Thermocouple

Sensor that converts thermal potential difference into electrical potential difference

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Libraries:
Simscape / Electrical / Sensors & Transducers

Description

The Thermocouple block represents a thermocouple using the standard polynomial parameterization defined in the NIST ITS-90 Thermocouple Database [1].

For thermocouples Type B, E, J, K (t<=0 degC), N, R, S or T, the voltage E across the device in mV is

$$E(mV)=c_0+c_{1}t+\mathrm{...}+c_n{t}^n$$

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.

For thermocouples Type K (t>=0 degC), the equation contains an additional exponential term:

$$E(mV)=c_0+c_{1}t+\mathrm{...}+c_n{t}^n+a_0{e}^{a_1{\left(t-a_2\right)}^2}$$

where a₀, a₁, and a₂ are additional coefficients, required only by the Type K thermocouple, defined by the Coefficients [a0 a1 a2] parameter value.

The following equation describes the thermal behavior of the block:

$$Q=K_d{t}_c\frac{dT}{dt}$$

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:

1. Connect the thermocouple to the B port of a Simscape™ Convective Heat Transfer block.

2. 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.

3. Set the Area parameter of the Convective Heat Transfer block to an approximate area A_nom.

4. Set the Heat transfer coefficient parameter of the Convective Heat Transfer block to K_d/A_nom.

Examples

Measure Temperature During Quenching of Steel Using Thermocouple Block

Use the Thermocouple block to measure the temperature of steel during quenching. During quenching, you heat the steel to a temperature above its recrystallization temperature and then rapidly cool the steel by submerging it in liquid. Because of the high temperature, you cannot use traditional temperature measurement apparatus such as thermistor-based or semiconductor-based temperature sensors or liquid thermometers. To measure temperatures in such harsh environments, industries and laboratories employ thermocouples, which convert thermal potential difference to electrical potential difference.

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Assumptions and Limitations

• 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.

• The maximum supported value for n in the Coefficients [c0 c1 ... cn] parameter is 14, that is, the vector cannot have more than 15 elements.

Ports

Conserving

A — Thermal port

thermal

Thermocouple thermal port.

+ — Positive port

electrical

Electrical conserving port associated with the thermocouple positive port.

- — Negative port

electrical

Electrical conserving port associated with the thermocouple negative port.

Parameters

Electrical

Thermocouple type — Thermocouple type

Type B, E, J, K (t<=0 degC), N, R, S or T (default) | Type K (t>=0 degC)

Select one of the modeling options:

• Type B, E, J, K (t<=0 degC), N, R, S or T — This option is equivalent to the block functionality in previous releases.

• Type K (t>=0 degC) — This option adds an exponential term to the block equations when the temperature difference is greater than 0 degrees Celsius.

Coefficients [c0 c1 ... cn] — Coefficients [c0 c1 ... cn]

[0, 0, 0, 0, 0, 0, 0, 0, 0] (default)

Vector of coefficients c in the equation that describes voltage as a function of temperature. The maximum length of the vector is 15 elements.

Note

You can download parameters for standard thermocouple types from the NIST database [1]. For information on how to do this, see the Simulink^®Approximating Nonlinear Relationships: Type S Thermocouple example.

Coefficients [a0 a1 a2] — Coefficients [a0 a1 a2]

[0, 0, 0] (default)

Vector of additional coefficients a₀, a₁, and a₂, required only by the Type K thermocouple.

Dependencies

This parameter is visible only when you select Type K (t>=0 degC) for the Thermocouple model parameter.

Thermal

Reference temperature — Reference temperature

0 degC (default)

Temperature the block subtracts from the temperature at the thermal port in calculating the voltage across the device.

Thermal time constant — Thermal time constant

1 s (default)

Time it takes the thermocouple temperature to reach 63% of the final temperature change when a step change in ambient temperature occurs.

Dissipation factor — Dissipation factor

1e-3 W/K (default)

Thermal power required to raise the thermocouple temperature by one K.

Initial temperature — Initial temperature

25 degC (default)

Temperature of the thermocouple at the start of the simulation.

References

[1] Dean C. Ripple, NIST ITS-90 Thermocouple Database, December 21, 1999 (https://srdata.nist.gov/its90/main/)

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

Version History

Introduced in R2008a