SimElectronics™

Thermocouple

Model sensor that converts thermal potential difference into electrical potential difference

Library

Sensors

Description

The Thermocouple block represents a thermocouple using the standard polynomial parameterization defined in the NIST ITS-90 Thermocouple Database [1]. The voltage E across the device in mV is

E(mV) = c0 + c1*t + ... + cn*tⁿ

where:

ci is the i^th element of the Coefficients [c0 c1 ... cn] parameter value.
t is the temperature difference in degrees Celsius between the temperature at the thermal port A and the Reference temperature parameter value.

Note The equation for voltage across the device as a function of temperature difference is defined in mV. The units of the voltage across the actual device is V.

The following equation describes the thermal behavior of the block:

where:

T is the temperature at port A.
Q is the net heat flow into port A.
K_d is the Dissipation factor parameter value.
t_c is the Thermal time constant parameter value.
dT/dt is the rate of change of the temperature.

To model the thermocouple in free space:

Connect the thermocouple to the B port of a Simscape™ Convective Heat Transfer block.
Connect the A port of the Convective Heat Transfer block to a Simscape Ideal Temperature Source block whose temperature is set to the ambient temperature.
Set the Area parameter of the Convective Heat Transfer block to an approximate area A_nom.
Set the Heat transfer coefficient parameter of the Convective Heat Transfer block to K_d/A_nom.

Basic Assumptions and Limitations

The model is based on the following assumptions:

The high-order polynomials this block uses are very sensitive to the number of significant figures used for computation. Use all available significant figures when specifying the Coefficients [c0 c1 ... cn] parameter.
Coefficients [c0 c1 ... cn] are defined for use over a specified temperature range.
This block does not include the additional exponential term that Type K thermocouples use when parameterized for t > 0.

Dialog Box and Parameters

Electrical Tab

Coefficients [c0 c1 ... cn]

The vector of coefficients c in the equation that describes voltage as a function of temperature. The default value is [ 0 0.0054031 1.2593e-05 -2.3248e-08 3.2203e-11 -3.315e-14 2.5574e-17 -1.2507e-20 2.7144e-24 ]. This value specifies a Type S thermocouple, which is valid in the range -50 to 1064 degrees C.

Note You can download parameters for other standard thermocouple types from the NIST database [1]. For a demo of how to do this, see the Simulink^® Approximating Nonlinear Relationships: Type S Thermocouple demo, sldemo_tc_script.m, and the associated model file, sldemo_tc.mdl.

Thermal Tab

Reference temperature: The temperature the block subtracts from the temperature at the thermal port in calculating the voltage across the device. The default value is 0 °C.
Thermal time constant: The time it takes the thermocouple temperature to reach 63% of the final temperature change when a step change in ambient temperature occurs. The default value is 1 s.
Dissipation factor: The thermal power required to raise the thermocouple temperature by one K. The default value is 0.001 W/K.
Initial temperature: The temperature of the thermocouple at the start of the simulation. The default value is 25 °C.

Ports

The block has the following ports:

A: Thermocouple thermal port.
+: Positive electrical port.
-: Negative electrical port.

References

[1] NIST ITS-90 Thermocouple Database http://srdata.nist.gov/its90/main