THERMAl RESISTORS(RTD)

THERMAl RESISTORS(RTD)
Thermistors are special solid temperature sensors that behave like temperature-sensitive electrical resistors. No surprise then that their name is a contraction of "thermal" and "resistor". There are basically two broad types, NTC-Negative Temperature Coefficient, used mostly in temperature sensing and PTC-Positive Temperature Coefficient, used mostly in electric current control.

There's even more history of the name and development of thermistors and facts about some key NTC parameters at the Kele Electronics website, just be prepared for some strong opinions about one brand of thermistor.

They are mostly very small bits of special material that exhibit more than just temperature sensitivity. They are highly-sensitive and have very reproducible resistance Vs. temperature properties.

During the last 60 years or so, only ceramic materials (a mix of different metal oxides) was employed for production of NTC thermistors. In 2003, AdSem, Inc. (Palo Alto, CA) developed and started manufacturing of Si and Ge high temperature NTC thermistors with better performance than any ceramic NTC thermistors.

Thermistors, since they can be very small, are used inside many other devices as temperature sensing and correction devices as well as in specialty temperature sensing probes for commerce, science and industry.

Some of those new-fangled digital medical thermometers that get stuck in one's mouth by a nurse with an electronic display in her other hand are based on thermistor sensors. They are probably inside your cell phone, automobile, stereo and television, too, but you'd never know it unless you were an engineer or visited here.

Thermistors typically work over a relatively small temperature range, compared to other temperature sensors, and can be very accurate and precise within that range, although not all are.


Thermistor Terminology
A glossary slightly modified from that given in a US government publication: MIL-PRF-23648D. Note that the term being described is in bold typeface.
A thermistor is a thermally sensitive resistor that exhibits a change in electrical resistance with a change in its temperature. The resistance is measured by passing a small, measured direct current (dc) through it and measuring the voltage drop produced.

The standard reference temperature is the thermistor body temperature at which nominal zero-power resistance is specified, usually 25°C.

The zero-power resistance is the dc resistance value of a thermistor measured at a specified temperature with a power dissipation by the thermistor low enough that any further decrease in power will result in not more than 0.1 percent (or 1/10 of the specified measurement tolerance, whichever is smaller) change in resistance.

The resistance ratio characteristic identifies the ratio of the zero-power resistance of a thermistor measured at 25°C to that resistance measured at 125°C.

The zero-power temperature coefficient of resistance is the ratio at a specified temperature (T), of the rate of change of zero-power resistance with temperature to the zero-power resistance of the thermistor.

A NTC thermistor is one in which the zero-power resistance decreases with an increase in temperature.

A PTC thermistor is one in which the zero-power resistance increases with an increase in temperature.

The maximum operating temperature is the maximum body temperature at which the thermistor will operate for an extended period of time with acceptable stability of its characteristics. This temperature is the result of internal or external heating, or both, and should not exceed the maximum value specified.
.
The maximum power rating of a thermistor is the maximum power which a thermistor will dissipate for an extended period of time with acceptable stability of its characteristics.

The dissipation constant is the ratio, (in milliwatts per degree C) at a specified ambient temperature, of a change in power dissipation in a thermistor to the resultant body temperature change.

The thermal time constant of a thermistor is the time required for a thermistor to change 63.2 percent of the total difference between its initial and final body temperature when subjected to a step function


The resistance-temperature characteristic of a thermistor is the relationship between the zero-power resistance of a thermistor and its body temperature.

The temperature-wattage characteristic of a thermistor is the relationship at a specified ambient temperature between the thermistor temperature and the applied steady state wattage.

The current-time characteristic of a thermistor is the relationship at a specified ambient temperature between the current through a thermistor and time, upon application or interruption of voltage to it.

The stability of a thermistor is the ability of a thermistor to retain specified characteristics after being subjected to designated environmental or electrical test conditions.

Thermistor Types, including applications information and downloadable programs for resistance vs temperature

Thermisor Temperature Sensor Resources
Thermistor Types, including applications information and downloadable programs for resistance vs temperature
Thermistor-based Temperature Sensor Probes
Thermistor Applications
Silicon Thermistors
The Steinhart-Hart Equation and Miscellaneous Information

Thermocouples

Thermocouples (TCs)
Thermocouples are pairs of dissimilar metal wires joined at least at one end, which generate a net thermoelectric voltage between the the open pair according to the size of the temperature difference between the ends, the relative Seebeck coefficient of the wire pair and the uniformity of the wire-pair relative Seebeck coefficient

Thermocouples are among the easiest temperature sensors to use and obtain and are widely used in science and industry. They are based on the Seebeck effect that occurs in electrical conductors that experience a temperature gradient along their length. They are "simple", rugged, need no batteries, measure over very wide temperature ranges and more. They have their quirks, too, like everything else. The solution to thermocouple uses and problems lies in the details of a given application.
Have a read through, click on the key topics below to get more detail about the thermocouple (TC), how it works, color codes, recommended use limits and thermocouple standards. There is a link to an excellent article on how they work and notes on using thermocouples from a well-known expert on them.
Don't forget to check our temperature references pages for the many additional publications that provide a wealth of information on the theory and application of these very popular and rugged temperature sensors. The applications are where the seasoned TC user and the novice part company and the one with know-how usually acheives the desired result. If all else fails to impress you to be thorough and careful with thermocouples, consider these few facts:
Thermocouples measure their own temperature. You must infer the temperature of the obect of interest by being certain there is no heat flow between them when you take the measurement. That's easier than it sounds in some case.
Thermocouples can err in reading their own temperature, especially after being used for a while, or if the insulation between the wires loses its resistance due to moisture or thermal conditions, or there are chemical, nuclear radiation or mechanical effects with the immediate surroundings.
Beware of electrical hazards using thermocouples, they are electrical conductors, even refractory oxide sheathed models at high temperatures; should they contact another source of electricity, they can kill you!
Oh, Yes..Thermocouples DO NOT MEASURE AT THE JUNCTIONS! They can't, it is physically impossible to have a temperature gradient at a point. Also the Electric Field Strength (i.e. volts/meter) at such an impossible condition would be infinite, sufficient to tear the materials apart. So, if you want to understand TCs better, start with the very basics; learn about the Seebeck Effect and how thermocouples really work!
Also, if you use thermocouples, you need to have some way to interpret their small and non-linear output voltages. There's lots of ways to do it, the simplest being a measurement of the output voltage and looking up the value in a table of millivolts dc versus temperature AND correcting for the cooler junction not being at 0 °C, or 32°F, according to your units inclination.
The high end is to hook the thermocouples up to a modern readout display or a DAQ module plugged into a PC and read away!
However, it is not a fool proof business and there are many subtle things about thermocouples and their uses that have made well-intentioned engineers, who thought they understood them, look like fools. The hooking up and reading out are the easy parts of a measurement.
The selection, installation details and the conditions of use play a big role in obtaining a measurement that is accurate and reliable. It's like many measurement subjects, the devil is in the details and "simple" thermocouples have a lot of details!
Their context of use is perhaps the biggest, especially where relatively high temperature-above a few hundred degrees (on anyone's scale) are the object of measurement. Above a few thousand degrees, there are a whole range of additional problems.
Thanks for visiting and if you plan to use them, be careful, don't get fooled by their apparent simplicity. Learn the details, start with the basics (What measurement precision do I need?-Is it realistic for using thermocouples, etc.etc?) In other words always apply a systematic, measurement engineeering approach, please
Thermocouple Temperature Sensor Applications
APPLICATIONS ARE WHERE THE SENSORS MEET THE "REAL WORLD" OR THE RUBBER MEETS THE ROAD; WHERE THE RESULTS PROVE THAT YOU UNDERSTAND THEIR USE AND HAVE SELECTED WELL ENOUGH TO DO THE JOB!
There is a great deal of information on successful use of thermocouples temperature sensors under a great many different conditions of use. The cataloging of those proven applications can help in two ways. First, it saves one from reinventing the wheel. Second, it can make a new application easier to solve if it is analogous to a proven one, with perhaps only a change to one or two of the influencing conditions. Thanks for visiting.
Nanmac Corporation's Temperature Handbook This site offers a very useful collection of interesting and unique thermocouple applications stories. Featuring quite often the unique devices they make, it nonetheless is an impressive collection of solved temperature measurement problems using thermocouples!

Control and instrumentation engineer

Job description
A control and instrumentation engineer (CI engineer) is essentially responsible for designing, developing, installing, managing and/or maintaining equipment which is used to monitor and control engineering systems, machinery and processes.

The main objective of the work of CI engineers is to ensure that these systems and processes operate effectively, efficiently and safely. They usually work for the companies who manufacture and supply the equipment, or for the companies who use it.

CI engineers need a thorough understanding of the operational processes of an organisation and have a truly multidisciplinary role, working closely with colleagues across a number of functions, including operations, purchasing and design.

Typical work activities
Control and instrumentation engineers (CI engineers) will develop skills in specific control disciplines such as advanced process control (APC), distributed control systems (DCS), programmable logic controllers (PLC), and supervisory control and data acquisition (SCADA). The use of these disciplines will depend on the exact nature of individual job roles. Tasks and responsibilities which are common to many CI engineer positions, however, may include:

designing and developing new control systems;
maintaining and modifying existing systems;
managing operations;
working collaboratively with design engineers, operation engineers, purchasers and other internal staff;
liaising with clients, suppliers, contractors and relevant authorities (e.g. the Nuclear Decommissioning Authority);
project management within cost and time constrained environments;
troubleshooting and problem-solving;
understanding and ensuring compliance with the health and safety regulations and quality standards of the country in which work is undertaken;
providing advice and consultancy support;
purchasing equipment;
writing computer software;
developing new business proposals;
accepting responsibility and a level of accountability commensurate with the seniority of the position.