Thermal Divider

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

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.

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

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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)

--=20 Bill Sloman, Nijmegen

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.

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.

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.

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

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.

snipped-for-privacy@ieee.org scrobe on the papyrus:

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.

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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 =^.^=

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

an unknown resistor attached to each

resistors tell you about the source

is insoluble. There are an infinite

HOWEVER, if you let me measure the voltage

I've got a fighting chance if you know what

between your divider and the heat source /

in a high current circuit) your bar with

flow is a function of area, material,

tension transmission line there is some

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...

sensors at 1 and 2 inches. Betcha the

temp 2).

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

Are you assuming negligible heat loss from the bar itself?

Best regards, Spehro Pefhany

inches)

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 =^.^=

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.

--=20 Bill Sloman, Nijmegen

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:

I have one of these:

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

Rich S.

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.