Friday, July 6, 2018

Common Terminology Used in Temperature Measurement and Process Control

Terminology Used in Temperature Measurement
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

Monday, July 2, 2018

Happy 4th of July from Duro-Sense Corporation

"One flag, one land, one heart, one hand, One Nation evermore!"

Oliver Wendell Holmes

Tuesday, June 26, 2018

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:
Dissimilar Metal Junctions

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:

Dissimilar Metal Junctions

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 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.

Tuesday, June 19, 2018

Thermocouple Types, Materials of Composition, and Temperature Ranges

Thermocouple Types
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

Friday, June 8, 2018

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.

Friday, May 25, 2018

Power Plant Temperature Sensors

Power plants (generating facilities) transform the mechanical energy of a spinning generator into electrical energy. Heat (from flame, nuclear reaction, or chemical reaction) is used to create steam that, in turn, produces the mechanical energy to drive turbines. There are many areas where precision temperature measurement and monitoring is critical to keep power plant systems running. Thermocouples and RTD sensors provide accurate, repeatable, and reliable  measurement.

Phone: 310-533-6877

Wednesday, May 16, 2018

RTD and Thermocouple Selection and Location for Optimal Control

Loop diagram
Loop diagram *
A perfectly designed temperature loop would precisely balance the power required to heat the media to it's desired temperature while compensating for system losses. In the real world however, there are many external variables that upset the balance between energy input and desired temperature. To offset these external variables and ensure adequate power is available to do the work, energy calculations with liberal safety factors are coupled with temperature controllers that throttle or proportion the amount of energy added to the process.

Most temperature control loops have (5) four major components:

1) The media to be heated (e.g. metal platen, a tank of liquid, a stream of gas)
2) A energy source (e.g. electric heater, steam, hot oil, flame)
3) A temperature sensor (thermocouple, RTD)
4) A controller (e.g. electronic thermostat, PID controller)
5) Control element (e.g. control valve, SCR, SSR)

Temperature controllers provide sophisticated functions that "learn" or understand the relationship between available power and sensor temperature. They then adjust the amount of energy (heat) added, based on the current reading of the sensor and the desired temperature setpoint.  Unfortunately, temperature controllers are often relied upon to overcome the oversights and inadequacies of poor control loop design.

Lag time
Lag time *
In poor control situations the controller usually takes the blame, when in actuality,  the problem lies in the system design. The controller's actions are a function of the difference between setpoint and sensor reading, the availability of energy to eliminate that error, and the sensor lag time. The further the sensor or energy source is located from the process media, the wider the swings in energy input will be, and therefore produce a more difficult loop to balance. Considering this, it is important to optimize the sensor (thermocouple or RTD) placement. Any distance or barrier between sensor and process media introduces lag which is an impediment to close control.

In the most ideal situation, temperature sensors, the energy source and process media would all be at the same physical location. Since it's virtually impossible to accomplish this, compromises have to be made to allow for the mechanical, physical, and electrical realities of the application. Here are some practical recommendations for sensor selection and placement to improve temperature loop performance:
  • Thermocouples, because of their low mass generally have a response advantage over RTDs.
  • Exposed junction thermocouples are the fastest responding (least lag) sensor choice, but they are also the most prone to physical and chemical damage.
  • Narrow, sheathed, grounded junction magnesium oxide insulated thermocouples are nearly as fast as exposed junctions, and provide protection from the process media.
  • Applications that require protection sheaths and thermowells for RTDs and thermocouples increase sensor lag time.
  • An immersion length of at least 10 times the diameter of the thermowell or sensor sheath should be used to minimize heat loss along the sensor sheath or thermowell wall from tip to process connection.
  • Where possible, insert the temperature sensor in a pipe elbow into the oncoming flow.
  • Tapered, swaged, or stepped thermowells are  faster responding
  • Always make sure the fit between sensor outer diameter and thermowell inner diameter is tight, and that the tip of the sensor is in direct contact with the thermowell. 

* Images courtesy of Tony Kuphaldt and his book "Lessons In Industrial Instrumentation"