Fuel tank sender resistance?

[roger001]

Here's a question for the electical minded members. I am in the process of fitting a sender unit to my front mounted fuel tank. I am using the same VDO dip type sender unit as supplied for the Hawk tanks, however it is shorter as the tank is not so deep (length = 28cm).
With this sender when the tank is full it reads full on the guage - so far so good, unfortunately when empty it reads just over half full on the guage.

The resistance of the sender is 2ohm on full and 72 ohm when empty.

I presume I need to change the resistance of the wire in the sender - how do I work out which resistance I need and where do I source some?

One way to do this would be if someone with a Hawk sender can measure its resistance, presumably it is only empty reading that is needed.

Then I need to find wire with the same resistance but over a shorther length.......

[Sptwoman]

Roger, I measured my Hawk supplied sender unit because I was going down the VDU fuel gauge path. I needed a 301-035 fuel gauge as the sender reads 80ish ohms empty and zero when full.
There's a few threads explaining how to calculate the res wire if you want to open the sender to mod it.

http://www.stratossupersite.com/forum/showthread.php?t=4795&highlight=sender+resistance

[roger001]

Thanks

Opening the sender and changing the wire is not a problem - I had to replace the wire on my original unit when it broke some years ago, 80 ohm seems low as the short unit I have reads 72 ohm on empty and this is above the half way mark on the guage. Unless of couirse the guage is not linear....

Next question does anyone know of any suppliers for different resistance wires _ apparently the hawk guage uses 263 ohm/yd so i will need a higher value as the distance is shorter...

[Sptwoman]

Have you checked the spec of the gauge as the VDO fuel gauge specs vary. Some are 0 - 180!! You can check the serial number against the spec. as well as checking with Martin Smith.
He stated:
You can configure the dip pipes to your own configuration.
Any further questions then please contact me.

Kind Regards

Martin Smith
Furneaux Riddall
Alchorne Place
Portsmouth
PO3 5PA
02392 668621

[mudhut]

Roger,

Scientific Wire Company (http://www.wires.co.uk/)

It's going to be nichrome, constantan or manganin wire I think. The site abovew does provide a diameter conversion chart but unfortunately not a table of resistance per metre. Resistivity data can be looked up though.

[roger001]

Thanks for the info - will ring martin Smith in the week. I guess it's a case of getting the right wire.

[Mel Lewis]

Hi There
I think its the arc of the float that needs to be adressed, If your tank is shallower than the original the float will be touching the floor before it reads empty. The best way to modify this is either bend the float wire or shorten it.
A dry run with the sender out of the tank is advised before any chopping and changing is done.

[Sptwoman]

Roger's running a dip type sensor unit.

[roger001]

Looks like trying to find the correct resistance wire is going to be impossible - so the next best alternative is to fit a 50 ohm rheostat in series and asjust it so that empty on the tank = empty on the guage, tho it wont read full when full but I guess the main use of the guage is to tell you when the tank is nearly empty, looks like the best compromise to me......

[colin artus]

Alloy Wire international used to make this stuff (alloywire.com). Nowadays I think it's more spring wire, but it might still be worth giving them a try.

[roger001]

Thanks Colin I will give them a ring.

[pimms]

Hi Roger,
I just bought a VDO fuel gauge which has a potmeter you can ajust to get the gauge to read empty. The instructions say the gauge and sender have to be matched within a range of 60 to 90 ohms.

[roger001]

The lead Colin gave me can supply different resistance wires but it is quite pricey - quoted £80 for 4 different resistances in 2m lengths - so I will have to try and be a bit more scientific and narrow down my choices - can someone confirm the resistance of the Hawk sender unit when empty - Previous post (Sptwoman) quoted 80 ohm though this seems low as the sender I have is 72 ohm and reads half a tank on the guage when empty at the sender unit - or can 8 ohm make all the difference???

[Sptwoman]

My readings were from the orginal VDO unit without the res wire mod to match the Veglia gauge. If you look at the spec of some of their gauges (http://www.egauges.com/vdo_mult.asp?Type=Fuel_Level&Series=Cockpit&Units=E) they are made to match certain senders. For example your sender might match the 16-158 Ohm gauge (Part No: 301-037)

[lyndon]

Has any one managed to obtain the correct resistance wire as I'm wanting to sort out fuel gauge inaccuracy,
Lyndon.

[Elliott.L]

Roger has it sorted. He has ordered some of the correct wire and we are waiting for it to be delivered.

When I went up to Beverley to finalise the paperwork with Roger and to drive the car a couple of weeks ago, Roger took the controls first to show me the ins and outs of driving it.

He pointed out that although the fuel gauge wasn't reading correctly he could tell that there was enough fuel in the tank as the needle was just above the stop in 'positive' territory.

We ran out of fuel on a dual carriageway ten minutes later! :D

[lyndon]

you can't beat a good laugh ,
let me know if there's any available,
Lyndon.

[Elliott.L]

Sure, no problem Lyndon :)

[Mel Lewis]

Has anyone tried to use a variable resistor ( volume control from a radio ) available from Tandy,Maplins or the likes ??? just an idea !!!

[Sptwoman]

Lyndon, Gerry has the correct res wire to convert the sender to work with the Veglia gauge. He supplied me but I gave mine to Chris J...

[chris.richard]

Lyndon, Gerry has the correct res wire to convert the sender to work with the Veglia gauge. He supplied me but I gave mine to Chris J...
That won't work with Elliot's short sender in the front tank.
I measured mine last week when I had the tanks out to Kevlar sheath them, but I didn't write it down. IIRC it was about 220 ohms at the bottom

[roger001]

I used a varible resistor to zero my guage then measured its resistance at that setting and have worked out I need approx 400 ohm/metre wire. Currently resistancewire.co.uk are making a new batch which should be available in a few weeks......

[chris.richard]

That sounds about right, doesn't it? Your short front tank sender must be about 30cm long, = about 0.5m wire = about 220 ohms! Don't you love it when things add up!

[Mel Lewis]

Whats all this fuss with resistance wire, floats, gauges etc etc. Whats wrong with a good old piece of hardwood baton with a few knotches on it. Just like my Dad use to use on his Fordson Major.

[chris.richard]

Doesn't work if your tanks filled with foam Mel!

[richard]

Looking at all this amusement, I am honestly considering putting a microprocessor in there to map the sender reading input into a meter output. I suspect it would be the same amount of aggro overall, but would mean it could compensate for the fact that my tank is not a simple rectangle and make a 1/4 full reading actually meant 1/4 volume left not 1/4 of the way up in level.

All I need is some of that copious free time to knock up the interface electronics :D

[SUSIT]

what is wrong with a good old fashioned sight guage?

[mudhut]

Looking at all this amusement, I am honestly considering putting a microprocessor in there to map the sender reading input into a meter output. I suspect it would be the same amount of aggro overall, but would mean it could compensate for the fact that my tank is not a simple rectangle and make a 1/4 full reading actually meant 1/4 volume left not 1/4 of the way up in level.

All I need is some of that copious free time to knock up the interface electronics :D

I like this this idea and might set my mind to it at some time in the future.

A little aggro but once done, calibrating for other systems ought to be straightforward.

Too many pressing things like a refit to do first.

[chris.richard]

But not even a clever gizmo like that can allow for the fact that the Hawk sender doesn't reach near the bottom of the tank! :rolleyes:

[mudhut]

Well... :D

errr... :D

Its' a reserve supply then (notched stick not included) ;)

[mudhut]

Well... :confused:

Err... :(


It's a reserve supply then (notched stick not included) :D :D

[Mel Lewis]

"Doesn't work if your tanks filled with foam Mel!"


It will but you've got to push a lot harder and use a twisting action and a point on your baton.

[chris.richard]

I'm re-visiting this caper again.
Determined for zero on the guage to mean bottom of the dip tube, with the rest as spare and swirl pot. So while the tanks are out (for more thicker kevlar), I'm addressing it. 260 ohms with tank empty, 0 ohms with the tank inverted. Using a trimmer pot, I measured that my guage needs 100 ohms tank resistance to read zero. Thanks to recently supervising O-level (or whatever they call it now) physics homework, I calculate that if I put a 162 ohm resistance in parallel with the tank wire the guage should read correctly at empty.:) No idea what it'll do at full though!:rolleyes:
In the course of this I went one better than letting smoke out of the wires - I released some of the smoke that's kept in the battery!:eek: Somehow I bolted up cables to the battery that just shorted it. I was puzzled why the instruments didn't come to life, then smelled something horrible and realised the battery was beginning to melt and the terminals were too hot to touch - cue frantic spannering against the ticking clock to stop the flames leaking out too. Can batteries survive that treatment? Cheapest new red top 25 I can find is £88.48 inc.VAT:o Doesn't anybody realise there's a credit crisis going on? Bollox!:mad:

[richard]

At full the zero ohms (short circuit) will still be zero ohms.

The battery life is down to how long it was hot for. They don't like having their electrolyte boil away, they don't like having the plates buckle from heat. So down to just how hot it got I guess.

[chris.richard]

At full the zero ohms (short circuit) will still be zero ohms.

The battery life is down to how long it was hot for. They don't like having their electrolyte boil away, they don't like having the plates buckle from heat. So down to just how hot it got I guess.

Just checked the "full" resistance, and you're right Richard!

Battery got hot enough to start melting the plastic round the negative terminal. It's a sealed gel battery, so would it lose electrolyte? Should I charge it up and see what happens, or might that be too spectacular?:(

[SUSIT]

Just checked the "full" resistance, and you're right Richard!

Battery got hot enough to start melting the plastic round the negative terminal. It's a sealed gel battery, so would it lose electrolyte? Should I charge it up and see what happens, or might that be too spectacular?:(

Jez Chris your not having a great deal of luck:(
Know less about electricity than almost anything else in life
(exception being keeping women happy)
However read an article on using starter chargers and from reading that I would reckon your battery wont last much longer:( Why not give red top a call

Stephen

[CorseChris]

Chris, which type of instrument are you using for the fuel gauge? If it's he old Veglia one, then you can balance an incorrect sender resistance by putting a correction resistor in parallel with the internal balancing resistance of the meter.

Have a look here http://en.wikipedia.org/wiki/Wheatstone_bridge, this diagram is basically what the setup is. Your fuel gauge sender is the 'test' leg (Rx), you need to tweak 'R3' by adding a parallel resistance.......worked for me at least using a VDO dip tube. Some care is needed or the current flowing when you reach full tanks can become higher than you might like. I ended up with a 75 Ohm resistor across 'R3'

Same idea will apply for other types of gauge assuming they use the same design.

HTH

[AndyR]

You need to tweak 'R3' by adding a parallel resistance.......worked for me at least using a VDO dip tube. Some care is needed or the current flowing when you reach full tanks can become higher than you might like. I ended up with a 75 Ohm resistor across 'R3'
HTH
I calibrated mine about three weeks ago. VDO dip tube and Veglia gauge.

I agree the parallel resistor is the best way forward.
It's better to do the balancing on the gauge end which will take account of any cable/circuit resistance as well and of course keeps a warm/hot resistor away from the tank end!

I found that by removing the dip tube and using a test tank/empty beer keg filled with water gave better readings than tipping the tube in fresh air, obviously a factor of some resistance in fluid as opposed to air in the tube.
Raising and lower the tube in the water to test it each time I change a resistor value.

I started with only 5 resistor values from 35ohm up to 120ohm. Temp of the resistor is hot to the fingers. Rather than mess about I threaded the resistor legs onto the terminals and held them tight with small croc clips rather than solder each time.

At 80ohm mine gave the best compromise of readings.
At full it's perfect.
The hard one when is when you are nearing empty as you will find a sudden drop, as these gauges do. I played safe and at 80ohms I know I have enough to drive and empty my wallet for fuel as opposed to being worried and running on fumes.
That was the main benefit of using the test tank and knowing how far up the tube the fluid was at the empty end.

[richard]

Just checked the "full" resistance, and you're right Richard!

Battery got hot enough to start melting the plastic round the negative terminal. It's a sealed gel battery, so would it lose electrolyte? Should I charge it up and see what happens, or might that be too spectacular?:(

Yay, my electronics degree in action ;)

Gel batteries are intended to be more hardy than the usual wet cells, but I don't have any experience of them other than some basic theory so I couldn't say what your chances are. Cold cranking an engine isn't that far off a short circuit (under 0.1 ohm) as far as the battery is concerned so they can take some punishment, it is just the length of time of that punishment.

[CorseChris]

Adjusted mine using a decade resistance box so zero faffing about! The resistor sits across the gauge on mine - it's simple to add it to the instrument loom.


....but I see we have been here before...we talked about this back in 2004 didn't we Chris...... http://www.stratossupersite.com/forum/showthread.php?t=1396&highlight=fuel+gauge

[chris.richard]

Thanks for the input, one & all.
My calculations come out with a 162 ohm resistance in parallel to the diptube wire. I've done all the measurements at the tank end, so was going to put a trimmer in the loom there somewhere. The current through the diptube won't change, and the trimmer won't get as hot as the exhaust/brakes that are a similar distance from the tank! Thought I'd adjust it to the right reading, then encase it in resin or silicone. Unfortunately I can't take the diptube out of the tank as the foam would stop me getting it back in again. It's the empty end of the scale that I want to be accurate, so fluid reistance shouldn't affect it. What's the relative resistivity of petrol & water??

[chris.richard]

....but I see we have been here before...we talked about this back in 2004 didn't we Chris...... http://www.stratossupersite.com/forum/showthread.php?t=1396&highlight=fuel+gauge

So we have - unfortunately my solution didn't, well solve things!

[AndyR]

It's the empty end of the scale that I want to be accurate, so fluid reistance shouldn't affect it.
Watch out for the non linear effect of the guage and tube combination.

You may find you want at least an inch or two of inches of petrol up the tube to show a reading just going onto the red section heading down!
Before it suddenly drops quickly falls off the RED into the panic zone.

I found it was a best compromise for nerves on the tube and guage combination, so no walking head down with a petrol can in hand !

[chris.richard]

And the very non-linear fuel tank shape!

My Veglia guage doesn't have a red section; there is an audible warning though - it goes all quiet behind you when the fuel runs out!!:D

[CorseChris]

I made up a wee voltage comparator circuit to drive the red warning light on mine. Not as accurate as a float switch, but close enough for a reminder.

I still think you need a resistor in parallel with your gauge, not the sender Chris, just need to get the right value. I'm feeling lazy....remind me what value it gives at full and empty and I'll do some sums for you.

A minor bonus on the Corse tank - it's cross section gets bigger the lower down you go, so it doesn't catch me out.

[richard]

What's the relative resistivity of petrol & water??

Am I missing something here? The float at the top of the fluid shorts out all the resistance wire that is immersed, hence making the resistance drop until with a full tank only the short circuit across the two wires is conducting and you get zero ohms.

As the immersed wire has no current flowing through it, its resistivity doesn't matter.

[chris.richard]

Am I missing something here? The float at the top of the fluid shorts out all the resistance wire that is immersed, hence making the resistance drop until with a full tank only the short circuit across the two wires is conducting and you get zero ohms.

As the immersed wire has no current flowing through it, its resistivity doesn't matter.


Err yes, my mistake - it was at the end of a night-shift!

Chris S - 0 ohms at the top, 260 ohms at the bottom. guage needs to see 100 ohms at the tank end of the loom to read 0/4

[CorseChris]

Chris,

Your idea of putting the 160 R in parrallel with the sender should work to give you a correct 'empty' reading, as long as the gauge is wanting 100 R to read zero as you determined. Trouble is, it's not going to be particularly linear between the extremes. I had the reverse problem - the sender resistance was too low, hence having to balance the other side. Sadly, it's not possible to get at the right arm of the bridge to balance your situation in a more elegant way.

...but, going back to your very first post on this in '04, you say it reads 3/4 for a 1/4 tank of fuel. So, that implies the sender resistance is too low, not too high...so we come back to wanting a resistor across the gauge, not the sender.....have you tried putting, say that 160 R between the +12v and the sender teminal? What did it do?? Have you definately got a solid earth on the sender body as well - don't rely on the tank for earthing it...


IIRC, the Fiat sender is about 330 R at empty??

If you get totally stuck, you can always send me the gauge and I'll calibrate it for a 0-260 R sender. I might have a spare one kicking about - will have alook at the weekend. If so, I can work out waht oyu need on that.

[strat6v]

Going off at a tangent here, i have fitted a chassis earth to each tank as they are rubber mounted, same to each cap assembly as they are joined to the tank with a hose. I'm assuming this is a sensible idea??:confused:

[CorseChris]

Good question.....presume you are thinking about static buildup. Isn't fuel hose slightly conductive??....or did I just make that up!! I'll measure some.

It seems like a sound notion to me... (waits for contrary reply...)

[strat6v]

I have some fuel hose that has a earth strip bonded in but instead i've used the rubber connector from the 164 filler neck, definitely no earth bit in it. Static was my thinking.:confused:

[CorseChris]

I was thinking it had a bit of carbon in it to make it sufficiently conductive to dissipate static....

....but don't ask me why I think that!!!

[strat6v]

you are probably correct:o

[chris.richard]

I was thinking it had a bit of carbon in it to make it sufficiently conductive to dissipate static....

....but don't ask me why I think that!!!


Is it coz I is black?:cool:

[chris.richard]

Chris S - 0 ohms at the top, 260 ohms at the bottom. guage needs to see 100 ohms at the tank end of the loom to read 0/4

Now I'm really confused. in the 2004 thread I said

upside down is <2ohms empty is about the 90 ohms.:confused:

That's the same sender as I'm now measuring at 260 ohms empty. I've a feeling that I've got a resistor added to the back of the guage already, but I don't want to take the dash out just to check. That's one reason I was going to add a resistor to the sender end. I did put a new wire in the sender at one stage, wonder when that was...

[chris.richard]

Well, I think I'm happy with where I've got to. :)
I put a 162 ohm resistor across the sender, and the guage reads 0/4 on empty, leaving below the dip-tube as a reserve, and 4/4 when full. Or over full as it may turn out. I'll worry about that when the engine is running again! On with the glassfibreing.....:(

[CorseChris]

Now I'm really confused. in the 2004 thread I said

:confused:

That's the same sender as I'm now measuring at 260 ohms empty. I've a feeling that I've got a resistor added to the back of the guage already, but I don't want to take the dash out just to check. That's one reason I was going to add a resistor to the sender end. I did put a new wire in the sender at one stage, wonder when that was...

Ah, now that would explain why it wants 100 R to show empty......looks as if you may have done two fixes!

Full will always be correct as the sender will be 0 R regardless, so 12V across that leg of the meter, current through the meter will be the same, plus whatever parallel resistance is there as well..but that's outside the movement, so doesn't matter.

Just the linearity that'll be a bit odd...but then they aren't great at the best of times.

[chris.richard]

Gel batteries are intended to be more hardy than the usual wet cells, but I don't have any experience of them other than some basic theory so I couldn't say what your chances are. Cold cranking an engine isn't that far off a short circuit (under 0.1 ohm) as far as the battery is concerned so they can take some punishment, it is just the length of time of that punishment.

Well, the battery is Donald Ducked. After 24 hours of charging, it is holding 10V :(