Purpose: To familiarize the student with the Clausius-Clapeyron relation as it relates to the operation of a pressure cooker.

Theory: The increase in the pressure...

Safety Note 1: Steam burns can be extremely painful and, if misused, the pressure cooker's pressure release valve can go off sending a stream of superheated steam towards the cieling. Don't let this happen! Never put too much wieght on the pressure cooker's regulator.

Safety Note 2: Never try to open the pressure cooker when it is pressurized! Also, never sudenly remove the weight on the pressure cooker's regulator; this will cause the water to flash steam and lead to a geyser of suerpheated water and steam. carefully follow all the directions regarding safety in the proceedure and do the readings in the order suggested!


Proceedure: Verify the operation of the thermometer inside the pressure cooker. Verify the operation of the hotplate. Fill the pressure cooker with about 2 qts. of water, enough to cover about 2/3 of the thermocouple stick. Note that the thermocouple's active  section is a small area near the end of the metal tube. It must remain submersed throughout the entire laboratory.

Measurement 1: Measure the diameter of the regulator tube.

Measurement 2: As the water heats towards the phase transition ('Boiling') measure the mass of the regulator block, and each of the mass rings that are to be used to increase the weight on the regulator. these will be carefully dropped and removed from the regulator during the course of the laboratory.

Measurement 3: As the water boils, verify that the temperature doesn't change but stays at 100 degrees C. Record the temperature reading of the temperature probe as the water boils.

Measurement 4: Now place on the regulator pipe the empty pressure regulator block. You will note that slowly the plastic overpressure release plug is slightly elevated and the cylindrical pressure plug is raised (this may take a few minutes). Wait until the pressure cooker starts bouncing the regulator and then record the temperature on the probe.

Measurement 5: Now, one at a time, slip on the various mass rings and mass ring combinations, being careful each time to record the temperature at which the pressure cooker starts bouncing the regulator. When you wish to reduce the pressure the safe way to
do that is to depress in short blasts the metal cylindrical pressure release stem until the pressure is low enough to gently slip a weight off. NEVER REMOVE THE REGULATOR IN TOTO WHILE YOU ARE RUNNIN THE EXPERIMENT.

Now, after you have assembled a collection of masses and temperatures, you are ready to graph steam pressure (newtons/m²) -vs- temperature (^oK). Measure the slope of that line. Now use  the Clausius-Clapeyron relation to turn your measurement of the slope  into a determination of the latent heat of vaporization. You may use the fact that you may neglect the molar volume of the liquid and that the molar volume of the steam is about






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       Original logbook of experiment. March 17, 2004.

About 2qts of water, maybe a bit less.
Used Tom's instrumented hotplate....very nice. Cost $800 I am told. Ouch.

Size of the opening. 2.7 mm diameter.

Used epoxy on the feed through and vacuum grease on the gasket. It should be tighter now.
started boiling right around 99.7. Things seem to be functioning well. No dripping. No steam leaks.
Took ~30 minutes to get to temp.

PROBLEM: slow temp change of water body.
ANSWER  :  mount the temp sensor in the vapor phase: lower heat capacity, faster changes.
    starts refluxing at 101
   
    full weights....steam temp kept rising....now it looks like good data !! Easily get to 106 range.
hot plate at 250 degrees target. Waited five minutes after each change of wieght and took reading.


full                                     111.7      47g
less delta ring  only           111.2      34 g
less flared only                 
less all                                109.9      21g

SO, putting this together with CC relation,

dT/dp = (v_g-v_l)T/L

v_g is the molar volumes of the gas phase, and in this case is much larger than that of the water
phase. T is the absolute temperature, here about 380 degrees K.

The pressure difference is mg/pi/r^2 = 9.4e4, 6.8e4 and 4.2e4 newtons/m^2 for the three masses above.

The graph gives a fit of dp/dt = 6.6 +/- 2.1 KPa/Kelvin in MKS units and since v_g \sim 1.28 m^3/Kg (from
CRC) and take v_l \sim 0 approx. Then we arrive at a latent heat of

L = dp/dT (v_g) T  = (6600 Pa/K)(1.28 m^3/Kg)(380 K)  = 3200 +/- 1000 KJ/Kg

within a sigma of the accepted 2250 Kj/Kg.






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