Measurement in Engineering

Transcription

1 Measurement in Engineering Responsible person for the course: Ing. Martin Novak, Ph.D. Report on the laboratory experiment Measurement of temperature of the made by Sebastian Kößler

2 Report on measurement of temperature Before using temperature sensors, it is necessary to define some important terms concerning this matter. First of all we will need a definition of heat as it always is the input parameter when measuring temperatures. Heat is basically the total amount of kinetic energy of molecules and atoms in an object. When temperature of an object changes, the average velocity of the molecules in an object changes as well. There a different ways of processing the input such as measuring a change of length, of resistance or of color. Depending on the desired precision of the measurement and the willing amount of investment into the setup it is possible to find a suitable temperature-sensor out of the many various types of possible sensors. Most commonly used is a sensor-setup which bases on resistance respectively voltage due to the advantage of not obligatory having to transform the output signal of the sensor in order to process it in further electrical control units. Transformation should always be avoided, for they will always negatively influence the accuracy and the reaction time of the control system. Another aspect of choosing a suitable sensor is the reaction time. Many sensors are virtually useless if being used in a dynamic environment (=unsteady-state) they may only be used in steady-state environment. Sensors which react really fast and are capable of measuring dynamic changes in temperatures are more costly or have other severe disadvantages such as lower precision or more complex integration into the control systems. In the following case, we measure the temperature of heated water with in total four different kinds of temperature-sensors. The experimental setup was build like following sketch visualizes: We only measure in steady-state; therefore it is necessary to wait at least three minutes after reaching the desired temperature in the experimental basin.

3 The sensors are immersed into the water, and then the temperature is set. After waiting for the steady-state, it is possible to read the output values of the sensors with a measurement device such as a multimeter or any other capable device. The water is being heated in steps of ten degree Celsius. The measuring starts at 30 C and ends at 80 C, which means each sensor will give six measuring points. In order to get more precise results, there are two experimental setups. Except for the processing of the output parameter with different multimeters, they are set up equally. In the following, I will present the results of the measuring. In this experiment, following sensors were used: 1. Thermocouple 2. Resistive sensor KTY10 3. Thermistor sensor 4. Pt 100 For protection of the fragile sensors, a thermowell is being used on two of the sensors (the thermocouple and the Pt 100 ) during the experiment. Picture: thermowell for protecting purposes. (source: colonial instruments)

4 1. Thermocouple type J The Thermocouple type J consists basically of two metals of different alloys. On one side they are welded together, on the other side it is possible to measure a voltage once the temperature changes. This is possible because every junction of dissimilar metals will produce an electrical potential while being exposed to a temperature-delta. The thermocouple works with three principles: - The Thomson voltage (When heating a e.g. bar locally, the warmer part becomes positively charged which leads to measurable voltages) - The Peltier effect (Diffusion of free electrons trough the joint of two different metals with different energy levels will cause a potential difference which causes voltage) - The Seebeck effect (basically, the Seebeck effect is the combination of both, the Thomson voltage and the Peltier effect) Just like every sensor, the Thermocouple has some advantages and some disadvantages: + very easy to build - it measures temperature difference not temperature + wide range - needs calibration (due to measuring only ΔT) In this experiment, the thermocouple sensor is protected by a thermowell. This means a safer environment for the sensor but also a longer time for the steady-state to generate.

5 voltage in [µv] Following values were measured in the experiment: Thermocouple type J t ( C) experiment a U (µv) experiment b U (µv) Thermocouple type J experiment a experiment b Linear (experiment a) S = 508,97 µv/ C temperature in [ C] Conclusion: The measurement data results in a linear dependence of the measured voltage to the temperature. The sensitivity evolves to: 508,97 µv/ C

6 resistance in [kω] 2. Resistive sensor KTY 10 The features of the KTY10 are: - Fast response - Independent polarity (due to symmetrical construction) - Partially linear gradient of resistance The output parameter of the resistive sensor KTY 10 is resistance. Resistive sensor KTY 10 t ( C) experiment a R (kω) 2,09 2,26 2,44 2,62 2,82 3,02 experiment b R (kω) 2,08 2,24 2,41 2,59 2,78 2,99 3,30 Resistive sensor KTY 10 3,10 2,90 2,70 2,50 2,30 experiment a experiment b Linear (experiment a) 2,10 S = 0,186 kω/ C 1, temperature in [ C] Conclusion: The measurement data results in a linear dependence of the measured resistance to the temperature in the observed range. The sensitivity evolves to: S = 0,186 kω/ C

7 resistance in [kω] 3. Thermistor sensor The thermistor sensor is based on a semiconductor. It measures resistance and doesn t have a linear, but an exponential gradient. However, it is possible to linearize the graph near the operation point. Therefore it has a rather small range. Some advantages and disadvantages are: + price (very cheap) + very high sensitivity (up to 10x higher than metal based sensors + fast response - not exchangeable: manufacturing impurities make it rather impossible to build two identical sensors - due to the not given interchangeability, it needs calibration - not very precise (exponential gradient) Thermistor sensor t ( C) experiment a R (kω) 5,09 3,63 2,64 1,96 1,47 1,13 experiment b R (kω) 2,48 1,80 1,34 1,01 0,77 0,60 5,50 Thermistor sensor 4,50 3,50 experiment a 2,50 1,50 experiment b 0,50-0, temperature in [ C] Conclusion: The measurement data results in a negative, exponential gradient of the resistance over the measured temperature. For predicting data near the operation point, one may linearize locally.

8 resistance in [Ω] 4. With a Pt 100 Just like the thermocouple, the Pt 100 was also placed in thermowell for better protection of the sensor. It belongs to the type of resistive temperature detectors (RTD) and its precision is divided into two classes Class A and Class B. The designation Pt 100 implies the information that the sensor output will be 100 Ω at 0 C. There are different types of this sensor-principle one may also use nickel, copper or constantan for this kind of sensor. Near the operation point, it has a highly linear (positive) gradient, which makes it easy to implement. For it basically works with the strain gauge effect and it is very fragile due to glass housing, it will need a thermowell. On the other hand it is very simple to produce a sensor of this kind. Pt 100 t ( C) experiment a R1 (Ω) 111,00 115,40 119,20 123,20 127,08 130,97 experiment b R2 (Ω) 111,62 115,42 119,28 123,10 126,90 130,70 135,00 Pt ,00 125,00 experiment a 120,00 experiment b Linear (experiment a) 115,00 S = 3,9683 Ω/ C 110, temperature in [ C] Conclusion: The measurement data results in a highly linear dependence of the measured resistance to the temperature in the observed range. The sensitivity evolves to: S = 3,9683 Ω/ C