Jump to content

JPI Fuel Flow Accuracy


Recommended Posts

What is the typical accuracy people are seeing with the fuel readings on a JPI?  I have a JPI 900 and since installation this summer, it seemed like it was reading high and I was using less fuel than indicated.  Its tough to really measure since I rarely fill the tanks all the way to get a good benchmark.  I did some long distance flying over the holidays and finally had a chance to take one sample point.  It indicates that the JPI is reading 3% more fuel used than actual.  So, at least the error is in the conservative direction.

There is a procedure for adjusting the K factor.  I'm curious though - what accuracy are other people seeing?  3% seems pretty good to me and I'm leaning towards leaving it alone.  But if it should be more accurate than that, maybe I need to do some more flying to pick up a couple more data points and do the procedure.

Link to comment
Share on other sites

Two other factors to consider - thermal expansion and compression and the accuracy of the fill input.  We’re they up to the exact same spot on a day with the same temps? 

My k factor is within 1 gal for about every 40 used from history 

Link to comment
Share on other sites

K factor, aka calibration, is everything...

the rest of the accuracy is consistency when filling the plane...

the sensor is a paddle wheel...

1% seems to be the target...

 

compare with fuel level...

with ceis digital sensors... 1% seems to be the target...

 

The JPI is just digitally reporting what it sees from these sensors...

 

pp thoughts...

Best regards,

-a-

Link to comment
Share on other sites

In my experience FF is scary accurate once the K factor is properly set. The only real difficulty is the fact that "full" tanks can vary quite a lot from one fill-up to the next. I keep a spreadsheet comparing the actual fuel added and the fuel used data from the JPI. Adjust the K factor by the %age the difference is between the two info sources over several hundred gallons. E.g., say FF accumulator over 10 flights might total 300 gallons. The actual fuel added might be 309 gallons. We would want to increase the K factor by 3%. If the actual was only 291 gallons we'd want to reduce the K factor by 3%.      

  • Like 2
Link to comment
Share on other sites

The sensor can be off as much as 3 to 7% without calibration.  Once calibrated, you will likely see within 1%.

Just record fuel used and fuel bought over a period of time and you should be pretty close.  Start with a full tank and end with one.

If you want to do a test on only a few tanks be sure to fill the tanks carefully, let them sit for a time (15 minutes or so) and top. Also be sure the plane is sitting on the same slope.  If the plane has bladders, they do take a little longer to fill and stabilize.

Once you have numbers you can adjust the k value. It is always mentally nice to have more fuel than what the computer says you have, so most error on the conservative side while adjusting the k value.

 

Link to comment
Share on other sites

1 hour ago, hypertech said:

Thanks, all.  Sounds like it is possible to do a bit better.  I'll just have to take a few more trips worthy of filling the tanks all the way up to get a few more data points and then do the adjustment.

If the 900 is anything like the 700, there is a setting to prevent the fuel used (USD) from being reset when you add fuel.  That way, you can total up the fuel added over 3-4 refuelings, and the equivalent measured fuel to adjust your K-factor.  Alternative, you could just write it down both and add it up, but I kept forgetting to do that.  This way, there's nothing to forget other than keeping your fuel receipts.

Link to comment
Share on other sites

I’ll have to look for that. I’ve got 89 gallon tanks though and unless I’m going over 500 nm, I rarely fill them full to get a base point. I usually fill to somewhere around 60 gallons but the exact level is really hard to confirm in that range.

Is it better to keep track over weeks or months or just go with the single flights that use a lot of fuel?

Link to comment
Share on other sites

Thanks, all.  Sounds like it is possible to do a bit better.  I'll just have to take a few more trips worthy of filling the tanks all the way up to get a few more data points and then do the adjustment.


The best way to get an accurate K factor is do the calibration using several substantial fuel burns. I’m typically within 0.5 gallons. Did some approaches tonight and the fuel burn on the JPI showed 15.5 gallons used. The receipt showed 15.41 gallons. Can’t get much better than that.


Sent from my iPad using Tapatalk Pro
Link to comment
Share on other sites

13 minutes ago, hypertech said:

I’ll have to look for that. I’ve got 89 gallon tanks though and unless I’m going over 500 nm, I rarely fill them full to get a base point. I usually fill to somewhere around 60 gallons but the exact level is really hard to confirm in that range.

Is it better to keep track over weeks or months or just go with the single flights that use a lot of fuel?

Mathematically, it would be better to do it over one single large refueling than multiple small refuelings, since your refueling error could be additive.  Practically, though, over multiple small refuelings, your errors are more likely to even out rather than add up, so I don't think it makes too much of a difference.

On my J, I usually fill to the tabs rather than the filler neck.  The tabs are a very discrete volume mark, whereas whenever I fill to the neck, I found I was inconsistent depending on how hard I was trying not to spill fuel.

Link to comment
Share on other sites

The transducer is made by Floscan which was purchased by JPI last year. The transducer spec for flow rate accuracy is +/- 0.5% @ 16 gph. It should be good enough if you set the JPI K-factor to match the K-factor written on the transducer which should be around 29K. 

Link to comment
Share on other sites

A couple of details come to mind...

1) the Ovation tanks probably have 102 gallons of useable in them....

2) The filling instructions are in the POH....

3) when calculating the K factor, the tanks don’t need to be filled to the top, just the same way from the first to the last filling...

4) using the mechanical wing gauges for this exercise would just induce a huge error and wouldn’t make any sense...

5) this isn’t a one and done exercise.... the K factor doesn’t change over time... but, you can collect data every time you fill your tanks twice... collect 10 k-factor sets of data... throw away the least and the most, average the rest....

6) why throw out the outliers?  This is from experimental error... We just aren’t that good at collecting data...  :)

7) If you didn’t do it the same way each time... you can’t expect to get the best results....

8) so... collect a lot of data, see how well you do....

9) Now... That thing at the top of the tank that causes all the confusion... call it the fill neck.... its job is primarily to protect the aluminum sheet metal when filling your plane.  Mooney has near a dozen variations of different designs... it’s secondary job is to inhibit filling the tanks to their 102 gallon possibilities...  (ask an Eagle owner how to handle proper fill neck venting...)

10) It is near impossible to over fill a Mooney fuel tank... the tank vent is so far up hill from the fill hole...

11) New owners get to go through three exercises... mostly, because it is a simple job that takes a lot of time....

  • Determine the k-factor, and verify it working... before relying on it...
  • Determine the useable fuel in the tanks... before relying on it...
  • Determine how to reset the low fuel light on the annunciator panel... before it can be used...for every flight...

Just a few thoughts that come to a PP’s mind...  of a mechanic...

Best regards,

-a-

Link to comment
Share on other sites

1 hour ago, hypertech said:

I don’t have tabs. There’s a wing gauge that gets you close to 30 gallons but it’s not very precise. The only visual marker is all the way full. 

My site gauges are dang near spot on.  I know this because I have run a tank dry and filled it up gallon by gallon expecting to make a fuel dipstick only to realize that the wing site gauges were so dead on that I did not need the stick.  M20K 252 with stock 75 gal tanks.

  • Like 1
Link to comment
Share on other sites

5 minutes ago, Bryan said:

My site gauges are dang near spot on.  I know this because I have run a tank dry and filled it up gallon by gallon expecting to make a fuel dipstick only to realize that the wing site gauges were so dead on that I did not need the stick.  M20K 252 with stock 75 gal tanks.

And it's a good thing the wing gauges are so good because with the damn flappers in the tanks of the K's, it's impossible to measure full fuel. It's different every time. 

  • Like 1
Link to comment
Share on other sites

On ‎1‎/‎4‎/‎2019 at 4:47 PM, hypertech said:

What is the typical accuracy people are seeing with the fuel readings on a JPI?  I have a JPI 900 and since installation this summer, it seemed like it was reading high and I was using less fuel than indicated.  Its tough to really measure since I rarely fill the tanks all the way to get a good benchmark.  I did some long distance flying over the holidays and finally had a chance to take one sample point.  It indicates that the JPI is reading 3% more fuel used than actual.  So, at least the error is in the conservative direction.

There is a procedure for adjusting the K factor.  I'm curious though - what accuracy are other people seeing?  3% seems pretty good to me and I'm leaning towards leaving it alone.  But if it should be more accurate than that, maybe I need to do some more flying to pick up a couple more data points and do the procedure.

I have a JPI 830.  I recorded fuel use and found the indicator readings  to be within 0.25% . Don't be afraid to tweak the K factor to dial it in.. 

At 0.25%, I thought that was close enough...   :)

Nav

Link to comment
Share on other sites

Unless you have a great reference point, anything within 2% on average is within tolerance. Bear in mind these are *volume* meters and not *mass* meters.

You have to manage at least:

  1. Consistent filling level
  2. The retail dispensing equipment (which when I used to be in fuel retail, the allowable tolerance was -0.5% to +1%, most retailers tried to get close to the minimum, and some were a little over zealous! Back then they were checked by weights and measures officials, at 90% or more of maximum flow - the error at low flow rates could be shocking) What gets "lost" in the hose doesn't even have to be accounted for! (ever wondered why when you turn the pump on it jumps straight away off the zero?)
  3. Relative temperature of fuel dispensed vs. fuel consumed (Density alters ~1% per 10dC)

Take fuel in the afternoon from a bowser that's been sitting in the sun all day, and then go climb to FL250 and burn it - you won't get the quantity out that you put in! Conversely, get fuel in the morning from an underground tank and scud around in the warm and wonder why you can't fit in what you thought you burnt.

 

 

Edited by Awful_Charlie
  • Like 1
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.