Thermal Divider

Now THERE'S a hell of a thought. Consider a "bar" three inches long with
sensors at 1 and 2 inches. Betcha the temperature at the head (zero inches)
is (temp 1) plus delta (temp 1 minus temp 2).

Jim


how about two temp sensors at measured intervals along the insulated bar,
then assuming the temperature gradient is constant along the bar, you can
work out the temp at the cylinder head.

Colin =^.^=

"Capt. Geoffrey Thorpe" The Sea Hawk at wow way d0t com wrote in message
...
"RST Engineering" wrote in message news
Think of a series voltage divider with two known resistors in the middle and an unknown resistor attached to each
end. What does the votage measured at the junction between the two known resistors tell you about the source
voltage?


Absolutely nothing. An equation in one unknown with two degrees of freedom is insoluble. There are an infinite
number of correct answers and an infinite number of incorrect answers. HOWEVER, if you let me measure the voltage
ACROSS one of those known resistors and THEN the voltage at the junction, I've got a fighting chance if you know what
the bottom end resistor is tied to.


Think of heat flow as current, temperature as voltage, the actual connecton between your divider and the heat source /
sink like unknown resistors (area, contact, material all make a difference as in a high current circuit) your bar with
the sensor in the middle is like the voltage drop in a transmission line - flow is a function of area, material,
potential. Heat loss from the bar is a little harder - I guess in a high tension transmission line there is some
leakage to ground across the insulators?

And you thought you didn't know thermodynamics...

fwiw, I think I would just buy a different sensor, eh?

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.


Maybe use two sensors, one way out at the cold end, and the other 'half way out'. Use some sort of logic to sort out the
cold end and adjust from there...

"RST Engineering (jw)" wrote in message ...
Now THERE'S a hell of a thought. Consider a "bar" three inches long with sensors at 1 and 2 inches. Betcha the
temperature at the head (zero inches) is (temp 1) plus delta (temp 1 minus temp 2).

Jim


how about two temp sensors at measured intervals along the insulated bar,
then assuming the temperature gradient is constant along the bar, you can
work out the temp at the cylinder head.

Colin =^.^=





Hmmm, just posted that...must be a fairly good way. Think cold junction compensation....

On Mon, 29 May 2006 17:48:07 -0700, the renowned "RST Engineering
\(jw\)" wrote:

Now THERE'S a hell of a thought. Consider a "bar" three inches long with
sensors at 1 and 2 inches. Betcha the temperature at the head (zero inches)
is (temp 1) plus delta (temp 1 minus temp 2).

Jim

Are you assuming negligible heat loss from the bar itself?




Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
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"Spehro Pefhany" wrote in message
...
On Mon, 29 May 2006 17:48:07 -0700, the renowned "RST Engineering
\(jw\)" wrote:

Now THERE'S a hell of a thought. Consider a "bar" three inches long with
sensors at 1 and 2 inches. Betcha the temperature at the head (zero
inches)
is (temp 1) plus delta (temp 1 minus temp 2).

Jim

Are you assuming negligible heat loss from the bar itself?


yes if the bar is insulated well enough between the sensors and the head,
maybe with 3 sensors you could comepnsate for the heat loss through the
insulation wich would make for a curved temperature profile along the bar.

Colin =^.^=

John B wrote:
scrobe on the papyrus:


RST Engineering (jw) wrote:
I have a thermometer that reads from 0 to 200°F. I'm trying to
measure a cylinder head on an aircooled engine where my range of
interest goes from, say, 200 to 400°F. The classic way of
measuring this temperature is with a thermocouple mounted on a
copper washer underneath the spark plug of the cylinder that you
determine (by trial and error) to be the hottest.

I can think of several ways of measuring a cooler spot on the
cylinder that will probably be in rough proportion to the actual
temperature at the plug seat, but most of them are dependent on the
airflow over the cylinder(s) remaining constant from day to day.
With the baffling on the engine being rather thin and wobbly, I
can't count on this airflow being truly constant.

The sensor on my thermometer is a plain old silicon diode that won't
directly take the heat that I'm trying to measure. Anybody got a
clever way of making a thermal divider that won't be subject to the
day to day shuffle of the airflow over the cylinders?

Why don't you get a better sensor? Either a thermocouple or a platinum
resistance sensor will go up to 400F (204C). Farnell do a thin-film
Pt100 sensor that is only 5 mm by 2mm by 1.1mm.

Omega do similar looking sensor with leads attached (good o 500F)

http://www.omega.com/ppt/pptsc.asp?r...D-2&Nav=temc06

A type K thermocouple will go much higher than that. Look at the
MAX6675, it will resolve to 0.25C from 0C (32F) to 1024C (1875F) with a
type K and it has an SPI bus.

But the OP only wants to go up to 400F, and thermocouples are pretty
nasty temperature sensors at the best of times - the output voltage is
low and you have to provide cold junction compensation. The Maxim part
may promise all kinds of nice performance, but it would still be
digitising the output voltage of thermocouple at the other end of at
least a foot of so of wire, if you could buy the thing in the first
place - Farnell doesn't stock the MAX6675, which means that it isn't
exactly a commodity part.

--
Bill Sloman, Nijmegen

On Mon, 29 May 2006 09:51:40 -0700, "RST Engineering \(jw\)"
wrote:

I have a thermometer that reads from 0 to 200°F. I'm trying to measure a
cylinder head on an aircooled engine where my range of interest goes from,
say, 200 to 400°F. The classic way of measuring this temperature is with a
thermocouple mounted on a copper washer underneath the spark plug of the
cylinder that you determine (by trial and error) to be the hottest.

I can think of several ways of measuring a cooler spot on the cylinder that
will probably be in rough proportion to the actual temperature at the plug
seat, but most of them are dependent on the airflow over the cylinder(s)
remaining constant from day to day. With the baffling on the engine being
rather thin and wobbly, I can't count on this airflow being truly constant.

The sensor on my thermometer is a plain old silicon diode that won't
directly take the heat that I'm trying to measure. Anybody got a clever way
of making a thermal divider that won't be subject to the day to day shuffle
of the airflow over the cylinders?

Jim

Jim would it be possible to tackle the problem another way and use
one of the off contact laser aimed temperature sensors?

Jim.................

This might be fun to play with:
http://www2.towerhobbies.com/cgi-bin...?&I=LXHEM4&P=0

I have one of these:

http://www2.towerhobbies.com/cgi-bin...&I=LXEMG5&P=ML

I use it for all sorts of things, BBQ, smoker, engines, electronic
equipment, et cetera.

Rich S.

"Rich S." wrote in message
...
Jim.................

This might be fun to play with:
http://www2.towerhobbies.com/cgi-bin...?&I=LXHEM4&P=0

Now you just need to figure out how to connect all these sensors to a
bluetooth transceiver so's you don't have to string any wires. . .

Rich S.