12 V DC: Matching resistance/ohm between devices

melvin

melvin

Surin Legend
In a 12 V DC system I have a fuel tank and in it a measuring device that transmits its views
of whether the tank is full/half or whatever through a signalling cable to an analogue gauge.

The measuring device operates in the range 10-180 ohm.
The gauge operates in the range 0-190 ohm.

Some people say that this should work fine and the gauge should display correct values.

Some people say that this will not work, the resistance ranges do not match.

I say; dunno, don't know enough about ohm'ish resistances in DC circuits.

What would you say? Should work or should not work?

(my practical experience is that the gauge displays way too high values)
 
I spent my whole life on circuits such as this one measuring meteorological parameters, you have what is called the classical span and zero problem and linearity or lack there of it.
Theoretically the two linear lines should run parallel and there should always be a standard offset if you convert this to a formula of trigonometry and use complex numbers you will see that your set up will show a consistent error of 8% from 0 to 100% so when your gauge reads full you will still have 8% left. Now this is where it gets complicated if there is capacitance or inductance in the circuit you will not have a nice straight line but it will have a weird bend in it some where along the line we call these non linearity errors these come about through bad transmission and electrical noisy cables. Generally you can assume the 8% but unless you know the characteristic curve of your output you are stabbing in the dark. If everything is set up fine I would go for the 8% and be done with it.
In a nut shell No they will never show correct and but it will work of sorts.
 
Thanks a heap Rice.
Although not an electro buff I can relate to what you explain.

Now:
ONE:
To get a grip of capacitance and inductance is way out I think. It would be necessary to get in touch with the actual designers in large companies to get a hold of that.:
TWO:
Some people say that I should never play with ohmish resistance linked in to
signaling wires in such systems in order to get the gauge to render correctly,
some say I should.
As usual I don't have the foggiest.

======

The measuring device has 3 resistance ranges on offer of which I can chose one.
It has 10-180 ohm and 240-33 ohm and a range that I have forgotten.

There are gauges available that operates in the 240-30 ohm range.

If I got myself a 240-30 ohm gauge and programmed the measuring device
to 240-33 ohm, would that improve life significantly?

Is it at all meaningful to think along the lines of splining in an omish resistance on the signalling wire from measuring device to rendering device in order to
compensate for the gap between 30 ohm and 33 ohm?
 
John, obviously you just have to continue drinking.

Given a few more hours you'll see the light...or a frying pan...or the commode. :sweat:
 
Melvin, I don't think I ever asked the obvious question. Why did, you buy mismatching units in the first place. ?
Sender fitted the tank and not the dash. One fitted the dash and did not fit the tank???? Me thinks.
 
Rice said:
Melvin, I don't think I ever asked the obvious question. Why did, you buy mismatching units in the first place. ?
Sender fitted the tank and not the dash. One fitted the dash and did not fit the tank???? Me thinks.
Click to expand...

Pertinent Q, by all means. Do I have any good answeres? Some, and some might be a surprise to you.

Firstly, I don't have an electro background myself. The nearest is from my university days when I side kicked
in a power utility company, asessing various configurations of the grid and its effect on cosine fi.

When I decided to embark on this boat adventure in LoS I did buy a heap of things back home that I brought with me to LoS.
In the heap quite a few gauges, temp gauges, tacho meter, speed gauge, pressure gauges and this Wema fuel gauge.
At the time, didn't even realise that there were potential for problems. Couldn't dream of that there would be problems with finding a sender for
a world wide brand as Wema.

When I got around to install the tank/fuel staff I instead opted for a Swedish product which did not require any holes in the tank.
It used asdic type techniques to measure the fuel level.
Very complicated setup, not anywhere near IP67. Tore it off after a year and half.

When I started on my 2nd boat I was determined to do it properly, with my Wema gauge.
Bought a so called TS1 (ultra sonic tank sender) from BEP in NZ, famous brand.
Didn't occcur to me that BEP and Wema could not talk.

Well they could not.

The instructions for the BEP thing states clearly which ohm ranges can be selected.
The instructions for gauges mostly does not even mention ohm at all.
Pretty dark this.
I contacted the two maritim shops in Pattaya that I use and two shops in Europe
that I use. They were able to inform me about the ohm range for the gauges.

3 out of these 4 shops firmly stated that 0-190 ohm gauge combined with 10-180 ohm
sensor/sender should work just fine, no problem.
Note, these are not dumb peopleat all, they have been in the business for years and
know (for most parts) what they ate doing.
The 4th shop, in Pattaya, Marin electro/electronic shop run by an Australian engineer
was clear that this would be a problem.
His approach was to hammer on BEP to change their software in the TS1 to include
the 0-190 ohm range. He was doubtful re trying to sort it out with resistors.

Before I asked my Q here I have explained the problem and asked advice on a couple
of very active yachting forums in my home country.
Turned out that this is quite simply a problem area people know zilch-to-little about.
The guys on these two fora have been playing with boats and marin equipment all their
life without coming across this.
Strange.

Of course, most people still use mechanical arms bobbing up and down in the tank
to send signal. But still!

Well, roughly; thats why.

Thanks for your help.
 
Last edited:
Rice said:
Melvin, I don't think I ever asked the obvious question. Why did, you buy mismatching units in the first place. ?
Sender fitted the tank and not the dash. One fitted the dash and did not fit the tank???? Me thinks.
Click to expand...

Also, not surprised that clear info came from the weather-spook hold.

Having used Admirality list of Radio Signals for many years I have seen all the normal, strange, weird and very weird ways in which
met information is distributed/broadcasted around the world. There ain't no protocol that the weather-spook hold do not use.

Assuming that the same diversification applies to remote controlling and information gathering from met measuring devices
you must have seen a myriad of technical solutions for this.
 
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