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boatnut
08-17-2005, 06:05 PM
:confused: In another thread I have been discussing the unexplained data concerning use of a Bravo 1 28P prop on my classic 22. In summary, the most logical explanation would be an inaccurate tach. The suggestions from others and the merc shop manual is to attach a "service tach" to check the accuracy. I don't have a service tach yet, but I checked the 4,6,8 cyl setting on the back of the tach (VDO) and moved that and the tach adj screw to ensure good contact -- In doing all of this I realized what a flimsy electrical instrument some of these tachs are. The way these boats bang around at times it is a wonder any of these tachs remain accurate. A 5% error is 250 rpm at 5000 (+-5% is 4750 to 5250), and from my experience (I was an instrument calibration tech many moons ago) that amount of error, or more, is within reason. We make lots of expensive prop decisions based on rpm readings, and make decisions that effect the longevity of our motors. The purpose of all of this babbling is to make a suggestion to the marine motor companies (in case we have any Mercruiser people on the registry). It would be relatively cost free (if done in the original design) to have a test mode in the engines control module circuitry whereby a test signal could be sent out on the tach wire that simulates a specific tach reading (e.g. 3000 rpm). One could then put the engine computer module in the test mode (engine not running) and pulses would be sent out on the tach wire and one could check the tach to see what it is reading and adjust the screw on the back if needed to get the correct reading. A built-in calibration. I would bet that many tachs are not reading correctly. The old mechanical tachs either worked or they didn't so accuracy was probably better on them. Just a thought. Ed

gcarter
08-17-2005, 06:34 PM
Someone on this board suggested I get a "photo" tach. I found several on eBay and bought one. They come with several feet of reflective tape, and I tried it out on a ceiling fan and it worked great (high speed is 240 RPM)! :rlol:
I still haven't tried it yet on my engine. By the time I got around to trying it the batteries were dead. Later I read the fine print, where it said that if you don't use it frequently, remove the batteries! :confused:
Any way the accuracy is 2.5 RPM to 99,999 RPM +/- 0.05 RPM.
Ought to work pretty good.Oh yeah, cost was about $60.00.

gcarter
08-17-2005, 09:01 PM
G, keep in mind that the diameter of the object you attach the tape to will affect the readings yu get on the photo tach.. Most come with a correction chart.
.
Scott...I don't get this...
I understand how this would be true with a wheel roller, but it is only reading a reflected light, how can the diameter affect it?

txtaz
08-17-2005, 09:38 PM
Beats me :) :)

Larger diameter, tape has to travel faster??



I skipped that physics lesson :) :)


Anywayz, just what my Snap on driver told me about ours... But then again, he may be tying to upsell me to the autocorrect self righting model :) ;)
Uhmmmm, huge difference between speed and frequency. Each blade travels past a point once per revolution no matter how long it is. The longer the blade, the faster the edge travels. So does the farthest edge pass this point before the closest edge??? Only if there is deflection, but that would then become a constant at the speed at which the blades travel. So counting the revolutions does not matter where on the blade the tape is located. Now if there is a harmonic of 60Hz and you are under flourescent lights, there may be a problem, but that is for another lesson.
Taz...Dr. of Physics

gcarter
08-17-2005, 09:44 PM
Mr Snap On was probably talking about the combination photo/roller tachs.
This one is light only.
Has a nifty laser pointer built into it.

MOP
08-17-2005, 11:01 PM
Silver tape on a crank shaft is in the same place all the time no matter how big the balancer is, I have been looked into LED photo tach's that fit the currnet tach hole some nice ones out there and super accurate.

Phil

boatnut
08-17-2005, 11:48 PM
This is kind of long to read but may be an inexpensive way to check the calibration of our tachs, it was taken from a Catalina 36 sailboat writeup, only test gear required is a flourescent lamp that can be placed in the engine compartment, what do you think: (I haven't tried it yet but plan to)
:biggrin:
Tachometer Calibration
One of the problems we encounter is a tachometer that is not accurate. Our Universal 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. This method was also written by Chuck Husick in the March/April issue of Ocean Navigator, entitled: “Tachometer Calibration.” I have included a copy of the calibration instructions as well as a copy of the performance curves of typical engines. 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 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.

MOP
08-18-2005, 12:01 AM
That is a WOW really neat! It will be bright in a few motor holes over the next few days :biggrin:

MOP

Tidbart
08-18-2005, 06:45 AM
That does sound neat. The only problem is that I see is that you now have to have a very accurate adjustment of the throttle and you need to purchase a flourescent light.

If you are going to spend $20 on a flourescent shop light, why not spend 15-20 more and get a dwell/tach meter. Take the leads and hook them up to the back of the tach on the boat. Run the boat to any constant rpm and turn the adjustment screw to match the meter. Accuracy will be as good as the meter, which is close enough for a boat.

B

boatnut
08-18-2005, 07:25 PM
That does sound neat. The only problem is that I see is that you now have to have a very accurate adjustment of the throttle and you need to purchase a flourescent light.

If you are going to spend $20 on a flourescent shop light, why not spend 15-20 more and get a dwell/tach meter. Take the leads and hook them up to the back of the tach on the boat. Run the boat to any constant rpm and turn the adjustment screw to match the meter. Accuracy will be as good as the meter, which is close enough for a boat.

B
I hear you---that is another way to look at it. However I have the light and don't have the tach. I think the important point (assuming this process works) is that it is a true calibration of the tach. As opposed to comparing two tachs --- in some cases one might be changing an accurate tach in the boat to match an inaccurate tool?? We do base a lot of decisions on tach readings. Ed