At 10:14 AM 6/29/2008, you wrote:

>Lots learned a lot over the past few days, and it really does seem that
>thermocouples (especially open-elements) are very very fast.  Type T
>thermocouples are apparently the most accurate and available in two classe=
s,
>with 0.5 deg-C error versions commonly available.  But thermocouples need a
>lot (relatively) of support circuitry,

The cold-junction compensation also adds an error term that could be
a problem in applications where the ambient varies widely (such as
automotive). It adds an error that is approximately 1:1 (eg. a 1 degree
error in the secondary sensor causes approximately a 1 degree error in the
reading. For this reason, T/Cs are better suited for EGT than intake. ;-)

>  especially since this is in an
>automotive application (measuring air temp at a couple points in the intake
>tract).  1sec to 2sec works fine, since this is a monitoring application,

Hmm.. what would cause the air temperature to change so quickly? Could
you use a milled out PCB as a combination mounting method, electrical
connection and heatsink? Maybe they you could use a thermistor or SMT
semiconductor sensor.

Monitoring is a *lot* easier than control. In control of air temperature
the sensor adds a pole to the response that can cause overshoot/undershoot
of the actual (not the measured) air temperature unless you grossly detune
the controller to make the response sluggish. In cases where there
is an object being heated or cooled that responds faster than the sensor
you can get serious problems if the actual air temperature does that.

>and 1% error would be nice, although a bit more would still be acceptable.
>
>Mounting (for the end-user) will be a major factor as well, so any basic
>element/component that needs to be PCB-mounted becomes less desirable, as
>I'd have to factor in the labor cost to build those to a user-friendly
>format ... and that's still be decided upon.
>
>I had missed the graphs with response time in those datasheets (LM35 etc) =
--
>I was looking for electrical parameters.  But checking back now, they're
>relatively quite slow.  I found RTD's that are available in better than 1
>sec response time using open elements, and they require minimal support
>circuitry.  They also come mounted to a stem/flange/etc so end-user install
>is ready-to go.  The problem is cost -- so far they're in the $35-$40 price
>range each (in 50-100pc quantities).
>
>I always thought that thermistors were not-so-accurate, but I've been
>corrected again it seems.

They can be okay (sometimes very good), but not so trustworthy at higher
temperatures. If your maximum is going to be 40 or 50=B0C, that's much less
of a worry.

>  I'm looking at various datasheets now in the 0.5%
>to 1% range, and they seem promising.  Very low cost (<$1 ),

1% of a thermistor is a lot fewer degrees than 1% of a substantially linear
sensor such as a thermocouple or RTD. Thermistors are extremely nonlinear,
so they're more appropriately used over a relatively narrow temperature ran=
ge.
  Watch that there is also a tolerance on the beta. The tolerance is kind o=
f a
conical tunnel that is narrowest at the reference temperature (eg. 25=B0C) =
and
quite a bit wider at both ends.


>  and I'm working
>out a mounting structure to see just how much it would cost if assembled
>in-house.  I also found some suppliers of thermistors in flanged probes, so
>I have some calls to make this week.
>
>Thanks,
>-Neil.
>
>
>
>--
>View this message in context: =

>http://www.nabble.com/Fast-temp-sensor--tp18104658p18181248.html
>Sent from the PIC - [EE] mailing list archive at Nabble.com.
>
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d"
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