Temperature Measurement
Temperature measurement is tricky at best, lease of all is the reading of the device. The location of the sensor in relation to what is being measured as well as the heat flow in the measured substance and the heat flow through the sensor encapsulating materials. The accuracy of the sensor is fairly academic. An interesting article can be found at the following address:
https://blog.beamex.com/calibration-uncertainty-for-dummies-part-1
Traditional Glass Thermometers
A traditional glass thermometer is versatile and reasonably accurate, if you have good eyes and a few seconds to prevaricate over the reading. They don't need batteries but are brittle and break rather easily.
Digital Thermometers
These are usual quite expensive but can be put together for a few rand.
Thermistor Thermometer
The most basic digital thermometer can be made by connecting a thermistor to a multi meter reading the resistance and using the manufactures data sheet to convert the resistance to a temperature. Accuracy is quoted at between 0.01° and 0.5° Celsius
The photograph below shows a resistance of 9810 ohms (9.81K), and by looking this value up on the table on the right we can see the temperature is between 25,56 and 25,97 degrees Celsius, say 26 degrees. This example is based on an NTC thermistor (Resistance decreases as temperature increases), but PTC thermistors are also available. The relationship between the resistance and the temperature is non linear and is given by the Stein Hart equation.
Temperature Sensor IC - LM335
The LM335 IC is a temperature sensor with a linear voltage response to a change in temperature from 0 to 100 degrees C. i.e. for each change in one degree Celsius the voltage will change by 0,01 volts. 25'C = 0,25V.
The thermometer can be made by connecting the LM335 to a 9V battery with a 200K resistor between the GND and signal wires and then to the multi meter with a chocolat block.
LM335 measuring ice water showing the temperature as 0'C.
By soldering the components, using a micro battery and encapsulating the resistor and battery in shrink wrap a convenient thermometer is created.
Digital Thermometer Kits
The kits consist of a printed circuit board and all the components needed to make the thermometer. the sensor is usually mounted on the circuit board but can be soldered to a length of wire to make it more versatile. However the accuracy will be affected by the length of the wires.
Micro Controller Thermometers
Once a temperature sensor has been connected to a micro-controller the signal can be processed recorded and utilized in various ways.
The temperature can be displayed in Celsius or Fahrenheit and on a LCD, 7-Segment display or a location remote from the sensor.
The temperature can be recorded and reviewed later.
The display could also be graphical to monitor the temperature profile of a thermal process.
Thermistor Temperature Sensor
The conversion table or formula to convert the resistance into temperature can be done by a micro controller and the output can be displayedon an LCD display or a 7 segment display.
ANALOG TEMPERATURE SENSORS
Analog Temperature Sensors
LM35 Analog Sensor
The LM35 series are precision integrated-circuit temperature devices with an output voltage linearly proportional to the Centigrade temperature. The LM35 device has an advantage over linear temperature sensors calibrated in Kelvin, as the user is not required to subtract a large constant voltage from the output to obtain convenient Centigrade scaling. The LM35 device does not require any external calibration or trimming to provide typical accuracies of ±¼°C at room temperature and ±¾°C over a full −55°C to 150°C temperature range. Lower
cost is assured by trimming and calibration at the wafer level. The low-output impedance, linear output, and precise inherent calibration of the LM35 device makes interfacing to readout or control circuitry especially easy. The device is used with single power supplies, or with plus and minus supplies. As the LM35 device draws only 60 μA from the supply, it has very low self-heating of less than 0.1°C in still air. The LM35 device is rated to operate over a −55°C to 150°C temperature range,
MPC9700 Analog Sensors
The MCP9700/9700A and MCP9701/9701A family of Linear Active Thermistor™ Intergrated Circuit (IC) is an analog temperature sensor that converts temperature to analog voltage. It’s a low-cost, low-power sensor
with an accuracy of ±2°C from 0°C to +70°C (MCP9700A/9701A) ±4°C from 0°C to +70°C (MCP9700/9701) while consuming 6 μA (typ.) of operating current.
Digital Temperature Sensors
DS18B20 Sensor
This thermometer provides 9 to 12–bit centigrade temperature measurements and has
an alarm function with nonvolatile user-programmable upper and lower trigger points. The DS18B20
communicates over a 1-Wire bus that by definition requires only one data line (and ground) for
communication with a central microprocessor. It has an operating temperature range of –55°C to +125°C
and is accurate to 0.5C over the range of –10°C to +85°C. In addition, the DS18B20 can derive power
directly from the data line (“parasite power”), eliminating the need for an external power supply.
Each DS18B20 has a unique 64-bit serial code, which allows multiple DS18B20s to function on the same
1–wire bus; thus, it is simple to use one microprocessor to control many DS18B20s distributed over a
large area. Applications that can benefit from this feature include HVAC environmental controls,
temperature monitoring systems inside buildings, equipment or machinery, and process monitoring and
control systems.
BLUE TOOTH DISPLAY
Once the signal from the sensor has been processed in the micro processor it can be out put to a LCD display, seven segment LED display or sent to a mobile via bluetooth.
