Temperature Sensor

The most commonly used type of all sensors are the types of sensors that detect temperature or heat.

Such temperature sensors range from simple ON/OFF thermostatic devices that control hot water heating to highly sensitive semiconductor types that can control complex process-controlled furnace plants.

We remember from our school science lessons that the movement of molecules and atoms produces heat (kinetic energy) and the larger the movement, the more heat is generated. Temperature Sensors allow us to detect any physical changes in that temperature by measuring the amount of heat energy and even coldness produced by an object or system, producing an analog or digital output.


Hence You can reach our temperature controlled fan project with DHT11 temperature sensor and Arduino.

Hence With orange pi PC's built-in temperature sensor, you can reach the temperature controlled processor fan project.

Here you can find the Temperature Sensor application we use with mBlock


There are many different types of Temperature Sensors available, and all have different features depending on their actual applications.A temperature sensor consists of two basic physical types:

  • Contact Temperature Sensor Types – Such temperature sensors must be in physical contact with the detected object and use transmission to monitor changes in temperature.They can be used to detect solids, liquids or gases at a wide range of temperatures.
  • Non-contact Temperature Sensor Types – Such temperature sensors use convection and radiation to monitor temperature changes.They can be used to detect radiant energy emitting fluids and gases as heat rises in convection currents and the cold sinks to the bottom, or to detect radiant energy transmitted from an object in the form of infrared radiation (sun).

The two main types of contact and even non-contact temperature sensors can be divided into the following three groups of sensors: Electro-mechanical, Resistant and Electronic:

Thermostat

The thermostat is a contact type electro-mechanical temperature sensor or switch, consisting mainly of two different metals such as nickel, copper, tungsten or aluminum, connected together to form a two metallic strip. Different linear expansion rates of two different metals produce a mechanical bending motion when the strip is exposed to heat.

The bi-metallic strip can be used as a mechanical way to operate an electric switch or an electric switch in thermostatic controls and is widely used to control hot water heating elements in boilers, furnaces, hot water storage tanks and vehicle radiator cooling systems.

Bimetallic Thermostat

The thermostat consists of two thermally different metals glued together in a row.When it is cold, the contacts turn off and the current passes through the thermostat.When warmed up, one metal expands more than the other, and the connected bimetallic strip bends up (or down) and opens contacts that prevent the current from flowing.

When exposed to temperature changes, there are two main types of bimetallic strips that are mainly based on their movements.There are types of "snap-action" that instantly produce an action of type "ON/OFF" or "OFF/ON" on electrical contacts at a set temperature point, and slower types of "creep-action" that gradually change their position as the temperature changes.

Snap-action thermostats are widely used in our homes to control the temperature setting point of oven, iron, immersion hot water tanks, and can also be found on walls to control the domestic heating system.

Creeper species usually consist of a bi-metallic coil or spiral, which slowly relaxes or curls as the temperature changes.In general, creeper-type bimetallic strips are more susceptible to temperature changes than standard ON/OFF types, as the strip is longer and thinner, ideal for use in temperature gauges and dials, etc.

Although they are very inexpensive and can be found in a wide working range, one main drawback of standard pass type thermostats when used as temperature sensors is that they have a wide range of hysteresis from the moment the electrical contacts are turned on to the moment they close again.For example, 20 o can be set to C, but 22 may not turn on up to O C or turn off again up to 18 o C.

Therefore, the range of temperature oscillation can be quite high.Bimetallic thermostats, which are commercially available for home use, have a more precise desired temperature setting point and temperature adjustment screws that allow pre-adjustment of the level of hysteresis.

Thermistor

The theristor is another type of temperature sensor whose name is a combination of therm-ally sensitive res-ISTOR words. A thermist is a special type of resistance that changes its physical resistance when exposed to temperature changes.

Thermisors are usually made of glass-coated ceramic materials such as nickel, manganese or cobalt oxides, which makes them easily damaged.Their main advantage over sudden movement types is the speed at which they react to any changes in temperature, accuracy and repeatability.

Most thermist types have a negative resistance coefficient or (NTC), that is, resistance values decrease with an increase in temperature, and of course those with a positive temperature coefficient (PTC) also increase resistance values with an increase in temperature.

The thermisors are made of a ceramic semiconductor material using metal oxide technology such as manganese, cobalt and nickel etc. Semiconductor material is usually formed into small pressed discs or balls that are airtightly sealed to react relatively quickly to any change in temperature.

The thermisors are rated according to resistance values at room temperature (usually at 25 o C), time constants (time to react to temperature change) and power ratings according to the current passing through them.Like resistors, thermists have resistance values from 10 MΩ at room temperature to just a few Ohms, but types with weight-ohm values are often used for sensing purposes.

Thermists are passive resistant devices; this means that we have to pass a current through it to produce a measurable voltage output.Then, the thermists are usually serially connected with an appropriate pre-redevestion resistance to form a potential dividing network, and the choice of resistance gives a voltage output at or at a predetermined temperature point or value.

Temperature Sensor Question Sample 1

In the following circuit 12V DC welding is used, the therist has a resistance value of 10KΩ at 25 o C and a resistance value of 100Ω at 100 o C.Calculate the voltage drop on the theristor and therefore the output voltage (Vout) for both temperatures when connected serially with 1kΩ resistance.

 25 at O C

100 o at C

By changing the constant resistance value of R2 (1kΩ in our example) to a ponciometer or a preset value, a voltage output can be obtained at a predetermined temperature setting point, for example at 5v output at 60 o C and by changing the pontiometer at a certain output voltage.

However, it should be noted that the thermists are nonlinear devices, and the standard resistance values at room temperature differ between different thermists, mainly due to the semiconductor materials from which they are made. The thermistorhas a exponential change with temperature and therefore, it has a beta temperature constant, β can be used to calculate its resistance for any temperature point.

However, when used with a series of resistances, such as voltage dividing network or Wheatstone Bridge type arrangement, the current obtained in response to a voltage applied to the divider/bridge network is linear with temperature.Then, the output voltage on the resistance becomes linear with the temperature.

Resistant Temperature Detectors (RTD).

Another type of electrical resistance temperature sensor is the Resistance Temperature Detector or RTD.RTDs are precision temperature sensors made of high purity conductive metals such as platinum, copper or nickel wrapped in a coil, and whose electrical resistance varies as a function of temperature, similar to that of the theristor.Thin film RTDs are also available.These devices have a thin film of platinum paste precipitated on a white ceramic substrate.

Resistant temperature detectors have positive temperature coefficients (PTC), but unlike the thermistor, their output is extremely linear and produces very precise temperature measurements.

However, they have very weak thermal sensitivities, that is, a change in temperature produces only a very small output change, for example 1Ω/ o C.

The more common RTD types are made of platinum and are called Platinum Resistant Thermometers or PRTs, the most common of which are all Pt100 sensors with a standard resistance value of 100Ω per 0 o C. The downside is Platinum, it is expensive, and one of the main drawbacks of such a device is its cost.

Like the thermist, RTDs are passive resistant devices, and it is possible to achieve a linearly increased output voltage by passing a constant current through the temperature sensor.A typical RTD has a base resistance of about 100Ω at 0 o C and rises to about 140Ω at 100 o C in a working temperature range from -200 to +600 o C.

Since RTD is a resistant device, we need to pass a current through it and monitor the resulting voltage.However, any change in resistance due to the spontaneous warming of the resistant wires as the current passes through it causes an error in readings I 2 R ,(Ohm Law).To prevent this, RTD is usually connected to a Wheatstone Bridge network with additional connection cables for lead compensation and/or connection to a stationary current source.

Thermocouple

Thermocoupl is by far the most widely used type of all temperature sensor types.Thermocoupls are popular because of their simplicity, ease of use and their speed of reacting to changes in temperature, mainly due to their small size.Thermocouple also has the widest temperature range of all temperature sensors from -200 o C to well above 2000 o C.

Thermocouple are thermoelectric sensors consisting mainly of two combinations of different metals, such as copper and constantan, welded or curled together.One port is held at a constant temperature, called a reference (Cold) port, while the other is called the measurement (Hot) port.When the two connections are at different temperatures, a voltage is developed along the connection used to measure the temperature sensor, as shown below.

Thermocoupl Structure

The principle of operation of a thermocouplun is very simple.When fused together, the combination of two different metals, copper and constant constant, produces a "thermo-electric" effect that gives a fixed potential difference of only a few millivolts (mV) between them.The voltage difference between the two connections is called the "Seebeck effect" because a temperature gradient is produced along the conductive wires that produce an emk.Then the output voltage from a thermocoupl is a function of temperature changes.

If both junction points are at the same temperature,the potential difference at the junction point is zero, that is, there is no voltage output in V1 = V2. However, when the junction points are connected in a circuit and both are at different temperatures, two bAccording to the temperature difference between the point of examination, a voltage output will be detected according to V1 – V2. This voltage difference,bthe point of improvement will increase with temperature until the peak voltage level is reached, and this will be determined according to the characteristics of the two different metals used.

Thermocoupls can be made from a variety of different materials that allow the measurement of extreme temperatures between -200 o C and +2000 o C.With such a wide choice of materials and temperature ranges, internationally recognized standards have been developed along with thermocoupl color codes to allow the user to select the right thermocouple sensor for a particular application.The English color code for standard thermocoupls is given below.

Thermocoupl Color Codes

The three most common thermocoupl materials used for general temperature measurement above are Iron-Constanta (Type J), Copper-Constanta (Type T) and Nickel-Chromium (Type K).The output voltage from a thermocoupl is very small, the temperature difference varies only a few millivolts (mV) for a change of 10 o C, and due to this small voltage output, some kind of amplification is usually required.

Thermocoupl Amplification

The type of amplifier in the form of a discrete or a Transactional Amplifier needs to be carefully selected, since good slip stability is required to prevent the thermocouple from being recalibrated at frequent intervals.This makes the clipper and instrumentation type amplifier preferred for most temperature sensing applications.

Other Temperature Sensor Types not mentioned here include Semiconductor Connection Sensors, Infrared and Thermal Radiation Sensors, Medical Type Thermometers, Indicators and Color-Changing Inks or Paints.

In this tutorial on "Temperature Sensor Types", we looked at several examples of sensors that can be used to measure changes in temperature.In the next lesson, we will look at sensors used to measure the amount of light, such as Photodiode, Phototransistors, Photovoltaic Cells and Light-Dependent Resistance.