMs to to calculate my gear ratio.

groundhog
10-29-2007 04:15 PM
BarryL Hello,

If I had to change my tach I would try one of these:

http://www.tinytach.com/tinytach/diesel.php

Good luck,
Barry
10-29-2007 02:03 PM
jones2r I've used a Veederoot handheld counter in conjunction with the sweep hand on my wristwatch; very accurate. It would probably be held against the harmonic balancer retaining bolt (inside of the crankshaft pulley). It's not dependent on the calibration of the test equipment (if your wristwatch is close to doing its job). Just offering an alternative in the event you know someone who has a Veederoot counter.
10-28-2007 01:46 PM
sailboy21 For about $30 from ebay or autozone you can get a handheld laser tach. They are extremely accurate and easy to use.
10-28-2007 01:22 PM
pedsto
diesel rpm

just had to buy a new tach. since not same as original equipment have to calibrate. how does one determine the rpm of a diesel engine? thanks

 
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