Experiment 3. Temperature Measurement

Experiment 3.  Temperature Measurement



The objectives of the experiment are the following:

- to measure thermal time constants of different temperature sensors
- to determine minimum waiting time before reading measurements of temperature
- characterize response of different thermal sensors with temperature

Introduction

Thermal equilibrium is the state wherein no more changes in the warmness or coldness occur when two objects, one warm and one cold are placed in contact with each other. Temperature is the quantity which is the same for the two bodies that are in contact and in thermal equilibrium.

The time constant is the time it takes for a temperature sensor to have a reading equal to its initial reading plus 0.632 times the difference between its initial and final reading.

T(t=z) = Ti + 0.632 (Tf-Ti)  (1)

It can be used to specify how long one must wait to get a reliable temperature reading. It is common engineering practice to wait for three to five time constants before reading the output of a thermometer. A material with a linear thermometric property has identical responses to heating and cooling - its cooling and heating time constants are the same.

In measuring temperature, we measure the degree of change in the properties of a material to heat and cold. In this experiment, three temperature sensors were used and studied: alcohol thermometer, mercury thermometer and a thermocouple.


Methodology

The experiment is divided into two parts for each of the three temperature sensors: the heating procedure and the cooling procedure.

Heating Procedure

A pot was filled with water up to 3/4 of its size. The water was kept boiling throughout the activity. The thermometer was dipped into the hot water. When the temperature stopped increasing, the final temperature Tf was recorded. The thermometer was then dipped on ice water. When the temperature reading stopped decreasing, the temperature was recorded as Ti.
Value for T(z) was computed and tabulated.
With the initial temperature Ti, the thermometer was dipped in boiling water and the time it took for the reading to reach T(z) was recorded.

Cooling Procedure
A pot was filled with water up to 3/4 of its size. The water was kept boiling throughout the activity. The thermometer was dipped on ice water. The final temperature, Tf, was recorded. The thermometer was dipped on boiling water until its temperature stopped increasing. The initial temperature Ti was recorded on the worksheet. Values for T(z) was computed. Initially with temperature Ti, the thermometer was dipped on ice water and the time it took for the reading to reach each T(z) was calculated. These values were recorded.

Results and Discussion

The following data were gathered:

Table W1. Alcohol thermometer

(  a)    Heating

Trial	Tf (°C)	Ti (°C)	T(z) (°C)	z(s)
1	95	1.0	60.408	6.0
2	95	1.0	60.408	5.8
3	95	1.5	60.592	5.7
Average				5.83

(b)   Cooling

Trial	Tf (°C)	Ti (°C)	T(z) (°C)	z(s)
1	2.0	94	35.9	5.8
2	0	94	34.6	6.2
3	0	94	34.6	6.0
Average				6.0

Table W2. Mercury thermometer

(a)    Heating

Trial	Tf (°C)	Ti (°C)	T(z) (°C)	z(s)
1	100	3.0	64.3	2.3
2	100	4.0	64.7	1.9
3	100	4.0	64.672	2.0
Average				2.07

(a)    Cooling

Trial	Tf (°C)	Ti (°C)	T(z) (°C)	z(s)
1	1.0	100	37.432	1.8
2	2.0	100	38.064	2.12
3	3.0	100	38.696	2.0
Average				1.9733

Table W3. Thermocouple

(b)   Heating

Trial	Tf (°C)	Ti (°C)	T(z) (°C)	z(s)
1	96.5	1.70	61.6136	0.3
2	96.5	1.70	61.6136	0.3
3	96.5	1.70	61.6136	0.3
Average				0.3

(b)   Cooling

Trial	Tf (°C)	Ti (°C)	T(z) (°C)	z(s)
1	96	-2.0	34.1	0.2
2	96	-2.0	34.1	0.2
3	96	-2.0	34.1	0.2
Average				0.2

It was shown that the thermocouple had the lowest values of time constant among the three, with a time constant of 0.3 for heating and 0. 2 for cooling. Among the three, it responses the fastest for during changes in temperature. The thermocouple is more linear with close time constants of 0.3 and 0.2 for heating and cooling, respectively.

Summary

The thermometric sensor that registered the fastest response to changes in temperature is the thermocouple, followed by the mercury thermometer and the slowest was the alcohol thermometer.