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nita
ICE Expert
Joined: June 30, 2006
Posts: 164
Location: Hiroshima,Japan
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 Posted:
Aug 07, 2006 - 09:32 AM |
 
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| Post subject: Resistance Temperature Detectors (RTDs) |
What are RTDs?
Resistance Temperature Detectors or RTDs for short, are wire wound and thin film devices that measure temperature because of the physical principle of the positive temperature coefficient of electrical resistance of metals. The hotter they become, the larger or higher the value of their electrical resistance.
They, in the case of Platinum known variously as PRTs and PRT100s, are the most popular RTD type, nearly linear over a wide range of temperatures and some small enough to have response times of a fraction of a second. They are among the most precise temperature sensors available with resolution and measurement uncertanties or ±0.1 °C or better possible in special desions.
Usually they are provided encapsulated in probes for temperature sensing and measurement with an external indicator, controller or transmitter, or enclosed inside other devices where they measure temperature as a part of the device's function, such as a temperature controller or precision thermostat.
The Advantages of RTDs
The advantages of RTDs include stable output for long period of time, ease of recalibration and accurate readings over relatively narrow temperature spans. Their disadvantages, compared to the thermocouples, are: smaller overall temperature range, higher initial cost and less rugged in high vibration lenvironments.
They are active devices requiring an electrical current to produce a voltage drop across the sensor that can be then measured by a calibrated read-out device.
RTD Error Sources
The lead wires used to connect the RTD to a readout can contribute to their measurement error, especially when there are long lead lengths involved, as often happens in remote temperature measurement locations. Those calculations are straight forward and there exist 3-wire and 4-wire designs to help minimize or limit such errors, when needed.
Often the lead error can be minimized through use of a temperature transmitter mounted close to the RTD. Transmitters convert the resistance measurement to an analog current or serial digital signal that can be sent long distances by wire or rf to a data aquisition or control system and/or indicator.
RTDs, as mentioned above, work in a relatively small temperature domain, compared to thermocouples, typically from about -200 °C to a practical maximum of about 650 to 700 °C. Some makers claim wider ranges and some construction designs are limited to only a small portion of the usual range.
Insulation resistance is always a function of temperature and at relatively high temperature the shunt resistance of the insulator introduces errors into measurement. Again, error estimates are straight forward, provided one has a good estimate of the thermal properties of the insulator.
Insulator material such as powdered magnesia (MgO), alumina (Al2O3) and similar coumpounds are carefully dried and sealed when encapsulated in probes along with an RTD element.
ASTM has standards related to insulation resistance testing to help determine the performance of such sealed probes, specifically E 1652-00.
RTDs Other Than Platinum
RTDs can be made cheaply in Copper and Nickel, but the latter have restricted ranges because of non-linearities and wire oxidation problems in the case of Copper.
Platinum is the preferred material for precision measurement because in its pure form the Temperature Coefficient of Resistance is nearly linear; enough so that temperature measurements with precision of ±0.1 °C can be readily acheived with moderately priced devices. Better resolution is possible, but equipment costs escalate rapidly at smaller error levels.
All RTDs used in precise temperature measurements are made of Platinum and they are sometimes called PRTs to distinguish them.
Standard Platinum RTDs(SPRTs)
The ITS-90 (International Temperature Scale of 1990- used as a worldwide practical temperature scale in national metrology labs like NIST, NPL et al) is made up of a number of fixed reference points with various interpolating devices used to define the scale between points. A special set of PRTs, called SPRTs, are used to perform the interpolation in such labs over the ranges 13.8033 K (Triple point of Equilibrium Hydrogen) to the Freezing point of Silver, 971.78 °C.
The Hart Scientific website provides a glimpse into the realm of precision SPRTs and readout equipment used in calibration labs. They operate one of the very few labs in the USA with accreditation under NVLAP to the ISO/IEC 17025 standard. |
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fundu_instru
ICE Expert
Joined: June 20, 2006
Posts: 180
Location: Singapore
Status: Offline
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| Posted:
Aug 07, 2006 - 11:08 AM |
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STANDARDS FOR RESISTANCE TEMPERATURE SENSORS (RTDs)
There are numerous standards around the world and the goal of this part of the site is to eventually catalog most, if not all, of them. This is a start with links to many of the organizations involved, if they are on the Web. The ones we know the best are those published by ASTM and those standards that deal with radiation thermometers. We shall be searching the Web and the Net for more details. If you can add to this page, just email your information and we will include it as we are able to verify and edit the page.
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ASTM Standards Related to Temperature and Calibration:
E 1594-99 Standard Guide for the Expression of Temperature
E 344-01a...Terminology Relating to Thermometry and Hygrometry
E 563-97 Standard Practice for Preparation and Use of Freezing Point Reference Baths
E 1502-98 Standard Guide for the Use of Freezing Point Cells for Reference Temperatures
E1750-02 Standard Guide for Use of Water Triple Point Cells
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ASTM Standards Related to Resistance Temperature Detectors
E 644-98 Standard Test Methods for Testing Industrial Resistance Thermometers
E 1137-97 Standard Specification for Industrial Platinum Resistance Thermometers
E 1652-00 Standard Specification for Magnesium Oxide and Aluminum Oxide Powder and Crushable Insulators Used in the Manufacture of Metal-Sheathed Platinum Resistance Thermometers, Base Metal Thermocouples, and Noble Metal Thermocouples
Other Standards Related to Resistance Temperature Detectors
German Industrial Standards Organization (DIN)
DIN 43760 references nickel precision
DIN IEC 751 reference platinum precision resistance thermometers. |
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nita
ICE Expert
Joined: June 30, 2006
Posts: 164
Location: Hiroshima,Japan
Status: Offline
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| Posted:
Aug 08, 2006 - 12:05 PM |
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| Post subject: RTD |
Info regarding Measuring Temperature with RTDs .. |
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nita
ICE Expert
Joined: June 30, 2006
Posts: 164
Location: Hiroshima,Japan
Status: Offline
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Oct 04, 2006 - 12:04 PM |
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nita
ICE Expert
Joined: June 30, 2006
Posts: 164
Location: Hiroshima,Japan
Status: Offline
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Oct 04, 2006 - 12:11 PM |
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fundu_instru
ICE Expert
Joined: June 20, 2006
Posts: 180
Location: Singapore
Status: Offline
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Oct 05, 2006 - 02:47 AM |
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instruite
ICE Expert
Joined: May 29, 2006
Posts: 321
Location: Japan/India
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Oct 18, 2006 - 12:43 AM |
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instruite
ICE Expert
Joined: May 29, 2006
Posts: 321
Location: Japan/India
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Oct 18, 2006 - 12:59 AM |
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prabakaran
ICE Newbee
Joined: July 16, 2007
Posts: 3
Location: Chennai
Status: Offline
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July 28, 2007 - 12:58 AM |
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what is the differnce between thermocouple J type and K type. i used PT 100 sensor but i dont know what is the difference.
i Measured in terms of mV. |
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prabakaran |
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instruite
ICE Expert
Joined: May 29, 2006
Posts: 321
Location: Japan/India
Status: Offline
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| Posted:
July 31, 2007 - 07:54 AM |
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Have a look at following Links
I guess the above two links will answer your query in case you have more specific questions feel free to ask |
_________________
Instruite 
======================
Ideas that work,
Concepts that sell.
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