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1.11 User-defined Types
For a typical thermistor, the relationship between resistance and
temperature can be expressed by the Steinhart–Hart equation
where R
is the resistance in ohms at temperature T in degrees
T
Kelvin (K = °C + 273.15). The values of A, B, and C are called
Steinhart Coefficients. The error of the equation is less than +/-
0.01°C in a 100°C span.
The tM-TH8 supports user-defined types by specifying the
Steinhart coefficients using the @AASxTttC(data) command, see
Section 2.33. The data sent is a 32-bit hexadecimal value in
IEEE-754 standard format:
Bits
31 (sign)
30-23 (exponent) The exponent base is 2. The actual exponent is
22-00 (mantissa) The mantissa is expressed as 1.f where f is the
Example:
For the hexadecimal value C3694000h
1. Bit 31is 1, indicating a negative number.
2. The value of bits 30-23 is 10000110 binary or 134 decimal.
The exponent is 7, 134-127.
3. The mantissa is 1.11010010100000000000000 binary.
4. Adjust the mantissa for the exponent. The adjusted mantissa
is 11101001.0100000000000000 binary. That is 233.25
decimal.
5. Considering the sign bit. The floating-point number of
C3694000h is –233.25.
The following methods can be used to obtain the Steinhart
Coefficients of a thermistor.
22
tM-TH8 User Manual, Rev: A1.1 7MH-018-A11
1/T = A + B lnR
Description
0 = positive, 1 = negative
calculated by subtracting 127 from the stored
value.
fractional part and is stored in this field.
3
+ C (lnR
)
T
T
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