I've dealt with this quite a bit. I have the old Faria unit. I was able to find NOS on Ebay several years ago and have since bought a NOS sensor unit just in case. I have it on my C-dory (50 hp outboard), which has a 23 gallon gas tank. It is very accurate and useful, not so much for the burn rate, but for the total usage. Instead of having a bouncing needle between 1/4 and E, it gives me a usage figure like 19.3 gallons. That means that it is time to head for the fuel dock. Another advantage is that when in Canada, I switch the gauge to liters and it then reads 73 liters used. Nice to know I can put in 73 liters without spilling.
Having a system that reads through the MFD on my trawler seems kind of silly, especially at the price asked for NMEA 2000 gadgets. It just isn't important enough to have instantaneous fuel burn on the screen. Here's why. If I go out and run my trawler at 1,000 rpm with a simple fuel flow meter, I get something like .63 gph. I note the burn rate and speed and after a minute I go to 1,200 rpm. I do this up through WOT if I want. I make a graph (Excel makes this easy). I can see where there is a sweet spot (or
if there is a sweet spot) and I have the graph for future reference. I then remove the fuel rate sensor from the fuel line. Next time I'm running at 1,652 rpm, I can just look at my graph and see my fuel burn rate. No need for an expensive NMEA gadget, a complex install, or cluttering up my MFD, or extra fittings in my fuel line.
Although the Faria unit isn't made anymore, there are similar devices for under $100. They also have totaling capabilities, but I wouldn't need them for that on my trawler. My sight glasses give me accuracy within several gallons and I never get as close to empty as I do in the C-Dory.
Here is a $50 unit that I bought. The one I bought had a Viton o-ring (and the model was listed as fuel resistant), but some of the other models don't say fuel resistant. If left permanently installed (which is unnecessary as per above), one could switch out the o-ring.
Similar flow meters are available even cheaper, but one has to calculate the K parameter to insure accuracy. Also, the sensor materials aren't listed so whether it is fuel resistant long enough to make your graph could be an issue.
One issue in a DIY setup is getting the right flow rate range. Flow rate monitors usually list something like 10 to 150 gallons per hour. Obviously way over what is required for my 80hp Lehman. They also usually have a an accuracy figure like plus or minus .5%. What that means is that as between 10 to 150 gph, that particular hall sensor is accurate to .5%. The sensors are also generally listed as liters per minute, and for my Lehman, I bought one that was accurate down to .03 liters per minute (which is .47 gallons per hour, about what I burn at idle). Of course, it is also accurate up to about 25 gallons per hour, which I could never use.
Another issue on a $30 DIY setup is the "K parameter." Think of this as basically how many "clicks" on the sensor rotor makes for one gallon. These little units use a Hall sensor and are sometimes provided with a K parameter. With units like the Faria, it is already programmed in the gauge. For instance, if 100 revolutions is one ounce, the meter then simply times the revolutions to calculate gph (and can also convert to liters per hour or minute.) Since there is no adjustment of the K parameter for Faria, it uses the same one for gas and diesel. On most DIY sensors, the given K parameter will likely be for water. Water, gas, and diesel are in theory close enough in viscosity that the same K parameter will keep you within the .5% accuracy from X to XX flow rate as stated in the specifications. However, for increased accuracy, one could pump a liter of gas or diesel through the sensor at approximately the anticipated flow rate and adjust the K parameter. For instance, if the unit measured 1.123 liters, one would adjust the K parameter down 12.3%. This type of accuracy is generally needed on flow meters for medical usage, not so much for engines.
None of this would show up on an I-pad nor is it necessary. Keep a simple graph on the I-pad and the money saved will pay the fuel bill for the doing the Loop.
