Download Thermometry: Temperature Measurement Techniques and Instruments and more Assignments Physics in PDF only on Docsity!
Talk Topics Due Wednesday March 5
HW-2 Problem
- An ideal Linde liquefier runs on air between 1 atm. and 200 atm. at 20 C. What is the liquefaction rate y (2 significant figures)?
Ans: y = = 0.082 = 8.2% (Reference source: Thomas M. Flynn, “Cryogenic Engineering”, second edition, New York: Marcel Dekker, p. 394, 2005.)
1
1 2
H H f
H H
HW-2 Problem #2a
- Calculate the Joule-Thomson coefficient for a gas which obeys a. the ideal gas law: pv=nRT
HW-2 Problem #2b
b. For the Dieterici equation of state: p(v-b)exp(a/vRT)=RT, where a and b are constants, find
we define
You try to get the answer: (^) vRTa^ = u
v
a
u
RT
2
1 v^ v
v v
JT P
b a RT
C RT a b
⎢⎣ −^ − ⎥⎦
a is associated with the interaction between molecules and b takes into account the finite size of the molecules, similarly to the Van der Waals equation.
HW-3 Problem
Ans: With normal hydrogen liquid in dewar, it consists 75% ortho and 25% para. The ortho fraction after 24 hours is
So the total fraction of para-hydrogen present after 24 hours is 1- x=37.64%
(Reference source:
- Thomas M. Flynn, “Cryogenic Engineering”, second edition, New York: Marcel Dekker, p. 193-202, 2005. )
+ ×
x =
Thermometry
OUTLINE
**1. Goals
- Thermometry
- Temperature Ranges** **of Application
- Constant Volume** **Thermometer
- Thermocouples
- Thermometer Time** **Response Data
- 4 Terminal Resistance** Measurement 8. Pt (pure metal) Thermometer 9. Typical Resistive Thermal Sensor 10. Typical Resistance and Sensitivity Curves 11. Thermal Regulation 12. Data Collection/Wheatstone Bridge
Thermometer Characteristics
Temperature Ranges of Application Constant Volume Thermometer
- Named after Sir Fancis Simon
- Helium is an ideal gas down to 5 K
- Modern versions use in situ pressure gauges at low T with electrical read- out (no gas line from RT to Low T)
Low T
RT
Helium gas
Gas line
Thermocouples
Figure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (1997), p. 530
Wires of 2 different metals (pure or alloy) when joined and connected to a volt meter produce a voltage related to temperature. At right is the sensitivity of various common thermocouples, perhaps the simplest, least expensive, and most common thermometer in use. Usually a reference junction in an ice bath is used to make the measurement absolute. Calibrations are tabulated (e.g. type K)
Thermometer Time Response Data
Differences between wet / dry can be exploited for level detection
Figure adapted from Cryogenic Engineering by Thomas M. Flynn, Dekker:NY (1997), p. 495
Thermometer at low T
Leads from RT to Low T must have low heat leak (alloy)
Meter at RT must have high input impedance
4 Terminal Resistance Measurement
Current source must be stable and reversible. Average of readings with current flowing in opposite directions
gives correct voltage drop, canceling thermal emfs. Often, low frequency ac source is used, with lock-in
detector as volt meter, to improve sensitivity.
The purpose of a 4-terminal measurement of resistance is:
4 Terminal Resistance Measurement
A. To provide extra wires in case of breakage B. To make it easier to hook up the measurement instruments C. To make sure the current only flows in one direction D. To make it a Wheatstone bridge E. To avoid contact and lead resistance
