Video: Comparison of Thermocouples and RTDs

The video below describes the basic differences between industrial thermocouples and RTDs.

Duro-Sense Corporation provides the thermocouples, RTDs, thermowells, and accessories to the aerospace, aviation, process control, medical, R&D, power generation, alternative energy, plastics, primary metals, high-tech and OEM industries.

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Reliable, Robust, and Affordable Process Heating Sensors and Controls

The ability to effectively measure, monitor, and control process heating operations is essential to minimize product variability and maintain product quality. This level of control requires reliable and affordable sensors and control systems that can withstand harsh environments and not require recalibration for at least one year. Process heating could become far more effective with access to more reliable, robust, and affordable sensors and process controls. There is a need for reliable, cost effective sensors for harsh environments and for the real-time measurement of the chemical composition of the fuel, oxidant, and flue gas in combustion processes. Real-time combustion controls for multiple fuel applications could help maximize fuel flexibility, while improved sensors as part of smart control systems could increase efficiency, safety, and reliability. In electromagnetic processes, low cost, robust, and reliable sensors are needed to measure field strength, as well as sensors that can measure process parameters but are immune to direct excitation by the electromagnetic energy.

Technology opportunities for sensors and process controls to improve the overall control and
performance of process heating systems include the following:

Sensors for Harsh, High-Temperature Environments: Technologies and methods are needed to reliably monitor and control critical product parameters (temperature, chemistry, pressure, etc.), especially robust sensors to measure critical parameters in harsh combustion environments. This includes direct process measurement sensors, and more accurate and reliable thermocouples and other sensors. The development of sensors that can provide accurate readings in high-temperature environments could enable opportunities to optimize heat transfer and containment systems in those conditions.

Furnace Control: In fuel-fired equipment, reliable sensing and control technologies can provide better fuel utilization, energy savings, temperature control, and system performance over time. This includes sensors that can accurately measure compositional characteristics of fuels and oxidant; low-cost, highly reliable flame monitoring systems to control flame quality and stability; and continuous flue gas analysis. By regulating and stabilizing internal furnace pressure, pressure controllers can eliminate cold air infiltration, maintain uniform temperatures, and reduce wear that would require more frequent and costly maintenance.

Advanced Control Strategies to Optimize Process Heating: Cost-effective smart process controls that can be integrated with the overall manufacturing system are needed. Analysis of flue gases can be used to optimize the inlet fuel/air ratio. By using sensors to measure oxygen and carbon monoxide in the flue gas stream, conditions can be created for ideal combustion scenarios.

Common Terminology Used in Temperature Measurement and Process Control

Accuracy: The closeness of an indicator or reading of a measurement device to the actual value of the quantity being measured; usually expressed as ± percent of the full scale output or reading.

Drift: The change in output or set point value over long periods of time due to such factors as temperature, voltage, and time.

Hysteresis: The difference in output after a full cycle in which the input value approaches the reference point (conditions) with increasing, then decreasing values or vice versa; it is measured by decreasing the input to one extreme (minimum or maximum value), then to the other extreme, then returning the input to the reference (starting) value.

Linearity: How closely the output of a sensor approximates a straight line when the applied input is linear.

Noise: An unwanted electrical interference on signal wires.

Nonlinearity: The difference between the actual deflection curve of a unit and a straight line drawn between the upper and lower range terminal values of the deflection, expressed as a percentage of full range deflection.

Precision: The degree of agreement between a number of independent observations of the same physical quantity obtained under the same conditions.

Repeatability: The ability of a sensor to reproduce output readings when the same input value is applied to it consecutively under the same conditions.

Resolution: The smallest detectable increment of measurement.

RTD: Abbreviation for "resistance temperature detector". Resistance temperature detectors are temperature sensors that are widely used because of their high accuracy, stability, and linearity. They work on the principle that the resistivity of metals is dependent upon temperature; as temperature increases, resistance increases. Resistance Temperature Detector’s can withstand temperatures up to approximately 800 C (~1500 F).

Sensitivity: The minimum change in input signal to which an instrument can respond. Stability: The ability of an instrument to provide consistent output over an extended
period during which a constant input is applied.

Thermocouple: A temperature sensing device widely used because they are relatively low cost, self-powered, durable and capable sensing high temperatures. Thermocouples generate and micro voltage in relation to temperature change.

Zero balance: The ability of the transducer to output a value of zero at the electronic null
point.

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Understanding Dissimilar Metal Junctions and the Need for Reference Junctions

When two dissimilar metal wires are joined together at one end, a voltage is produced at the other end that is approximately proportional to temperature. That is to say, the junction of two different metals behaves like a temperature-sensitive battery. This form of electrical temperature sensor is called a thermocouple:

This phenomenon provides us with a simple way to electrically infer temperature: simply measure the voltage produced by the junction, and you can tell the temperature of that junction. And it would be that simple, if it were not for an unavoidable consequence of electric circuits: when we connect any kind of electrical instrument to the thermocouple wires, we inevitably produce another junction of dissimilar metals. The following schematic shows this fact, where the iron-copper junction J₁ is necessarily complemented by a second iron-copper junction J₂ of opposing polarity:

Junction J₁ is a junction of iron and copper – two dissimilar metals – which will generate a voltage related to temperature. Note that junction J₂, which is necessary for the simple fact that we must somehow connect our copper-wired voltmeter to the iron wire, is also a dissimilar-metal junction which will also generate a voltage related to temperature. Further note how the polarity of junction J₂ stands opposed to the polarity of junction J₁ (iron = positive ; copper = negative). A third junction (J₃) also exists between wires, but it is of no consequence because it is a junction of two identical metals which does not generate a temperature-dependent voltage at all.

The presence of this second voltage-generating junction (J₂) helps explain why the voltmeter registers 0 volts when the entire system is at room temperature: any voltage generated by the iron- copper junctions will be equal in magnitude and opposite in polarity, resulting in a net (series-total) voltage of zero. Only when the two junctions J₁ and J₂ are at different temperatures will the voltmeter register any voltage at all.

We may express this relationship mathematically as follows:  Vmeter = VJ₁ − VJ₂

With the measurement (J₁) and reference (J₂) junction voltages opposed to each other, the voltmeter only “sees” the difference between these two voltages.

Thus, thermocouple systems are fundamentally differential temperature sensors. That is, they provide an electrical output proportional to the difference in temperature between two different points. For this reason, the wire junction we use to measure the temperature of interest is called the measurement junction while the other junction (which we cannot eliminate from the circuit) is called the reference junction (or the cold junction, because it is typically at a cooler temperature than the process measurement junction).

For more information on this subject, contact Duro-Sense, Inc. by visiting https://duro-sense.com or by calling 310-533-6877.

Reprinted from "Lessons In Industrial Instrumentation" by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License.

Thermocouple Types, Materials of Composition, and Temperature Ranges

Nickel-Alloy Thermocouples

Type E: 
(pos) 90% Nickel / 10% Chromium;
(neg) 55% Copper / 45% Nickel (Constantan)
32 to 1600°F, 0 to 870°C

Type J:
(pos) 100% Iron
(neg) 55% Copper / 45% Nickel (Constantan)
32 to 1400°F, 0 to 760°C

Type K:
(pos) 90% Nickel / 10% Chromium
(neg) 95% Nickel / 2% Aluminum / 2% Manganese / 1% Silicon
32 to 2300°F, 0 to 1260°C


Type M:
(pos) 82% Nickel / 18% Molybdenum
(neg) 99.2% Nickel / 0.8% Cobalt
-58 to 2570°F, -50 to 1410°C

Type N:
(pos) 84.5% Nickel / 14% Chromium / 1.5% Silicon
(neg) 95.4% Nickel / 4.5% Silicon / 0.1% Magnesium
32 to 2300°F, 0 to 1260°C

Type T:
(pos) 100% Copper
(neg) 55% Copper / 45% Nickel (Constantan)
-328 to 700°F, -200 to 370°C

Platinum/rhodium-Alloy thermocouples

Type B:
(pos) 70% Platinum / 30% Rhodium
(neg) 94% Platinum  / 6% Rhodium
1600 to 3100°F, 871 to 1704°C

Type R:
(pos) 87% Platinum / 13% Rhodium
(neg) 100% Platinum
1000 to 2700°F, 538 to 1482°C

Type S:
(pos) 90% Platinum / 10% Rhodium
(neg) 100% Platinum
1000 to 2700°F, 538 to 1482°C

Tungsten/Rhenium-Alloy Thermocouples

Type C:
(pos) 95% Tungsten / 5% Rhenium
(neg) 74% Tungsten / 26% Rhenium
32 to 4200°F, 0 to 2315°C

Chromel–Gold/Iron-Alloy Thermocouples

Type P:
(pos) 55% Palladium / 31% Platinum / 14% Gold
(neg) 65% Gold / 35% Palladium
32 to 2543°F, 0 to 1395°C

Precision RTD's (Resistance Temperature Detectors)

Duro-Sense RTDs, thermowells, and accessories provide high quality solutions to the aerospace, aviation, process control, medical, R&D, power generation, alternative energy, plastics, primary metals, high-tech and OEM industries.

https://duro-sense.com
310-533-6877