Saturation Pressure Measurement and Effects of Thermal Lag

This lab report explores saturation pressure measurement and the effects of thermal lag. It discusses the principles behind saturation pressure measurement and how thermal lag affects accuracy. The methodology, results, and discussion of the experiment are presented, along with references for further reading.

LAB REPORT ON SATURATION PRESSURE MEASUREMENT AIMED AT
DETERMINING THE EFFECTS OF THERMAL LAG
ABSTRACT
This experiment aims at understanding of the principles behind saturation pressure
measurement with a view to determine the effects on accuracy of results as due to
unsteady conditions i.e. thermal lag effect. In a bid to determine this, using a pressured
vessel, the temperatures of water is measured against the corresponding pressure
values. The Platinum resistance thermometer used yields an electrical output
resistance for every pressure values measured by the semiconductor –type electronic
pressure sensor. These resistance values read from the Platinum Resistance
thermometer are thereafter converted, using suitable charts and relevant formulae
provided in the lab manual and formulae, to give the measured temperature readings.
The series of data obtained from the experimental results are used to plot various
graphs used to determine the thermal lag phenomenon.
INTRODUCTION
At a constant volume, the properties of water represented as a function of both volume
and pressure is as diagrammatically represented below.
The point at which a change in phase of water occurs i.e. from the vapour to the liquid
or from the liquid to the vapour is referred to as the saturation point. Saturation

temperature is the temperatures at which a liquid boils into water phase for a
corresponding saturation pressure. The liquid at this stage is saturated with thermal
energy thus phase transition occurs on any slight addition of thermal energy. For an
isobaric system (a system in which pressure is constant), as the thermal energy is
removed, condensation of vapour at saturation temperatures into liquid occurs.
Additional thermal energy would however boil a liquid at saturation temperature and
pressure into vapour phase.
The pressure at which the liquid boils into vapour phase at a saturation temperature is
referred to as saturation pressure. Saturation temperatures and saturation pressure are
directly proportional to each other i.e. an increase in saturation temperatures would
increase pressure. Vapour at saturation temperatures and pressure begins to condense
into liquid phase of an isothermal system. This happens when the pressure of the
system is increased.
METHODOLOGY
The apparatus used in conducting this experiment consists of semiconductor –type
electronic pressure sensor and platinum resistance thermometers. The latter apparatus
measures temperatures with an electrical output in Ohms. A measurable change in the
resistance of the semiconductor is produced due to the compression and tension in the
gauges.
To set up the experiment, the drain valve and the calorimeter valve were both confirmed
to be closed. A confirmation of the ‘óff’ state of the mains power to the console was
made before the boiler was filled. The filling point was opened and the equipment filled
with de-ionized water to halfway levels of the sight-glass. The console and its mains
power were then both switched off.
After the above described set up, heater was switched on and its control turned to
maximum levels. Intense movement of water at the surface and presence of steam
escaping from the filling point indicated that water had reached its boiling point. In a bid
to allow for the maintenance of non-excessive but steady stream of steam, the power of
the heater was reduced. When the steadiness of resistance Rm1 had been achieved,

the reading of sensor’s pressure (P1) was noted as the pressure inside the vessel. The
corresponding resistance value for this pressure was read from the Platinum Resistance
thermometer. The filler valve was then switched off and the heater returned to maximum
power. Thermometer output was recorded at two-minute intervals and the electronic
pressure sensor readings too. After the maximum working pressure was achieved, the
heater was turned off. The electronic pressure readings were recorded at every 5-
minute interval of the corresponding thermometer output readings. This step was
repeated till the readings stabilized and the results recorded in table 1.1.
To investigate the phenomenal effect of different heating rates, the heater was set at
lower power. Finally, the isolating valve of the calorimeter was opened.
RESULTS
Table 1.1: Experimental Results
Time
(min)
Measured
Resistance
(Ohm)
Absolute
Pressure
(kPa)
Pressure
in Bar
Absolut
e temp
Actual
Temp
Corrected
Resistance
0 146.5 164 1.6186 113.002 124 147.57
2 148.8 233 2.995 124.693 132 150.57
4 150.9 311 3.0693 134.852 140 153.58
6 158.9 398 3.928 142.922 170 164.78
8 154.6 492 4.8557 150.674 154 158.81
10 156.3 594 5.862 157.891 160 161.04
12 157.8 697 6.8789 164.249 166 163.274

Graphical Results
0 2 4 6 8 10 12 14
350
400
450
Absolute Temperature versus time
Absolute Temperatures
Linear (Absolute
Temperatures)
Fig 1.1 A graph of Absolute measured temperature against time
0 2 4 6 8 10 12 14
360
380
400
420
440
460
Actual Temperature versus time
Actual Temperature
Linear (Actual
Temperature)
Fig 1.2 A graph of Actual Absolute temperature against time
0 2 4 6 8 10 12 14
340
360
380
400
420
440
460
Absolute Temperatures
Linear (Absolute
Temperatures)
Actual Temperature
Linear (Actual Temperature)
Fig 1.3 A graph of Absolute and Actual temperature against time

390 400 410 420 430 440 450
360
380
400
420
440
460
Measured temperature against actual
temperature
Measured temp
Linear (Measured temp)
Fig 1.4 A graph of Absolute and Actual temperature against time
DISCUSSION
A graph of all the various dependent variables versus time plotted yielded linear graphs
showing that the dependent variables increase with time. The measured and actual
temperature graphs nearly coincided with each other when plotted against time. The
values of temperatures obtained from the actual measurements and from the platinum
resistance measurements were found to be approximately the same. The negligible
discrepancies between them could be attributed to several factors including the
presence of errors due to thermal lag. The thermal lag errors emanate from the possible
delays in the thermometer to respond to changes in temperatures.
Additionally, presence of lead resistance in the thermometer brought about these
differences. Being a two-wire device, an extension wire finally becomes part of the
thermometer. Therefore, then error in the lead resistance increases with the lengthening
lead. The errors of the two wire resistance thermometers could be overcome by using
four wire or three wire devices which cancels out the lead wire resistances.
Possible self-heating of the pressure resistance thermometer could have led to these
small deviations. Naturally, for resistance to be measured, current must be made to flow
through the sensing resistor. This flow of current produces losses inform of heat i.e I2R
losses which would finally produce less accurate results. Effects of thermal capacity due
to slight increment in loading may have interfered with, to an extent, the accuracy of the
obtained results. Moreover, thermoelectric effects due to accidental joining of metals

which are dissimilar during construction of the thermometer may have generated small
D.C voltages thus interfering with the accuracy of the obtained results.
As therein results, the nature of values of saturation temperatures indeed determined
the nature of the values of saturated pressure obtained. The values of pressure at which
the liquid boiled into vapour phase (saturation pressure) at various saturation
temperatures depicted a direct proportionality with these temperatures. An increase in
saturation temperature increased the saturation pressure while a decrease in saturation
temperature decreased the saturation pressure. This is a very instrumental relationship.
CONCLUSION
In a nutshell, connections were set up as therein procedure and various data obtained
recorded under results. A number of graphical plots were then made as presented.
These graphs include that of measured temperatures against time, actual temperatures
against time, measured temperatures against actual temperatures and that of measured
temperatures and actual temperatures both on the same axis against time. It was
evident that the plots conformed to their expected theoretical profile. Analysis of these
graphs verified the thermal lag effect on the accuracy of the data obtained in such
pressure measurements. In this respect, the experimental objectives were verified.
REFERENCES
Biko, F. B. (2015). Transient Pressure Measurement. Fundamentals of Temperature, Pressure,
and Flow Measurements, 375-391.
Calvert, J. G. (2016). Saturation Vapour Pressure. IUPAC Standards Online.
Peeps, E. O. (2017). Resistance Thermometers. Temperature Measurement, 2(1), 85-102.
Standard Test Method for Thermal Lag of Thermal Analysis Apparatus. (2015).
Tomczuk, K., & Werszko, R. (2013). Correction for thermal lag in dynamic temperature
measurements using resistance thermometers. Review of Scientific Instruments, 84(7),
074903.