Thermistor in a gas tank

C

cxturbö

Guest
I'm having problems with the "Fuel Reserve" light on my motorcycle.
When the fuel level drops to 1.3 gallons the Fuel Reserve light on the
dash board is supposed to light, few weeks ago it quit working.

I removed the fuel gauge sending unit from the tank which has the
gauge rheostat and a separate metal capsule for the Reserve sensor. I
split open the capsule and a very small thermistor...maybe a 1/16"
fell out. It broke off the two wires soldered at both ends.
After looking over the schematic in my shop manual, the circuit seems
rather disturbing. 12 VDC is supplied to the 4 watt bulb in the dash
which then leads to the negative coefficient thermistor in the gas
tank. Submerged in fuel the circuit is open. Out of fuel it heats
up, closes and completes the circuit to ground.
I always thought that there were very low voltage value's sent into a
tank in order to prevent explosions? When that thermistor broke off
the base wire could have easily touched the metal casing and it's at
least able to handle 4 watts @ ~14 volts?
Is the fuel to air ratio so high that an explosion is not possible?
Even so, being a motorcycle with a 5 gallon tank I would think there
would be a risk when removing the cap to refuel?

Next problem is repairing it. I managed to solder the wires back on
but that didn't last but a few weeks. This time the microscopic metal
tab came off the black thermistor body. The bike is a very limited
production 1983 Honda CX650 Turbo...needless to say the sending unit
assembly is no longer available.
After searching the net I managed to get some info from a Goldwing
owner. He recommended a:

NTC Thermistor TYPE RL2004-582-97-T10
Rated at 1,000 ohms at 25 degrees Celsius.
Cold resistance is between 900 and 1200 ohms.

This evening I made a mockup circuit using that thermistor. With a 12
volt power source and a 4 watt bulb I left it energized for a few
hours. What has me worried is that the thermistor gets too hot to
touch. Submerging it in a very small vile of gas causes the light to
turn off. It's hot enough that I hear a very faint hiss as it goes in
the gas.
It works exactly as it should but I'm worried about the heat.

Anyone have experience with these thermistors? I would assume the
same style setup is used in most cars with "Low Fuel" warning lights?

Thanks for reading,

George
 
I think it would be safest to look for a sensor form another bike that uses
a similar circuit but...

If' I'm looking at the right data sheets ...

http://www.thermometrics.com/assets/images/rl20.pdf

the NTC material is D9.7A and the data for that suggests the resistance is
on the high side..

http://www.hhv.co.uk/PDF's/thermistors/keystoned97.pdf

I made it 107 Ohms at around 85C.

If that's correct I'm surprised the bulb lit up for your test.

I think the cold temp is about right (could be a bit lower) but the graph
needs to be steeper with a lower hot temp. I think the resistance of the NTC
needs to be less than the resistance of the bulb at whatver temp you feel is
safe (say <10Ohms at 50C?)



"cxturbö" <vairxpert@hotmail.com> wrote in message
news:3f2209a2.13442658@news.netnitco.net...
I'm having problems with the "Fuel Reserve" light on my motorcycle.
When the fuel level drops to 1.3 gallons the Fuel Reserve light on the
dash board is supposed to light, few weeks ago it quit working.

I removed the fuel gauge sending unit from the tank which has the
gauge rheostat and a separate metal capsule for the Reserve sensor. I
split open the capsule and a very small thermistor...maybe a 1/16"
fell out. It broke off the two wires soldered at both ends.
After looking over the schematic in my shop manual, the circuit seems
rather disturbing. 12 VDC is supplied to the 4 watt bulb in the dash
which then leads to the negative coefficient thermistor in the gas
tank. Submerged in fuel the circuit is open. Out of fuel it heats
up, closes and completes the circuit to ground.
I always thought that there were very low voltage value's sent into a
tank in order to prevent explosions? When that thermistor broke off
the base wire could have easily touched the metal casing and it's at
least able to handle 4 watts @ ~14 volts?
Is the fuel to air ratio so high that an explosion is not possible?
Even so, being a motorcycle with a 5 gallon tank I would think there
would be a risk when removing the cap to refuel?

Next problem is repairing it. I managed to solder the wires back on
but that didn't last but a few weeks. This time the microscopic metal
tab came off the black thermistor body. The bike is a very limited
production 1983 Honda CX650 Turbo...needless to say the sending unit
assembly is no longer available.
After searching the net I managed to get some info from a Goldwing
owner. He recommended a:

NTC Thermistor TYPE RL2004-582-97-T10
Rated at 1,000 ohms at 25 degrees Celsius.
Cold resistance is between 900 and 1200 ohms.

This evening I made a mockup circuit using that thermistor. With a 12
volt power source and a 4 watt bulb I left it energized for a few
hours. What has me worried is that the thermistor gets too hot to
touch. Submerging it in a very small vile of gas causes the light to
turn off. It's hot enough that I hear a very faint hiss as it goes in
the gas.
It works exactly as it should but I'm worried about the heat.

Anyone have experience with these thermistors? I would assume the
same style setup is used in most cars with "Low Fuel" warning lights?

Thanks for reading,

George
Click to expand...
 
"CWatters" <colin.watters@pandora.be> wrote in message news:<a7uUa.33865$F92.3747@afrodite.telenet-ops.be>...
I think it would be safest to look for a sensor form another bike that uses
a similar circuit but...

If' I'm looking at the right data sheets ...

http://www.thermometrics.com/assets/images/rl20.pdf

the NTC material is D9.7A and the data for that suggests the resistance is
on the high side..

http://www.hhv.co.uk/PDF's/thermistors/keystoned97.pdf

I made it 107 Ohms at around 85C.

If that's correct I'm surprised the bulb lit up for your test.

I think the cold temp is about right (could be a bit lower) but the graph
needs to be steeper with a lower hot temp. I think the resistance of the NTC
needs to be less than the resistance of the bulb at whatver temp you feel is
safe (say <10Ohms at 50C?)



"cxturbö" <vairxpert@hotmail.com> wrote in message
news:3f2209a2.13442658@news.netnitco.net...

I'm having problems with the "Fuel Reserve" light on my motorcycle.
When the fuel level drops to 1.3 gallons the Fuel Reserve light on the
dash board is supposed to light, few weeks ago it quit working.

I removed the fuel gauge sending unit from the tank which has the
gauge rheostat and a separate metal capsule for the Reserve sensor. I
split open the capsule and a very small thermistor...maybe a 1/16"
fell out. It broke off the two wires soldered at both ends.
After looking over the schematic in my shop manual, the circuit seems
rather disturbing. 12 VDC is supplied to the 4 watt bulb in the dash
which then leads to the negative coefficient thermistor in the gas
tank. Submerged in fuel the circuit is open. Out of fuel it heats
up, closes and completes the circuit to ground.
I always thought that there were very low voltage value's sent into a
tank in order to prevent explosions? When that thermistor broke off
the base wire could have easily touched the metal casing and it's at
least able to handle 4 watts @ ~14 volts?
Is the fuel to air ratio so high that an explosion is not possible?
Even so, being a motorcycle with a 5 gallon tank I would think there
would be a risk when removing the cap to refuel?

Next problem is repairing it. I managed to solder the wires back on
but that didn't last but a few weeks. This time the microscopic metal
tab came off the black thermistor body. The bike is a very limited
production 1983 Honda CX650 Turbo...needless to say the sending unit
assembly is no longer available.
After searching the net I managed to get some info from a Goldwing
owner. He recommended a:

NTC Thermistor TYPE RL2004-582-97-T10
Rated at 1,000 ohms at 25 degrees Celsius.
Cold resistance is between 900 and 1200 ohms.

This evening I made a mockup circuit using that thermistor. With a 12
volt power source and a 4 watt bulb I left it energized for a few
hours. What has me worried is that the thermistor gets too hot to
touch. Submerging it in a very small vile of gas causes the light to
turn off. It's hot enough that I hear a very faint hiss as it goes in
the gas.
It works exactly as it should but I'm worried about the heat.

Anyone have experience with these thermistors? I would assume the
same style setup is used in most cars with "Low Fuel" warning lights?

Thanks for reading,

George

I recently replaced the sending unit and fuel pump in my aerostar van
Click to expand...
and was shocked to find out that all of the wiring to the pump and the
sending unit, which was basically a big potentiometer, were all
exposed and submerged into the gasoline. I can't see why these things
do not create a danger of explosion.
 
cxturbö wrote:

I'm having problems with the "Fuel Reserve" light on my motorcycle.
[...]
This evening I made a mockup circuit using that thermistor. With a 12
volt power source and a 4 watt bulb I left it energized for a few
hours. What has me worried is that the thermistor gets too hot to
touch. Submerging it in a very small vile of gas causes the light to
turn off. It's hot enough that I hear a very faint hiss as it goes in
the gas.
Click to expand...
Maybe you had the bulb replaced with a higher wattage type accidentally in
the past?
That might explain the failure and the not-so satisfying experiment.

Just a thought,
Michael


--
One smooth landing is mostly luck; two in a row is all luck; three in a row
is prevarication.

Promoting Penguin Power. Web home: http://www.qsl.net/dc1rn
 
On Sat, 26 Jul 2003 22:54:46 +0200, Michael Hofmann
<westbound@gmx.net> wrote:

cxturbö wrote:

I'm having problems with the "Fuel Reserve" light on my motorcycle.
[...]
This evening I made a mockup circuit using that thermistor. With a 12
volt power source and a 4 watt bulb I left it energized for a few
hours. What has me worried is that the thermistor gets too hot to
touch. Submerging it in a very small vile of gas causes the light to
turn off. It's hot enough that I hear a very faint hiss as it goes in
the gas.

Maybe you had the bulb replaced with a higher wattage type accidentally in
the past?
That might explain the failure and the not-so satisfying experiment.
Click to expand...
I think the original thermistor just failed after 20 years. Bulbs in
the dash are still factory.
Actually the experiment worked exactly as it's supposed to. From
ignition "on" the Fuel Reserve light typically takes 30 seconds to
start glowing. The new thermistor does the exact same thing. Touch
it with a piece of paper soaked in gas and the bulb goes off.

My biggest concern is the temperature of the thermistor getting hot
enough to ignite the fuel.
 
cxturbö wrote:

My biggest concern is the temperature of the thermistor getting hot
enough to ignite the fuel.
Click to expand...
The ignition temperature of gasoline is approx. 240°C, so I wouldn't be too
worried about that.

Michael
 
On Sat, 26 Jul 2003 12:04:22 GMT, "CWatters"
<colin.watters@pandora.be> wrote:

I think it would be safest to look for a sensor form another bike that uses
a similar circuit but...
Click to expand...
Correct. I stopped by the library today and went through quite a few
books. Unfortunately all they have are Clymers and Haynes which don't
go into detail. No shop manuals.

If' I'm looking at the right data sheets ...

http://www.thermometrics.com/assets/images/rl20.pdf

the NTC material is D9.7A and the data for that suggests the resistance is
on the high side..

http://www.hhv.co.uk/PDF's/thermistors/keystoned97.pdf

I made it 107 Ohms at around 85C.

If that's correct I'm surprised the bulb lit up for your test.
Click to expand...
Actually it works great. Even the warm up time from ambient until the
bulb starts to glow is about 30 seconds...exactly how the original
used to work. Just touch it with a piece of paper dabbed in gasoline
and the bulb goes out.


I think the cold temp is about right (could be a bit lower) but the graph
needs to be steeper with a lower hot temp. I think the resistance of the NTC
needs to be less than the resistance of the bulb at whatver temp you feel is
safe (say <10Ohms at 50C?)
Click to expand...
The specs on the thermistor I'm using mention it's good for up to
150şC.
I measured the temperature of the thermistor today. Highest reading I
obtained was ~85ş C. From what I understand it would need to reach
232ş C in a perfect environment to ignite gas vapors.

It should work perfect. I also have a 500 Turbo which is nearly an
identical bike with the same sending unit. I'm going to pop the
thermistor out of that one and do a comparison just to be safe.

I've never really thought about what goes on in the fuel tanks of cars
(or motorcycles) until now. Fuel pumps that are not hermetically
sealed, rheostats that are designed for far less than 12 volts and go
into meltdown from a short or failed resistor, heated thermistors,
ect...I guess the engineers get things right since we don't hear of
cars going off like bombs everyday :)

George
 
cxturbö wrote:

ect...I guess the engineers get things right since we don't hear of
cars going off like bombs everyday :)
Click to expand...
You're not watching Hollywood movies? They do :)

SCNR,
Michael
 
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