# MAX Update No. 65: Measuring Fuel by the Tablespoon

| 12/23/2010 3:09:32 PM

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Sure, your car came with a fuel gauge, but MAX didn't.

After-market fuel gauges are tricky things, which is why nice ones cost a hundred bucks or more. They have to translate the electrical signal from a contraption inside the fuel tank into useful information about the quantity of fuel remaining. Well, OK, it's not really all that tricky, and I don't know why fuel gauges cost so much—the contraption in the tank is a variable resistor (think volume control knob on a radio) with a float attached to it, and all the gauge has to do is measure that resistance; but instead of reading out in ohms, it reads from E to F.

One thing store-bought fuel gauges have in common is they are remarkably vague. At least they give you fair warning that they're not going to give you specific data about your fuel quantity; instead of reading in gallons or liters, they have a few lines on the face, showing fractions of a tank; halves, quarters, and usually eighths. We all know not to take that literally, we don't see the needle hit the 3/8 line and say, “Yep, I've got a 15 gallon tank, so I have five gallons, two quarts and a pint of fuel remaining.” We don't even believe the F and the E, we learn from experience if our own car's fuel gauge says F for the first hour of driving after we fill it, and if it's good for another 20 miles after the needle reads E.

Cabby
2/4/2011 3:23:55 PM

In situations where you find yourself needing to hook a lower voltage DC device to a higher steady voltage source, there's a little trick I've used in the past. If you hook a string of silicon rectifier diodes up so the current is flowing through in series (normally the stripe end towards negative), each diode will cause an approximate 0.6V drop. So if you want a quick and dirty voltage drop from 12V down to 9V just hook 5 diodes in series and they'll drop the available load voltage by about 3V. It's easy for small loads... less practical for larger loads because each diode converts the load amperage times its 0.6V drop into watts of heat, and you have to stay within the diode's rated wattage (typically 1-2W unless it's a stud-mount type - heat sink mounting complicates the series hookup.) The blocking voltage rating is irrelevant for this application, a 50v works as well as a 300v diode. Enjoying reading about Max, thanks.

Jack McCornack
1/12/2011 6:13:09 PM

Hi Craig, Sure, and I'm probably not the only friendly competitor you can find to run with you, providing you set up the regs for broad appeal--which means not loading them too much toward one type of vehicle. But I'm game, and if you're going to all the trouble of putting on the event, I should at least come down and cover it, and enter it if there's a place for four wheelers. > We run 70 mph, into a 30 mph headwind. Ah yes, "Vetter conditions," and with four bags of groceries, no less. MAX can match your bikes on grocery capacity but it couldn't run with you at 70mph and a 30 mph headwind. I cruise at 55. If I bring a co-driver and an extra steering wheel, can we enter as two motorcycles in close formation? :-)

JackSwagger
1/11/2011 3:56:07 AM

You really choose a good technic to measure quantity of fuel in vehicle..

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