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The Purpose of my sensing circuit is to regulate the temperature in a Steam Sauna which operates between the temperatures 30C - 60C

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Introduction A Sauna is a small room designed to be heated to very high temperatures, with well-controlled humidity. Saunaâs are used for both recreational and therapeutic purposes. There are different types of Sauna, all with different heat sources and temperature ranges. Types Of Sauna Finnish Sauna Steam Sauna Infrared Sauna Temperature range (°C) 70-90 30-60 26-52 The Purpose of my sensing circuit is to regulate the temperature in a Steam Sauna which operates between the temperatures 30°C - 60°C Sensor My sensor is a thermistor (a temperature sensitive resistor). There are two types of thermistors. 1. PTC (Positive Temperature Co-efficient) thermistors for which an increase in temperature increases its resistance 2. NTC (Negative Temperature Co-efficient) thermistors for which an increase in temperature decreases its resistance Through my coursework I will be making use of a Negative temperature coefficient thermistor because I will be attempting to put a limit on the maximum temperature in the Sauna at which the steam generator can operate (also limiting the maximum temperature of the Sauna) by varying the potential difference across the steam generator as temperature changes. Principle of operation of a Negative Coefficient Thermistor Negative Temperature Coefficient thermistors are usually made from semiconductors (beads of metal oxides). They decrease in resistance as temperature rises because, a temperature rise ionises atoms in their structure, increasing the number of mobile charge carriers (electrons) and this outweighs the effect of the increased vibration of atoms. A rise in temperature thus increases the overall conductance of a Negative Temperature Coefficient thermistor.


1. Before using the data logger put it in melting ice to check for zero error 2. I then took my readings for voltage out at 60°C, 58°C, 56°C, 34°C, 32°C and 30°C (varying the water temperature by adding hot water to increase it and adding cold water to or letting it loose heat to the environment to decrease it). The readings are shown in the table below. Temperature/°C V1 /V V2 /V V3 /V Mean/V 60 1.54 1.55 1.57 1.55 58 1.63 1.60 1.61 1.61 56 1.68 1.67 1.63 1.66 34 2.33 2.31 2.35 2.33 32 2.39 2.34 2.38 2.37 30 2.42 2.45 2.46 2.44 More Problems and improvements 1. Problem: Taking voltmeter readings was very hard as the reading on the voltmeter was hardly stable at high temperatures. I assumed this was due to the high rate of heat loss from the water causing the temperature of the water to drop quickly, thereby changing the resistance of the thermistor and thus affecting output voltage. Improvement : I put an insulator round the beaker, and increased the volume of water I was using for each temperature reading as an attempt to reduce the rate of heat loss, thereby reducing the rate at which temperature drops and enable me take more accurate readings of the voltage out at high and low temperatures 1. Problem: I decided to use two data loggers as an improvement in taking temperature readings, keeping them at opposite sides of the beaker.


Response time : I used a sensitive thermistor in order to increase the response time of my sensing circuit as a short response time is required in Saunaâs to prevent temperatures from getting too high causing scalds and burns for its users. Sensitivity : This is the ratio of change in output to change in input from graph Sensitivity (S) = change in output change in input -S = 2.44-1.85 = -0.0295V/°C 50-30 (Max Sensitivity=0.0330V/°C and Minimum sensitivity=0.0260 V/°C) Uncertainty in sensitivity = 0.030 - 0.026 = 0.004 V/°C 1. S = 0.030 ± 0.004V/°C Resolution : This is the smallest change a sensor can detect in the quantity it is measuring. The resolution is limited by the number of decimal places the voltmeter can display/ smallest reading the voltmeter can make (which for me was 0.01V) 0.03V= 1°C 0.01V= resolution 1. Resolution = 0.01__ Sensitivity = 0.01 0.030 = 0.333°C Lowest Possible Resolution = 0.01 =0.385°C 0.026 1. Uncertainty in resolution = 0.385-0.333= 0.052°C =0.06°C (1 d.p) Resolution = 0.33 ± 0.06°C Conclusion 1. My sensor system has small variations between repeated results with a largest voltage out variation of 0.4V and a smallest variation of 0.1V. 2. My sensor system has a short response time for its application purpose. 3. My sensor system has an appropriate output for a given input (0.030V/°C). 4. My sensor system has a reasonably high resolution of 0.33°C based on its application purpose (temperature regulation in a Sauna) All these are qualities of a good sensor system and since theyâre all present in my sensor system Iâll conclude that my sensor system in fit for purpose.

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