Foundation of Mems, Author Chang Liu, 2. Edition, ISBN 0132497360, 9780132497367

Question

Foundation of Mems, Author Chang Liu, 2. Edition, ISBN 0132497360, 9780132497367

Two pieces of metal wires are connected in a thermal couple configuration as follows. The temperature at point 1 is higher than the temperatures at points 2 and 3. There are three possible metals. (1) Chromel, with Seeback coefficient = 30 mu V/K. (2) Alumel, with Seeback coefficient = -11 mu V/K. (3) Iron, with Seeback coefficient = 10 mu V/K. Which of the following statement would be correct? Which one gives the best temperature sensitivity? Show your analysis steps. (1) Metal 1 = Chromel, Metal 2 = Chromel, produces maximum possible thermal couple sensitivity of 60 mu V/K (2) Metal 1 = Alumel, Metal 2 = Chromel, produces maximum possible thermal couple sensitivity of 30 mu V/K (3) Metal 1 = Iron, Metal 2 = Alumel, produces maximum possible thermal couple sensitivity of 21 mu V/K (4) Metal 1 = Chromel, Metal 2 = Alumel, produces maximum possible thermal couple sensitivity of 41 mu V/K.

Explanation / Answer

Voltage for thermocouple is given by linear expression

VM1-M2 = ( a + bT)M1 - (a + bT)M2

where a and b are constants.

b is Seeback coefficient of metal.

T is the absolute temperature for hot junction.

And hence, (b)Thermocouple = (b)M1 -(b)M2

1) M1 Chromel with Seeback coefficient 30 * 10-6 V/K and M2 Chromel with Seeback coefficient 30 * 10-6 V/K.

(b)Thermocouple = (b)M1 -(b)M2

= 30 * 10-6 V/K - 30 * 10-6 V/K. = 0V/K.

So,it is wrong.

2) M1 Alumel with Seeback coefficient -11 * 10-6 V/K and M2 Chromel with Seeback coefficient 30 * 10-6 V/K.

(b)Thermocouple = (b)M1 -(b)M2

= -11 * 10-6 V/K - 30 * 10-6 V/K. = -41 * 10-6 V/K.

So, it is wrong.

3) M1 Iron with Seeback coefficient 10 * 10-6 V/K and M2 Alumel with Seeback coefficient -11 * 10-6 V/K.

(b)Thermocouple = (b)M1 -(b)M2

= 10 * 10-6 V/K - 11 * 10-6 V/K. = 21 * 10-6 V/K.

So, it is correct.

4) M1 Chromel with Seeback coefficient 30 * 10-6 V/K and M2 Alumel with Seeback coefficient -11 * 10-6 V/K.

(b)Thermocouple = (b)M1 -(b)M2

= 30 * 10-6 V/K - (-11 * 10-6 V/K). = 41 * 10-6 V/K.

So, it is correct.

Hence (4) M1 Chromel with Seeback coefficient 30 * 10-6 V/K and M2 Alumel with Seeback coefficient -11 * 10-6 V/K gives the best temperature sensitivity of 41 * 10-6 V/K.

Related Questions

Navigate

• Browse (All)
• Browse F
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.