Temperature Measurement and First-order Dynamic Response

Purpose and Objectives

The main purpose of the lab experiment will be to gauge the response of different temperature measurement instruments to different inputs. The process will focus on meeting the following five specific objectives;

1. To understand concepts related to temperatures and measurement of temperatures
2. To understand how to carry out instrument calibration
3. To determine and understand time constants of a thermocouple and a thermometer and make comparisons between their dynamic responses
4. To understand the errors or problems that arise during temperature measurements

Procedure

The first step will involve opening the VI developed during the introductory step and then configuring it within the LabVIEW for the purpose of temperature measurement. The second step will involve creation of a calibration curve. To achieve this, thermometer temperature readings will be taken in both boiling water and ice water. After recording the temperatures, the same process will be repeated using a thermocouple. Then a calibration curve will be created to correlate the measurements derived using a thermocouple with the measurements that will be derived using a thermometer. The next step will involve modification of temperature sensors (VI) so that they can measure temperatures in dynamic conditions. The temperatures sensors will then be calibrated when connected to first-order systems. The next step will involve determining the time constant of the sensors in dynamic conditions using the following five methods:

1. Mathematic definition of tau,
2. Determining time elapsed at 63.2% sensor accuracy
3. Analyzing acquired data
4. Determining error fraction
5. Calculating least squared error

After conducting the tests stated above, the next step will involve removing the thermocouple from ice water and carrying out additional tests using it in room temperature air. The next step will involve calculating the time constant before and after transition from ice to water in order to compare them.

The experiment will require the use of the following instruments:

1. A thermometer
2. J-type or K-type thermocouple
3. A PC-based data acquisition system

The key purpose of using a thermometer in the experiment is to calibrate the thermocouple. The measurements that will be gathered using the thermometer will thus be used as standard for the thermocouple. In order to achieve effectiveness in calibration process, the thermometer should have a good resolution and should be accurately read. After achieving an accurate calibration of the thermocouple, it will be used for measuring temperatures during the next steps. The thermocouple is made up of two different metals that are connected at one end. The thermocouple reports temperature differences when voltage is applied at the free ends of the wires. The wires have differing properties and hence, they create a voltage at the junction that is proportional to temperature change. The sensitivity of the thermocouple is dependent on the placement of temperature sensors and accuracy of calibration. LabVIEW is a graphical programming language that is used for generating Simple Virtual Instrument meant for acquiring, charting and writing data to data files. The time constant for a thermocouple will be faster than the time constant for a thermometer. This is due to the fact that a thermometer senses change in temperature when there is a change in density of the fluid contained in it, whereas a thermocouple senses change when a voltage is applied on its two connected wires. The wires are quick to transfer the induced voltage to the junction.

The results of the experiment will be useful in various ways. Firstly, the knowledge gained through the experiment can be applied in attaining speed, efficiency and accuracy of temperature measurements. Secondly, thermocouples are capable of measuring extreme temperatures and thus, they are useful for measuring temperatures during steel making. Thermocouples can also be applied in the processes of detecting and adjusting temperatures during the prototype phase of manufacturing process.