Measurement of various quantities: Temperature Measurement notes For Instrumentation Engineering

In this article, you will find the study notes on Measurement of Temperature which will cover the topics such as  Thermocouple, Bolometer, RTD (3/4 wire), Thermistor, Pyrometer and Semiconductor.

TEMPERATURE MEASUREMENT

• Temperature Sensors measure the amount of heat energy or even coldness that is generated by an object or system, allowing us to “sense” or detect any physical change to that temperature producing either an analogue or digital output.
• These types of temperature sensor vary from simple ON/OFF thermostatic devices which control a domestic hot water heating system to highly sensitive semiconductor types that can control complex process control furnace plants.

1. Thermocouple

• Thermocouples are thermoelectric sensors that basically consists of two junctions of dissimilar metals, such as copper and constantan that are welded or crimped together. .
• One junction is kept at a constant temperature called the reference (Cold) junction, while the other the measuring (Hot) junction.
• When the two junctions are at different temperatures, a voltage is developed across the junction which is used to measure the temperature sensor as shown below.

• The voltage difference between the two junctions is called the “Seebeck effect” as a temperature gradient is generated along the conducting wires producing an emf. Then the output voltage from a thermocouple is a function of the temperature changes.
• Thermocouples can be made from a variety of different materials enabling extreme temperatures of between -200^oC to over +2000^oC to be measured.

Thermocouple Amplification

The amplifier, discrete or in the form of an Operational Amplifieris to be carefully selected, because good drift stability is required to prevent recalibration of the thermocouple at frequent intervals. This makes the chopper and instrumentation type of amplifier preferable for most temperature sensing applications.

2. Bolometer Temperature Sensor

• A bolometer is a device for measuring the energy of incident electromagnetic radiation.
• Bolometer consists of an "absorber" connected to a heat sink through an insulating link. The result is that any radiation absorbed by the absorber raises its temperature above that of the heat sink; then the higher the energy absorbed, the higher the temperature will be.
• Bolometer is a very sensitive thermometer whose electrical resistance varies with temperature which is used in the detection and measurement of feeble thermal radiation and is especially adapted to the study of infrared spectra.

 The study of bolometers can be studied in the following mode of operation

• Including investigations of properties of high temperature superconducting bolometers.
• Operating uncooled resistive bolometers in a closed-loop mode.
• Thin-film micro-bolometers with Si-Ge thermo-sensing films deposited from plasma discharge.

Power measurement using self balancing bolometer

• The measurement of unknown RF. power is done by using bolometer bridge in which a known A.F. power is superimposed on unknown RF. power.
• Using the variable resistance R and the d.c. bias voltage Yss, the current is adjusted till bolometer element is heated and its resistance equals R1. With this value, bridge achieves balance condition.
• The test RF. input is switched off which again unbalances bridge. To achieve the balance condition again, the A.F. voltage is increased till RF. power equals.

3. Resistance Temperature Detector (RTD-3/4 wire)

• RTD's  able to sense temperature with high accuracy, they have consistent and repeatable performance and low drift error (-200°C to +850°C).
• For precision measurement they also require a linearization look-up table in the microcontroller due to sensor non-linearities.
• The  RTD's technically includes thermistor devices, however the term ‘RTD’ has come to stand for the specialized pure metal detector rather than the more generic semiconductor resistance element.
• These pure metal devices are highly accurate and stable over long periods of time. Unlike the thermistor, the Platinum RTD is a linear device.
• The resistance of RTD resistance changes linearly proportionally to temperature.
• The temperature range of a Platinum RTD typically runs from -270°C to +850°C.This is a much wider range than that of the thermistor.

     R_t = R_o(1+aT+bT²)

where R_t is the the resistance at temperature t.

R_o  Temperature at 0⁰K.

a & b are calibration coefficient.

• If accuracy is a high priority, the thermistor should be the temperature sensor of choice. Thermistors are available in two varieties, NTC (negative temperature coefficient) and PTC(Positive temperature coefficient).
• The NTC thermistor is constructed of ceramics composed of oxides of transition metals (manganese, cobalt, copper, and nickel).
• Circuits can be configured to effectively use the 3-wire and 4-wire configuration to remove the error contribution of the lead wires completely.
• Two-wire RTDs are the least accurate because the contribution of the wire resistance and wire resistance drift to the measurement. With four-wire RTDs, this error can be eliminated by using force and sense techniques in the circuit design.

 4. Thermistor Sensor

• The Thermisrtor is a different type of temperature sensor, the words THERM-ally sensitive res-ISTOR. A thermistor is a special type of resistor which changes its physical resistance when exposed to changes in temperature.

• They are made from ceramic materials such as oxides of "nickel/manganese/cobalt" coated in glass which makes them easily damaged.

• Their main advantage over snap-action types is their speed of response to any changes in temperature, accuracy and repeatability.
• Most types of thermistor’s have a Negative Temperature Coefficient of resistance, and some are  Positive Temperature Coefficient. 

Advantage: Advantages of thermistors include a very high sensitivity to changes in temperature (having a thermal response of up to -100Ω/°C at 25°C), fast response time and low cost.

Drawback: drawback of thermistors is that the change in resistance with temperature is highly non-linear at temperatures below 0°C and
greater than 70°C.

Application of Thermistors

• Battery Chargers
• Power Supplies
• Cold Junction Compensation

5. Pyrometer Sensor

• Simplest & oldest non-contact way of estimating the temperature of a radiating body by observing its color .The radiation pyrometer is a non contact type of temperature measurement.
• Temperature measurement is based on the measurement of radiation either directly by a sensor or by comparing with the radiation of a body of known temperature. 
• The optical pyrometer is designed to respond narrow band of wavelengths that fall within the visible range of the electro‐magnetic spectrum.
• Thermal detectors are used as sensors. Their hot junction is the radiation sensing surface. Thermopiles can detect radiation of all wavelengths.

Radiation from real surfaces: Black body is an idealized concept in radiation. A black body absorbs all incoming radiation and transmits none. Thus, all real surfaces emit thermal radiations lower than black surface at any temperature such that  

  e = ε e_b

In which ε is emissivity and e is total radiation from a real surface. Obviously ε = 1 for a black body and ε<1 for all a real surfaces.

Limitations of Radiation Pyrometer

• Availability of optical materials limit on the wavelengths that can be measured.
• The surface of the hot object should be clean. It should not be oxidized. Scale formation allow to measure radiation accurately.
• Emissivity correction is required. Change in emissivity with temperature need to be considered.

6. Semiconductor Sensor

Basically the semiconductor sensor have been classified as the Silicon Output Temperature Sensors, they are categorized as follow:

Logic Output Temperature Sensors:

• Logic output temperature sensor families offer excellent temperature accuracy (±1°C, typical), with a very low operating current of less than 600 μA.
• These devices can replace mechanical switches in a variety of sensing and control applications.

Voltage Output Temperature Sensors:

• Voltage output temperature sensors develop an output voltage proportional to temperature, with a typical temperature coefficient of 6.25 mV/°C, 10 mV/°C and 19.5 mV/°C respectively.
• These temperature-to-voltage converters can sense a -40°C to +125°C temperature range and feature an offset voltage that allows reading negative temperatures without requiring a negative supply voltage.
• The extremely low operating current minimizes self-heating and maximizes battery life.

Serial Output Temperature Sensors:

• These type of sensor offer excellent temperature accuracy (±0.5°C, typical) with a very low operating current of 250 μA (typical).
• Communication with these devices is accomplished via an industry standard SMBus, I2C™ or SPI compatible interface protocol.
• They have fast temperature conversion rate, with temperature resolution for the entire family ranging from 0.0625°C to 0.5°C.

All the Best

Team Gradeup!