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'[EE]: Why Steinhart-Hart equation?'
2001\04\19@112850 by Jose S. Samonte Jr.

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Hello everybody! =)
In my project, I used Steinhart-Hart equation to linearize the output of the
thermistor, because I knew that it is the most commonly used. But my professor
wants me to give much much better reasons why I used it. He said that there
are other equations. He wants to know why I opted to use Steinhart-Hart
equation. Can anyone, please help me? What are the advantages of using
Steinhart-Hart equation?

Thank you very much.
Best regards.


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2001\04\19@113721 by David VanHorn

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At 09:28 AM 4/19/01 -0600, Jose S. Samonte Jr. wrote:
>Hello everybody! =)
>In my project, I used Steinhart-Hart equation to linearize the output of the
>thermistor, because I knew that it is the most commonly used. But my professor
>wants me to give much much better reasons why I used it. He said that there
>are other equations. He wants to know why I opted to use Steinhart-Hart
>equation. Can anyone, please help me? What are the advantages of using
>Steinhart-Hart equation?

I think what he's getting at here, is that "because everyone else does it
this way" is not a very strong reason.

I get this frequently in engineering meetings, and I see both sides of the
argument.
On one hand, if a lot of people do it one way, there may be a good reason
for it.
On the other hand, it's a lazy attitude, and I've seen many cases where the
way something is usually done is not the best way to do it.

I think he's looking for you to present him with a "strong" reason for
using it.

The only one I can think of is that it dosen't have to be calibrated to the
individual thermistor, like a table would be, for maximum accuracy.


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2001\04\19@114551 by Jose S. Samonte Jr.

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Sir David, I didn't get what you mean. =)
One more time, please?

Please, more help and more reasons would be very much appreciated...

David VanHorn <spam_OUTdvanhornTakeThisOuTspamCEDAR.NET> wrote:
At 09:28 AM 4/19/01 -0600, Jose S. Samonte Jr. wrote:
>Hello everybody! =)
>In my project, I used Steinhart-Hart equation to linearize the output of the
>thermistor, because I knew that it is the most commonly used. But my
professor
>wants me to give much much better reasons why I used it. He said that there
>are other equations. He wants to know why I opted to use Steinhart-Hart
>equation. Can anyone, please help me? What are the advantages of using
>Steinhart-Hart equation?

I think what he's getting at here, is that "because everyone else does it
this way" is not a very strong reason.

I get this frequently in engineering meetings, and I see both sides of the
argument.
On one hand, if a lot of people do it one way, there may be a good reason
for it.
On the other hand, it's a lazy attitude, and I've seen many cases where the
way something is usually done is not the best way to do it.

I think he's looking for you to present him with a "strong" reason for
using it.

The only one I can think of is that it dosen't have to be calibrated to the
individual thermistor, like a table would be, for maximum accuracy.


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2001\04\19@125533 by David VanHorn

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At 09:44 AM 4/19/01 -0600, Jose S. Samonte Jr. wrote:
>Sir David, I didn't get what you mean. =)
>One more time, please?
>
>Please, more help and more reasons would be very much appreciated...

Your prof is asking you to justify your design decision.
He may be looking for a "stronger" answer than "because everyone else did
it that way".
Or, he may be looking for honesty from an engineer, saying "I did it this
way because I ran out of time to research it, but I assumed that if many
others use this method, it must be usually ok."

Me, I'd go with the honest answer.

Engineering is about facts. If you start covering them up or distorting
them, then how is anyone supposed to make decisions on that distorted data?

Just my opinion.  It's gotten me in trouble more than once, but I stick by it.

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2001\04\19@170801 by Michael Rigby-Jones

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{Quote hidden}

Jose,

The transfer function of a thermistor (i.e. resistance versus temperature)
is highly non-linear as you have no doubt found out.  In fact, it turns out
to be quite a complicated function to model.  If you only need accuracy over
a small range, then a relatively simple exponential function can be used.
However because the transfer function of a thermistor does not exactly fit
the simple exponential model, this becomes quite inaccurate over larger
ranges.

The Steinhart-Hart equation effectively models a thermistors transfer
characteristics far more accurately over large temperature ranges than the
simple exponential function.  Steinhart and Hart examined the
characteristics of a thermistor, and discovered that a polynomial could
provide an excellent fit in most cases.  It was found that a third order
polynomial provided the best fit with the least complexity.

Hope that helps

Mike.

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2001\04\19@193436 by Jose S. Samonte Jr.

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THANK YOU SIR MICHAEL!=)


Michael Rigby-Jones <.....mrjonesKILLspamspam.....NORTELNETWORKS.COM> wrote:
{Quote hidden}

Jose,

The transfer function of a thermistor (i.e. resistance versus temperature)
is highly non-linear as you have no doubt found out.  In fact, it turns out
to be quite a complicated function to model.  If you only need accuracy over
a small range, then a relatively simple exponential function can be used.
However because the transfer function of a thermistor does not exactly fit
the simple exponential model, this becomes quite inaccurate over larger
ranges.

The Steinhart-Hart equation effectively models a thermistors transfer
characteristics far more accurately over large temperature ranges than the
simple exponential function.  Steinhart and Hart examined the
characteristics of a thermistor, and discovered that a polynomial could
provide an excellent fit in most cases.  It was found that a third order
polynomial provided the best fit with the least complexity.

Hope that helps

Mike.

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