Thermal Divider

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

RST Engineering (jw) wrote:
a thermal divider that won't be subject to the day to day shuffle
of the airflow over the cylinders?

Jim


Attach the sensor to a piece of metal. Bolt the metal to something on
the cylinder head. Wrap the metal/sensor with insulating tape to keep
cooling air off it. Change the size of the metal piece to vary the
temperature ratio.

--
John Kimmel

remove x

"He's dead, Jim."

Is it purely a function of the size of the metal piece, or is the type of
metal part of the equation?

The other thing I don't understand is that if I do a good job of insulating
the metal piece from ambient how it won't eventually come to the same
temperature as the head.

Thermo and I never did get along very well.

Jim



"John Kimmel" wrote in message
...
RST Engineering (jw) wrote:
a thermal divider that won't be subject to the day to day shuffle
of the airflow over the cylinders?

Jim
Attach the sensor to a piece of metal. Bolt the metal to something on the
cylinder head. Wrap the metal/sensor with insulating tape to keep cooling
air off it. Change the size of the metal piece to vary the temperature
ratio.

--
John Kimmel

remove x

"He's dead, Jim."

RST Engineering (jw) wrote:
I have a thermometer that reads from 0 to 200°F. I'm trying to measurea
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

--
Bill Sloman, Nijmegen

RST Engineering (jw) wrote:
Is it purely a function of the size of the metal piece, or is the type of
metal part of the equation?

The other thing I don't understand is that if I do a good job of insulating
the metal piece from ambient how it won't eventually come to the same
temperature as the head.

Thermo and I never did get along very well.

Jim

The insulation is to shield the sensor and heat sink from the effects of
transient airflow. The insulation won't be perfect in any case, but you
can vary the amount of insulation and the size of the heat sink to vary
your results. If the insulation is perfect, then you will need to bolt
the heat sink to something that doesn't get very hot.

This idea might not work at all, but it is something that would be easy
to try, and easy to change variables on.


--
J Kimmel

www.metalinnovations.com

"Cuius testiculos habes, habeas cardia et cerebellum." - When you have
their full attention in your grip, their hearts and minds will follow.

RST Engineering (jw) wrote:
Is it purely a function of the size of the metal piece, or is the type of
metal part of the equation?

The other thing I don't understand is that if I do a good job of
insulating the metal piece from ambient how it won't eventually come to
the same temperature as the head.


If you mount the sensor in a bar that is picking up heat at one end, and
losing heat at the other - you sensor will read somewhere between the two
temperatures. But, if you don't know a lot about the heat transfer rates at
each end, you will not know how the temperature your sensor sees is related
to the temperature you want to measure.

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?

--
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.

If you mount the sensor in a bar that is picking up heat at one end, and
losing heat at the other - you sensor will read somewhere between the two
temperatures. But, if you don't know a lot about the heat transfer rates
at each end, you will not know how the temperature your sensor sees is
related to the temperature you want to measure.

That's exactly the point, Geoff. When I do a resistive divider, it is always
between two "hard" voltages, either a variable voltage and a reference
voltage or a variable voltage and ground. I know what the reference is and
can easily calculate the variable. The thermal reference is not quite so
trivial. It is a function of airflow, ambient temperature, and phase of the
moon.



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.

Jim

"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.

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.

--
John B

"Capt. Geoffrey Thorpe" The Sea Hawk at wow way d0t com wrote in message
news
RST Engineering (jw) wrote:
Is it purely a function of the size of the metal piece, or is the type
of
metal part of the equation?

The other thing I don't understand is that if I do a good job of
insulating the metal piece from ambient how it won't eventually come to
the same temperature as the head.


If you mount the sensor in a bar that is picking up heat at one end, and
losing heat at the other - you sensor will read somewhere between the two
temperatures. But, if you don't know a lot about the heat transfer rates
at
each end, you will not know how the temperature your sensor sees is
related
to the temperature you want to measure.

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?

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 =^.^=