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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

Extracts from this document...

Introduction

Introduction

A Sauna is a small room designed to be heated to very high temperatures, with well-controlled humidity. Sauna’s 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).

...read more.

Middle

Measurements made

image10.png

My objective at this point was to collect as accurate readings as possible for the potential difference across the thermistor in circuit at specific temperatures between 30°C and 60°C.

  • Before starting I checked my voltmeter (multimeter) for zero error by measuring the potential difference across the no component and I checked my thermometer for zero error by putting it in melting ice.
  • I then started by setting up the circuit above and placed my thermistor in a beaker with a thermometer.
  • I boiled hot water and added it to the beaker

First problem I faced was collecting accurate temperature readings with the thermometer. I had to try to avoid parallax error and systematic error so I do not end up linking a voltage out (potential difference across thermistor) with a wrong temperature.

Improvement: I decided to use a data logger to replace the thermometer because it is more sensitive and also helps me avoid the problem of parallax error when taking readings.

  • Before using the data logger put it in melting ice to check for zero error
  •  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

  • 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

  • Problem: I decided to use two data loggers as an improvement in taking temperature readings, keeping them at opposite sides of the beaker. I noticed that there was a variation of 0.2°C to 0.3°C in their temperature readings

Improvement: I decided to stir the water in an attempt to keep the water body at a uniform temperature. I also decided to cello tape a data logger to the right and to the left of the thermistor only taking voltage readings when they are both on the specified temperature.

New readings taken with improvements

Temperature/°C

V1 /V

V2 /V

V3 /V

Mean/V

60

1.56

1.56

1.55

1.56

58

1.61

1.61

1.62

1.61

56

1.68

1.67

1.67

1.67

34

2.34

2.34

2.34

2.34

32

2.39

2.38

2.39

2.39

30

2.43

2.44

2.44

2.44

The new readings are more consistent than the previous ones

 I could now take all my readings of voltage out (potential difference across the thermistor) for every 2°C from 30°C to 60°C.

This I did and repeated 9 times and found the average

The readings were taken over three consecutive days using the same equipment throughout the experiment.

Safety Precautions in Carrying Out Experiment

  • Had to be careful with hot water, wore gloves to reduce chances of getting burnt
  •  Avoided spilling water with lots of wires and electrical appliances around. (If water  spilled switched of all appliances and cleaned it up with a cloth before proceeding on collecting readings)
  • Careful use of scissors (sharp object) to reduce chances of getting a cut and kept the scissors away from wires and other electrical appliances.
  • Avoided the use of any faulty or damaged instrument and use of wet hands on electrical appliances to reduce risk of electric shock.
  • Kept all glassware (beakers and thermometer) towards the centre of the table and handled them with caution.
...read more.

Conclusion

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

  • 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

  • Resolution =      0.01__

                                   Sensitivity

                     =  0.01

                        0.030  

                     = 0.333°C

Lowest Possible Resolution = 0.01   =0.385°C

                                                    0.026

  • Uncertainty in resolution = 0.385-0.333= 0.052°C

=0.06°C (1 d.p)

Resolution = 0.33 ± 0.06°C

Conclusion        

  • 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.
  • My sensor system has a short response time for its application purpose.
  • My sensor system has an appropriate output for a given input (0.030V/°C).
  • 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 they’re all present in my sensor system I’ll conclude that my sensor system in fit for purpose.

...read more.

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