Question  set 166

Response of a Thermocouple

1.      “The response of a thermocouple is defined as the time required for the thermocouple to reach the surrounding temperature when it is exposed to it”.
a) True
b) False
Answer: b
Explanation: It is the source temperature.

2.      The sensitivity of thermocouple is defined as the time required by thermocouple to reach how much percentage of its steady state values?
a) 43.3
b) 53.2
c) 63.3
d) 73.3
Answer: c
Explanation: The time constant of a thermocouple represents the time required to attain 63.2% value.

3.      The response time for different sizes and materials of thermocouple wires usually lie between
a) 0.04 to 2.5 seconds
b) 0.06 to 1.2 seconds
c) 0.02 to 0.04 seconds
d) 2.4 to 9.4 seconds
Answer: a
Explanation: Depending upon the type of fluid used, the response time for different sizes and materials of thermocouple wires usually lie between o.o4 to 2.5 seconds.

4.      A thermocouple junction of spherical form is to be used to measure the temperature of the gas stream. The junction is at 20 degree Celsius and is placed in a gas stream which is at 200 degree Celsius. Make calculations for time required by the thermocouple to reach 197 degree Celsius temperature. Assume the thermos-physical properties as given below
k = 20 W/ m K
h = 350 W/m² K
c = 0.4 k J/kg K
p = 8000 kg/m³
a) 1.094 seconds
b) 2.094 seconds
c) 3.094 seconds
d) 4.094 seconds
Answer: d
Explanation: t – t _a/t _{I –} t _a = exponential (- p V c T/h A).

5.      An egg with mean diameter of 4 cm and initially at 25 degree Celsius is placed in an open boiling water container for 4 minutes and found to be boiled at a particular level. For how long should a similar egg boil at the same level, when refrigerator temperature is 5 degree Celsius? Use lumped parameter theory and assume following properties of egg
k = 12 W/m K
h = 125 W/m² K
c = 2 k J/kg K
p = 1250 kg/m³
a) 251.49 seconds
b) 261.49 seconds
c) 271.49 seconds
d) 281.49 seconds
Answer: c
Explanation: T _{t –} T _INFINITY/T _{i –} T _INFINITY = e ^{– b T}.

6.      A person is found dead at 5 pm in a room whose temperature is 20 degree Celsius. The temperature of body is measured to be 25 degree Celsius, when found and heat transfer coefficient is estimated to be 8 W/m² K. Modeling the body as a 30 cm diameter, 1.7 cm long cylinder. Estimate the time of death of person.
a) 13.55 seconds
b) 12.55 seconds
c) 11.55 seconds
d) 10.55 seconds
Answer: d
Explanation: T _{t –} T _INFINITY/T _{i –} T _INFINITY = e ^{– b T}.

7.      The following data pertains to the junction of a thermocouple wire used to measure the temperature of the gas stream.
Density = 8500 kg/m³, specific heat = 325 J/kg K, thermal conductivity = 40 W/m K and the heat transfer coefficient between the junction and gas = 215 W/m² K.
If thermocouple junction can be approximated as 1 mm diameter sphere, determine how long it will take for the thermocouple to read 99% of the initial temperature difference
a) 9.86 seconds
b) 8.86 seconds
c) 7.86 seconds
d) 6.86 seconds
Answer: a
Explanation: t – t _a/t _{i –} t _a = exponential (- h A T/p V c).

8.      A thermocouple junction in the form of 4 mm radius sphere is to be used to measure the temperature of a gas stream. The junction is initially at 35 degree Celsius and is placed in a gas stream which is at 300 degree Celsius. The thermocouple is removed from the hot gas stream after 10 seconds and kept in still air at 25 degree Celsius with convective coefficient 10 W/m² K. Find out the time constant of the thermocouple. Assume the thermos-physical properties as given below
h = 37.5 W/m² K
p = 7500 kg/m³
c = 400 J/kg K
a) 6.67 seconds
b) 106.67 seconds
c) 206.67 seconds
d) 306.67 seconds
Answer: b
Explanation: T = p V c/h A = p r c/3 h = 106.67 seconds.

9.      What percentage of water an average human body can have?
a) 52%
b) 62%
c) 72%
d) 82%
Answer: c
Explanation: Average human body is 72% water by mass.

10.  Heisler charts are valid if
a) Fourier number is equal to 0.2
b) Fourier number is less than 0.2
c) Fourier number is greater than 0.2
d) Fourier number is equal to 0.4
Answer: c
Explanation: The solution to the transient heat flow in infinite flat plates are available in the form of these charts.