Monroy Tank Levels

[Pinecone]

I am too lazy to go out to the airport right now, so asking here.
 

Does anyone have the Monroy STC paperwork for a 252 handy?  I am trying to figure out how much fuel is in the tank when the senders say it is full.

I know there is fuel in the outboard (aux) section that is above "full" reading on the sender, but now much?

[Marc_B]

Monroy Aux Tanks M20C-K FMS.pdf

@Pinecone  The way that Monroy shows it, you dip the aux tanks to determine amount of fuel and add this (chart amount based on inches of fuel in aux) to the amount shown by the senders...which just occurs to me that this no longer works if you've re-calibrated your fuel senders after Monroy installation. 

For me, this is the only weakness of the Monroy tanks...it would have been nice to have a sender for the aux tanks.

[Pinecone]

Thanks, but you are right, it doesn't have what I need.

As part of my avionics upgrade, we put in CIES senders and with the G3X EIS, removed the stock gauges.  

But with the stock gauges, with a full 52 gallons (main + Monroy aux with speed brakes), they would read 37.5 gallons until I burned a certain amount, then they would start to go down.  But there would be more than 37.5 gallons on that side.  Then at some point, the aux would be dry and the fuel gauges would read correctly.

So what I need is that conversion from the stock gauges with Monroy to actual fuel on board.

Shop calibrated the gauges with full being the rated amount of fuel, but we know that part is not measured by the senders.

Yeah, another sender would have been nice, but not an easy install.  For dipping the tanks, I measured every 2.5 gallons for engine stumble to full.  Once there was fuel in the aux, I dipped both tanks  so can convert either tank to the amount of fuel.

WTF?!  DUH, I HAVE the dipping numbers, so can figure it out from there.  It looks like 42.5 gallons is full on the main tank with about 2 inches in the aux.  FYI, fuel starts in the aux is about 1/2 inch at 27.5 total gallons.

[Aerodon]

Pinecone,

Somewhere someone posted his accurate filling details.  I plotted the results and as expected found that it is definitely non linear -  from empty until the bottom is filled, then until the inside rib is filled, then to the top.

I worked out some really good quantities to enter into my JPI for maximum accuracy.  The idea is to choose calibration points with good linearity between them.

IIRC, the JPI allows 5 calibration points, so I have chosen 0,10,25,42.5,52.5 for mine.  Have not done it yet.

 

Aerodon

 

 

 

 

[Pinecone]

That is the problem. You can't set it to read 52.5 gallons.  The fuel senders don't go up enough.  

This is what the avionics shop did with mine.  And it makes the fuel readings wonky.

It looks from my numbers that "full" on the senders is 42.5 gallons.  So you have to set it up so that they read 42.5 gallons from 4.25 - actually full.  And as you burn fuel, nothing changes under you get down to 42.5, then they start reading the actual fuel.

[Ragsf15e]

1 hour ago, Pinecone said:

Thanks, but you are right, it doesn't have what I need.

As part of my avionics upgrade, we put in CIES senders and with the G3X EIS, removed the stock gauges.  

But with the stock gauges, with a full 52 gallons (main + Monroy aux with speed brakes), they would read 37.5 gallons until I burned a certain amount, then they would start to go down.  But there would be more than 37.5 gallons on that side.  Then at some point, the aux would be dry and the fuel gauges would read correctly.

So what I need is that conversion from the stock gauges with Monroy to actual fuel on board.

Shop calibrated the gauges with full being the rated amount of fuel, but we know that part is not measured by the senders.

Yeah, another sender would have been nice, but not an easy install.  For dipping the tanks, I measured every 2.5 gallons for engine stumble to full.  Once there was fuel in the aux, I dipped both tanks  so can convert either tank to the amount of fuel.

WTF?!  DUH, I HAVE the dipping numbers, so can figure it out from there.  It looks like 42.5 gallons is full on the main tank with about 2 inches in the aux.  FYI, fuel starts in the aux is about 1/2 inch at 27.5 total gallons.

I have the same exact setup as you.  I used your measurements and checked them from engine stumble to full on the left.  They were close, but it’s easy to get a 1/2” variation in measurement and that might be significant fuel.  As @Aerodon said, it’s not linear.  I know my cies are very accurate 20 gallons and below, so i fill and set my total starting fuel based on that, but there’s always a little question about exactly how much is in the tanks?  Especially since you can fill them up past what the paperwork seems to indicate…

[Ragsf15e]

5 minutes ago, Pinecone said:

That is the problem. You can't set it to read 52.5 gallons.  The fuel senders don't go up enough.  

This is what the avionics shop did with mine.  And it makes the fuel readings wonky.

It looks from my numbers that "full" on the senders is 42.5 gallons.  So you have to set it up so that they read 42.5 gallons from 4.25 - actually full.  And as you burn fuel, nothing changes under you get down to 42.5, then they start reading the actual fuel.

If i fill it completely, my reading is also a bit wonky for the first 20ish gallons.  It’s decreasing but not exactly in line with the totalizer.  Then magically they both line up when there’s almost no fuel in the money tank on that side.  It’s a little frustrating but not terrible.

[Pinecone]

To limit the variance I place the FuelHawk in the cutout nearest the fuselage.  And I am very careful how I put my thumb over the end.  I have found that to make a huge difference.

I really prefer the FuelStik as it has less variables, but it is harder to use on the inboards due to the anti siphon flapper.

[Ragsf15e]

1 minute ago, Pinecone said:

To limit the variance I place the FuelHawk in the cutout nearest the fuselage.  And I am very careful how I put my thumb over the end.  I have found that to make a huge difference.

I really prefer the FuelStik as it has less variables, but it is harder to use on the inboards due to the anti siphon flapper.

Agree with both.  A small difference in the outboard measurement is a lot of fuel though, and there’s rivets/screws sticking up from the bottom of the wing that can mess with the measurements.  Luckily, I rarely have more than 65gallons onboard, but even then care is required because as you said, i notice some fuel hitting the monroy tanks anytime i fill above about ~20/side.  Careful filling, measuring, etc is still required.

[Aerodon]

3 minutes ago, Ragsf15e said:

If i fill it completely, my reading is also a bit wonky for the first 20ish gallons.  It’s decreasing but not exactly in line with the totalizer.  Then magically they both line up when there’s almost no fuel in the money tank on that side.  It’s a little frustrating but not terrible.

It's not that the Monroy 'side' is empty, it's that the overall level of all tanks is starting to drop below the highest physical level the float in the tank can go.

The tank has two floats, one on inboard side that will measure down the bottom, the one on the outboard side measure up to the top.  Now Monroe moves the tank further outboard and the float cannot reach the top.   

 

 

 

[Ragsf15e]

1 minute ago, Aerodon said:

It's not that the Monroy 'side' is empty, it's that the overall level of all tanks is starting to drop below the highest physical level the float in the tank can go.

The tank has two floats, one on inboard side that will measure down the bottom, the one on the outboard side measure up to the top.  Now Monroe moves the tank further outboard and the float cannot reach the top.   

 

 

 

Agreed.  The float on the outside has a much more “sensitive” measurement though as well because the level of fuel it’s measuring for each say 1/2” is much greater volume than the inboard sensor.  There’s more slop in those initial measurements.

[Aerodon]

Looking at my numbers (and the chart that I drew), the first 'turning point' is at about 10G, this is when the feel fills the wedge at the bottom.  The second is at about 25G, this is when the inboard rib is filled to the top.  There is a bit of a turning point at 42G, I'm not sure why.

On a standard tank, the wedge at the top is never really filled because of the filler neck.  But with the Monroy tank, it is filled to the top at the old filler location and near the top at the new location.

I will go measure how far away the float is from th outboard side later today.

Aerodon

 

 

 

 

 

[Pinecone]

When my shop does my calibration again, I want to watch the CEIS output (input to EIS) as they add fuel.  When it stops going up, that is the point that will be made "full" for the gauges.

[Marc_B]

1 hour ago, Pinecone said:

When it stops going up, that is the point that will be made "full" for the gauges.

Is that going to be when it’s equalized with aux and full?  Or are you saying full with no back flow into aux?

The tops are fuzzy with exact amount of fuel without a calibrated dipstick. But the bottom half of the range can be pretty accurate. But if you have aux tanks you really have to let them equilibrate I think when you’re calibrating. 

[Z W]

9 hours ago, Pinecone said:

When my shop does my calibration again, I want to watch the CEIS output (input to EIS) as they add fuel.  When it stops going up, that is the point that will be made "full" for the gauges.

This is how my shop did it, and how I think it should be done. Start with empty tanks. Add known quantities in the inboard tanks to calibrate the gauges to the marks shown on the EIS. Mine is a G500TXi so looks like this:

Start with tanks empty, calibrate. Put 4 gallons in (low fuel light / red line on the gauge), calibrate. Put 6 more in, calibrate to the 10 mark. 20 more in, calibrate to the 30 mark. Continue on until full.

Obviously at some point it stops going up, and that's "full" on the gauge, but not necessarily "full" in the Monroy tank, and the gauge becomes inaccurate. However, during the travel range of the senders, this is as accurate as I believe it can possibly be. They seem to work very well and are accurate for the last 5-6 hours of fuel endurance which is plenty for me.

[Fly Boomer]

9 hours ago, Pinecone said:

When my shop does my calibration again, I want to watch the CEIS output (input to EIS) as they add fuel.  When it stops going up, that is the point that will be made "full" for the gauges.

I think that is the best you can do.  That’s how the factory gauges are supposed to work.

[Pinecone]

On 7/16/2025 at 11:50 PM, Marc_B said:

Is that going to be when it’s equalized with aux and full?  Or are you saying full with no back flow into aux?

The tops are fuzzy with exact amount of fuel without a calibrated dipstick. But the bottom half of the range can be pretty accurate. But if you have aux tanks you really have to let them equilibrate I think when you’re calibrating. 

There is no way to stop fuel flow to/from the aux.  You have to consider the system as one big tank that just happens to not have a senso for the range near full.

It seems that the C-182 has a similar issue where the top number of gallons is not gauged.

[Pinecone]

On 7/17/2025 at 7:44 AM, Fly Boomer said:

I think that is the best you can do.  That’s how the factory gauges are supposed to work.

They did.  They read 37.5 until the fuel got down to a point, then they started reading.  I used to know how much fuel there was when they just started moving.  But that was almost 2 years ago.

I flew until one gauge just started to move, then switched tanks.  Once I landed, I filled the tank that was reading 37.5 on the gauge, so knew what the actual amount was.

[Pinecone]

On 7/17/2025 at 7:21 AM, Z W said:

This is how my shop did it, and how I think it should be done. Start with empty tanks. Add known quantities in the inboard tanks to calibrate the gauges to the marks shown on the EIS. Mine is a G500TXi so looks like this:

Start with tanks empty, calibrate. Put 4 gallons in (low fuel light / red line on the gauge), calibrate. Put 6 more in, calibrate to the 10 mark. 20 more in, calibrate to the 30 mark. Continue on until full.

Obviously at some point it stops going up, and that's "full" on the gauge, but not necessarily "full" in the Monroy tank, and the gauge becomes inaccurate. However, during the travel range of the senders, this is as accurate as I believe it can possibly be. They seem to work very well and are accurate for the last 5-6 hours of fuel endurance which is plenty for me.

That is the plan.

Actually, start empty, add 1.5 gallons (unusable) and set that as Empty.  I think my low fuel is set more like 10 gallons right now.  IIRC original low fuel lights were 9 gallons. I will entertain a discussion of what to set them at.

I just checked at the Encore POH (mine is converted) says 2.5 - 3 gallons for low fuel lights.   Can anyone with a 252 POH check?  It is in the Systems section under Annunciator panel.  But it looks like your 4 gallons is reasonable.

Interesting, I found this video where it shows that at least on the Experimental world, you can calibrate at both flight and ground attitude.  I am sure my shop is going to be SO HAPPY when I ask about doing this.

But during the calibration, you can see the sender readout and see when it stabilizes.  So when you add fuel near the top reading, you can add small amounts and see if the sender output changes or not, then determine what amount of fuel is the top of the measured range.

 

 

 

[Z W]

Well, maybe I should have checked before posting, but my 1982 231 POH does say 2.5-3 gallons usable fuel remaining is when the factory low fuel lights come on. Add 1.5 gallons of unusable fuel to that and you're at about 4 gallons, so that's probably where I got that number.

[Fly Boomer]

6 hours ago, Pinecone said:

Can anyone with a 252 POH check?

For the 252, it's in Section III - Emergency Procedures

Left or Right Fuel     2 1/2 to 3 gallons(9.5 to 11.4 liters)
Low                    of usable fuel remain in the respec-
                       tive tanks. Switch to fuller tank