On Sat, 7 Mar 2009 12:21:57 -0800 (PST), put finger
to keyboard and composed:

Thanks for all the replies.

There is no anticipator. It's a digital. No pots. No adjustments.
There is a coil; a crystal; a relay; a box - not an IC- from what I
can tell.
Switches that (1) indicate gas or electric; (2) enable or disable
(fuzzy logic) temperature
swing compensation just before start of the next program. { its
purpose to raise comfort level
to meet the new setting on next program before the new program starts}
And a span range is handled by keypad entry.


Anyway it's not about whether or not the t-stat meets the set
programming cycles.

The thermostat is not reading room temperature correctly.
I have 5 thermostats nailed to the wall now.
Two old style HW round mercury with anticipators, and
three digital style.
And this one is off by the greatest degree.

At room temp. 70 it reads 72. The others read 70 -71.
At 72 , the Hunter44360 reads 75.
At 74 it reads 78. And so on.

There isn't much on the circuit board other than what I described and
a few caps. I originally thought that the thermistor sensing varying
temperatures
would produce varying resistance and somehow translate that into the
room temp display.


So if it's not the thermistor, what handles the reading of room temp?

The thermistor handles this. It is probably connected as part of a
potential divider driven from a regulated voltage source. Maybe there
is a factory calibration routine built into the uP inside the "box"
??? How does the thermostat read at lower temperatures? High or low?
Is there a possibility that self heating may be affecting the reading?
Does the thermostat read lower with its cover removed? Maybe you could
"calibrate" your thermostat by shunting the thermistor with a high
valued resistor, or by adding a low value in series with it. Or you
could take some resistance readings at various temperatures and look
for a matching replacement.

The following test results suggest that thermistor accuracy is not
affected significantly (less than 0.5 deg C) by aging:
http://www.springerlink.com/content/tn758n7285033298/

This document ...

http://www.thermistor.com/references...sign_Guide.pdf

.... suggests that a replacement thermistor manufactured with the same
RT curve should require no calibration.

================================================== ============
A thermistor can be defined as having an interchangeability
tolerance of ±0.1°C over the range from 0° to 70°C. This means
that all points between 0° and 70°C, are within 0.1°C of the
nominal resistance values for that particular thermistor curve.
This feature results in temperature measurements accurate to
±0.1°C no matter how many different thermistors are substituted
in the application.
================================================== ============

Here are some RT curves:
http://www.omega.com/temperature/Z/pdf/z256-257.pdf

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.