2 fuel senders per tank?

[wombat]

How does this work?   Are they wired in parallel, in series, or separately somehow?

I only have one fuel indication per tank, so the signals are being combined somehow.

Specifically, this is on a M20K 305 Rocket, but I think the senders are standard from the factory with the 231 that this plane was originally.

 

Edited June 12, 2023 by wombat

[PeteMc]

Yes, there are two and they are in series if I remember correctly. 

It has to do with the slope of the wing and how to properly measure the fuel since the bottom of the tank is not flat.  I believe it is the impedance between the two senders that gives the accurate reading on the gauge. 

Also, if you're having funny readings or flickering in the gauges, look for a bad ground before you start tearing out the senders.  I think they're a pretty basic system and don't usually go bad.  But a bad ground will totally mess up what the gauges shows.  Also remember they're only required to be correct at Full and down near empty.  Maybe 2 gal remaining or something like that.  (I'm sure someone will have the maintenance manual with the specifics.)

 

And BTW....   Just moved to Spokane and will probably be flying over to Lake Chelan later this summer when a friend is out from the east coast. 

 

Edited June 12, 2023 by PeteMc

[EricJ]

24 minutes ago, wombat said:

How does this work?   Are they wired in parallel, in series, or separately somehow?

I only have one fuel indication per tank, so the signals are being combined somehow.

Specifically, this is on a M20K 305 Rocket, but I think the senders are standard from the factory with the 231 that this plane was originally.

 

They're in series.    They're just potentiometers, i.e., variable resistors, so the resistances add.

[A64Pilot]

You need two because there is no one place in the tank that you could measure the total content

Meaning where you could measure the last bit the fuel hits the top of the tank long before it’s full, and where you could measure full, the fuel goes dry in the bottom long before the tank is empty

[N201MKTurbo]

They are each 0-30 ohms. The outer sensor has the flange grounded. The inner sensor has the flange isolated from ground. The wire from the outer sensor connects to the flange of the inner sensor. The wire from the inner sensor goes to the gauge.

[wombat]

OK, so they are wired in series.   That's great to know, thanks, everyone!

I've already pulled all the senders out of my tanks anyway, since I'm in the middle of a full tank reseal, but it wasn't because they were glitching although they were reporting incorrectly. That's just a calibration problem.

@PeteMc Stop on up to Twisp (2S0) sometime; we just got our crew car working again.    Or we can coordinate to meet up somewhere.     If you are into camping, Electric City (3W7) is a very easy airport with lakeside camping.

[PeteMc]

6 minutes ago, wombat said:

we just got our crew car working again. 

Now that's a classic Crew Car!!!

 

[Pinecone]

Hmm, how do the Ceis senders, in frequency mode, work with the two per tank????

And if you have the Monroy tanks, you still don't get a reading all the way full.  I wonder if it would work to move the outer sender from the normal place to the outer end of the Monroy?  Not that you could just do so, just wondering.

[A64Pilot]

44 minutes ago, Pinecone said:

 

And if you have the Monroy tanks, you still don't get a reading all the way full.  I wonder if it would work to move the outer sender from the normal place to the outer end of the Monroy?  Not that you could just do so, just wondering.

You might have a dead band in the middle where the top sensor is sitting on the bottom and the lower is still full, if so then I’d guess you would need three?

Maybe adding one as opposed to moving one would be better?

[N201MKTurbo]

The top of the outer cell is only about 2 inches above the top of the inner cell. The tanks are about 9 inches tall inside. Two is plenty.

[Pinecone]

1 hour ago, N201MKTurbo said:

The top of the outer cell is only about 2 inches above the top of the inner cell. The tanks are about 9 inches tall inside. Two is plenty.

I was thinking that.  As even when the level in the aux is almost to the top, you can add fuel and hear air bubbling from the main.

I wonder if that could be done as a minor alteration (moving the outboard fuel sensor to the outer end of the Monroy)?  And how much work it would be (probably a lot).

[N201MKTurbo]

52 minutes ago, Pinecone said:

I was thinking that.  As even when the level in the aux is almost to the top, you can add fuel and hear air bubbling from the main.

I wonder if that could be done as a minor alteration (moving the outboard fuel sensor to the outer end of the Monroy)?  And how much work it would be (probably a lot).

I can't see how modifying a fuel tank would be minor. You would need to cut a hole in the outer cell and add the sensor mounting ring and then seal the whole thing up. extend the wire to the new location and add a cover plate to the old location. 

I think you can get it done, but you will need to get a DER involved. If you want to spend a few AMUs, it can be done. This all depends on if the sensor float can swing unimpeded. The FAA doesn't approve anything, they are not engineers, they rely on certified engineers (DER) to approve the design change.

[A64Pilot]

It would depend on the IA’s opinion, if it were my aircraft and as long as the access used original parts I’d consider it minor. But I don’t think I’d do it for a few gallons of ungaugable fuel I don’t consider to be a problem, I assume if all it took was relocating the sender then who did the STC also didn’t consider it an issue.

‘But if you really want it done I think you can get there without a DER.

In my opinion if you use existing parts and simply change the location it’s not major

https://www.law.cornell.edu/cfr/text/14/appendix-A_to_part_43

[wombat]

I'll be recalibrating my fuel level sensors in the near future anyway since I know they are off by about 20% in the mid-teens right now.

And while it would be 'nice' to have the gauges measure 100% accurate all they way up to full long-range tanks, I agree with @A64Pilot here that it's just not worth spending money/effort on it.  If I was doing my own labor on this part, then *maybe*, but not being able to measure the top 5 to 10 gallons isn't a big deal to me.   Since I've got the totalizing the fuel flow measurement, I can fly with the gauges reading what is effectively "at or above X gallons" for the first while until they get down into their sensing range.

 

If I was embarking on a flight where there is a point of no return like an ocean crossing and there was a discrepancy between the actual fuel onboard and the totalizer's value the fuel gauges would be in their sensing range before I got to the point of no return.  So there is no concern there.  Unless I had 100+ knot headwinds on the return leg... Maybe then.  But that's not going to happen.

[Pinecone]

Just musing.

If it were a couple of AMU, I would consider it.  But it is not something I will lose sleep over.  I am fine knowing that there is X amount of fuel that is ungauged.

[Igor_U]

On 6/12/2023 at 3:40 PM, wombat said:

OK, so they are wired in series.   That's great to know, thanks, everyone!

I've already pulled all the senders out of my tanks anyway, since I'm in the middle of a full tank reseal, but it wasn't because they were glitching although they were reporting incorrectly. That's just a calibration problem.

@PeteMc Stop on up to Twisp (2S0) sometime; we just got our crew car working again.    Or we can coordinate to meet up somewhere.     If you are into camping, Electric City (3W7) is a very easy airport with lakeside camping.

Woha! Olds coupe is back online again!!

My wife and I were so disappointed (especially her) last year when we landed and were told city didn't want to insure it anymore! We have borrowed it on occasion for over 10 years. We chatted for half an hour with some great people including someone on airport board but flew elsewhere for a lunch. Were you there, by any chance?

We'll definitely fly in for lunch. Thanks.

 

[NicoN]

@wombat:

I can confirm that you have TWO fuel senders. One on the outer edge of the fuel tank. This one is connected to A/C GND via the screws which touch the senders body.

The next one is in the cabin wall and this one SHALL not have contact with the senders body and A/C GND. This is achieved by using "shoulder washers" which also isolate the screw shaft from connecting.

Both senders are wired in series, so their resistancde is 60 ohms (I believe) together.

If the inboard sender is connected to GND, then the outboard sender is useless.

We had the situation that a hangar idiot lost one of the shoulder washers. Therefore, foru screws were isolated, one was not. That results in an indcation of maximum 18 gals instead of 36 (we have 72 gals). Not a real problem, becuase it takes a long time, then the needle goes below 18.

But according to FAA rules your gauges only have to indicate the empty situation precisely - not the full.

[wombat]

On 6/15/2023 at 3:37 PM, Igor_U said:

Woha! Olds coupe is back online again!!

My wife and I were so disappointed (especially her) last year when we landed and were told city didn't want to insure it anymore! We have borrowed it on occasion for over 10 years. We chatted for half an hour with some great people including someone on airport board but flew elsewhere for a lunch. Were you there, by any chance?

We'll definitely fly in for lunch. Thanks.

 

The city never insured it; it's owned by and the insurance paid for by the Twisp Airport Improvement Club and the person who ran that was trying to get the city to pay for it; they declined to even ask their insurance agent what it would cost.    So I paid for the registration, insurance, and maintenance to get it going again.

There are a lot of great people on this airport, I'm glad we have such an active pilot community.

[hoot777]

Do all m20s have two senders per tank ??  I have a 68 M20G. I’ve sent the inboards out for work before.  Thanks. 

[EricJ]

3 minutes ago, hoot777 said:

Do all m20s have two senders per tank ??  I have a 68 M20G. I’ve sent the inboards out for work before.  Thanks. 

No, the smaller tanks only have one inboard sender each.    The larger tanks (e.g., 64 gallons+) have an inboard and outboard sender, since the inboard can't reach far enough to measure the entire tank and still reach the bottom of the inboard part.

[hoot777]

Thanks