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Applications of Thermocouple 3.

Advantages of Thermocouple over other Methods 4. Methodology 6.

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Introduction A Thermocouple is a device used for measuring temperature in which two wires of different metals are joined. The basis of thermocouples was established by Thomas Johann Seebeck in when he discovered that a conductor generates a voltage when subjected to a temperature gradient[1]. The potential difference between the wires is a measure of the temperature of the temperature difference between the two junctions. The different wires of different metals are joined at each end. When these two junctions are kept at different temperatures, a small electric current is induced.

Due to the flow of current, a voltage drop occurs.


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The voltage drop depends on the temperature difference between the two junctions. The measurement of the voltage drop can then be correlated to this temperature difference. It is extremely important to note that a thermocouple does not measure the temperature, but rather the temperature difference between the two junctions[2]. In order to use thermocouple to measure the temperature directly, one junction must be maintained at a known temperature.

This junction is commonly used and called the Reference Junction, and its temperature is called the Reference Temperature. The other junction which is normally placed in contact with the body of unknown temperature is called the measurement junction. Also, the joined end is referred to as the Hot junction, the other end of these dissimilar metals is referred to as the End or Cold Junction. The cold junction is usually formed at the last point of thermocouple material.

The voltage created by a thermocouple is extremely small and is measured in millivolt. Thermocouple can be used in measurement of Temperature very accurately where small voltage of different or dissimilar metals are joined to form a loop and the two wires are of different temperatures. They are less suitable for applications where smaller temperature differences need to be measured with high accuracy. Thermocouple is practically used for different measurement such as Voltage-Temperature relationship, cold junction compensation etc[3].

This research work investigated the various factors that could affect the output voltage and efficiency of a thermocouple. These factors included the volume of the containers and the temperature difference between the hot and cold junctions. For such applications, thermistors and resistance temperature detectors are more suitable.

Analog Dialogue, Volume 45, Number 3

Applications include temperature for kilns, gas turbine exhaust, diesel engines, and other industrial processes[4]. Thermocouple are the best options when the need is to measure range of temperature but the efficiency of the device decreases as and how it is used to measure smaller differences in temperature. They are generally used in the steel industry where it is used to measure temperature throughout the entire process of making steel.

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It is also used in Radioisotope thermoelectric generators to generate electricity, through in a series which is commonly termed as Thermopile[5] Thermocouples are largely used in the fields of science and electronics as a temperature sensor, which are quite easy to handle and use. The functioning of a thermocouple is based on the seeback effect that is common in case of electrical conductors that experience a temperature gradient along their length.

Thermocouples consist of two different metals, joined together at one end. Whenever the junction of the two metals is heated or cooled a voltage can be obtained that is proportional and can be correlated to the temperature that was given when the sensor was heated. Thermocouple alloys are usually available as wire. They are quite cheap but their main drawback is their accuracy[6]. They are widely available, standardized, reasonably cheap and able to measure Extreme Temperatures.

They are also capable of producing an electrical signal that has a fairly direct relationship with temperature, a requirement that was particularly critical in early temperature measurement instrumentation. Perhaps this is one reason the use of thermocouples has been so ingrained in the process and manufacturing Industries. Thermocouples are rugged devices that are immune to shock and vibration and are suitable for use in hazardous environments.

Because they are small and have low thermal capacity, thermocouples respond rapidly to temperature changes, especially if the sensing junction is exposed. Higher-order modulators greater than third order are difficult to stabilize and present significant design challenges. While integrating architectures dual - slope, triple - slope, etc. These converters offer excellent power-line common-mode rejection and resolutions up to 24 bits as well as digital conveniences such as on- chip calibration.

Many have programmable-gain amplifiers PGAs , which allow small signals from bridge - and thermocouple transducers to be directly digitized without the need for additional external signal conditioning circuits and in-amps. Figure 9 shows a simplified diagram of a precision load cell. This particular load cell produces mV full-scale output voltage for a load of 2 kg with 5-V excitation.

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The diagram shows the bridge resistance values for a 2-kg load. The output voltage for any given load is directly proportional to the excitation voltage, i. Evaluation boards and software can greatly assist in this process. Nevertheless, there are still many instrumentation and sensor signal- conditioning applications that can be efficiently solved with a traditional in-amp for signal amplification and common-mode rejection followed by a multiplexer and a SAR ADC.

In addition, the ease of adding digital functions to a CMOS-based converter makes features such as digital-filter programmability practical with only small increases in overall die area, power, and cost. Digital techniques for voiceband audio began in the early days of PCM telecommunications applications in the s. For more discussion on input-referred noise and noise-free code resolution see Further Reading 1.

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Ratiometric operation eliminates the need for a precision voltage reference. A traditional approach to digitizing this low-level output would be to use an instrumentation amplifier to provide the necessary gain to drive a conventional SAR ADC of bit to bit resolution. Appropriate filtering circuitry is needed due to the noise of the auto-zero in-amp. Except for this small region, the applications considered high speed are most often served by a pipelined ADC.

Although low-resolution flash converters remain an important building block for the pipelined ADC, they are rarely used by themselves, except at extremely high sampling rates—generally greater than 1 GHz or 2 GHz—requiring resolutions no greater than 6 bits to 8 bits. Also requiring highspeed converters are video, radar, communications IF sampling, software radio, base stations, set-top boxes, etc. The pipelined ADC has its origins in the subranging architecture, first used in the s. This waveform is typical for a low-frequency ramp signal applied to the analog input of the ADC. In order for there to be no missing codes, the residue waveform must not exceed the input range of the second-stage ADC, as shown in the ideal case of Figure 12A.

The ADC output under such conditions might appear as in Figure At this point it is worth noting that there is no particular requirement—other than certain design issues beyond the scope of this discussion—for an equal number of bits per stage in the subranging architecture. In addition, there can be more than two stages.

Nevertheless, the architecture as shown in Figure 11 is limited to approximately 8-bit resolution unless some form of error correction is added. The error-corrected subranging ADC architecture appeared in the mids as an efficient means to achieve higher resolutions, while still utilizing the basic subranging architecture.

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The extra range in the second-stage ADC allows the residue waveform to deviate from its ideal value—provided it does not exceed the range of the second-stage ADC. A basic 6-bit subranging ADC with error correction is shown in Figure 14, with the second-stage resolution increased to 4 bits, rather than the original 3 bits. The input SHA remains in the hold mode during the time required for the following events to occur: After the digital data passes through the error correction logic and output registers, it is ready for use; and the converter is ready for another sampling-clock input.

Are you an author? Help us improve our Author Pages by updating your bibliography and submitting a new or current image and biography. Learn more at Author Central. Please Select a Language. Welcome to the September issue of Analog Dialogue. When to Use a Double Transformer Configuration. Is No Noise Good Noise? Measuring small differential voltages in the presence of large common-mode voltages.

Please turn JavaScript on and reload the page. Analog Dialogue, Volume 45, Number 3 The output voltage for any given load is directly proportional to the excitation voltage, i. Analog Dialogue - Wikipedia Ratiometric operation eliminates the need for a precision voltage reference. This latency may or may not be a problem, depending upon the application. What's it all about? The chart shown in Figure 1, taken from the middle of the data stream, shows the behavior:.

The transient behavior of the screen is shown in the second chart, Figure 2. This shows how the resistive touch panel takes some time to come to equilibrium when pressure is applied. With the parameters used for this test, the settling time was about reporting intervals. This graph was generated with the driver reporting the full bit position resolution. The effect of pressure changes when the stylus is first applied can be clearly seen at the leading edge; a similar effect happens at the trailing edge although it is less pronounced in this sample of data.

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