RTDs Resistance Temperature Detectors are sensing temperature devices that are used to measure temperature by correlation the resistance of the RTD element with temperature.
RTDs are widely used in various industrial and scientific applications due to their high accuracy, stability, and repeatability.
This article will discuss the principle of RTDs , their construction, classification of RTDs , their applications, testing, calibration method, advantages, disadvantages and selection criteria...
Principle of Operation
RTD works on the principle that the electrical resistance of a conductor changes as its temperature varies. RTD consists of a very small piece of metal whose electrical resistance changes in a known way with changing temperature.
The change in the resistance for 1 deg C change in the temperature is known as 'temperature coefficient of resistance' which is constant over some range of temperatures.
All the metals generally have PTC (Positive Temperature Coefficient) i.e. their resistance increases with increase in temperature. Some materials like carbon and germanium have NTC (Negative Temperature Coefficient), their resistance decreases with increase in temperature.
Classification of RTDs
RTDs are classified based on the material used to make the RTD element, the number of wires used to connect the RTD element, and the type of RTD element:
1. Based on Material:
The RTD element can be made of various materials, including platinum, copper, and nickel. Platinum RTDs are the most common due to their high accuracy, stability, and wide temperature range. Copper RTDs are less expensive than platinum RTDs but have a lower temperature range and accuracy. Nickel RTDs are the least expensive but have the lowest accuracy and temperature range.
2. Based on Number of Wires:
RTDs can have two, three, or four wires. Two-wire RTDs are the simplest and least expensive but have the lowest accuracy due to the lead resistance effect. Three-wire RTDs provide better accuracy than two-wire RTDs by compensating for the lead resistance effect. Four-wire RTDs offer the highest accuracy by providing separate current and voltage leads, eliminating the lead resistance effect.
3. Based on Type of RTD Element:
RTD elements can be thin-film or wire-wound. Thin-film RTD elements are made by depositing a thin film of the RTD material on a ceramic substrate. Wire-wound RTD elements are made by winding a wire of the RTD material around a ceramic or glass core. Thin-film RTD elements have a faster response time and are more robust than wire-wound RTD elements.
Applications of RTDs
RTDs are used in various industrial and scientific applications, including:
- Temperature Control Systems: RTDs are used in temperature control systems for industrial processes, such as chemical plants, food processing, and pharmaceutical manufacturing.
- Medical Equipment: RTDs are used in medical equipment, such as incubators, dialysis machines, and MRI machines.
- Aerospace: RTDs are used in aerospace applications, such as aircraft engine monitoring and spacecraft thermal control systems.
- Automotive: RTDs are used in automotive applications, such as engine and transmission temperature monitoring.
- Laboratories: RTDs are used in laboratories for temperature measurement and control in various experiments and research.
Construction
Used resistance materials are platinum, nickel, copper etc. out of which platinum is used in most of the applications. Platinum has linear characteristics over much of its range.
Common RTDs are made of a very small diameter platinum wire wound on a ceramic core.
The wire wound care is sealed with molten glass to protect it and then it is placed in a metal housing to allow it to be used for measurement in the process plant.
P1100 is most widely used temperature sensor worldwide and designates platinum RTD whose resistance at 0 deg C is 100 ohms.
Connectivity to other devices
RTDs are available in 2/3/4 wires configurations, which are used, based on the accuracy required and depending upon the type of secondary measurement circuit. It is connected as a bridge element to measure the resistance.The effects of lead (connecting wires) resistance can be reduced/ eliminated using 3/4 wire configurations.
Characteristics of RTD
- Sensitivity
- Response Time
- Construction
- Signal Conditioning
- Range
Advantages
- High Accuracy
- Good Reproducibility
- Stable and accurate over long period
- Fast Response
- Low cost
Disadvantages
- Self heating
- Mechanically less strong
- Bulkier than thermocouple
RTD Testing The LCSR Method
The response time of installed RTDs can be measured using the Loop Current Step Response (LCSR) method. This method is based on heating the RTO's sensing element with a small electric current (about 35 to 80 milliamperes) applied to the RTD's extension leads.
The current causes a temperature transient in the RTD as shown in this figure. This transient is sampled by a computer and analyzed to give the response time of the RTD. The analysis involves a mathematical fitting of the LCSR transient data to an exponential series that has been developed based on a general heat transfer model of RTDs..
The validity of the LCSR method has been established by numerous laboratory tests performed over a five year period in the mid-1970s. The LCSR method has been approved by the U. S. Nuclear Regulatory Commission (NRC) for in-situ response time testing of nuclear plant RTDs.
The advantage of LCSR test is it's permits remote testing of RTD as installed in operating process, and thus provides the actual "in-service" response time of the RTD. The test accounts for all installation and process condition effects on response time.Most PWR plants, especially those which have removed their RTD by-pass manifolds, have large temperature fluctuations in their hot legs and to lesser extent in their cold legs. This is due to temperature stratification which is inherent in PWRs and originates in the core because different streams of water that exit the core are at different temperatures.
This problem can interfere with accurate measurement of RTD response time using the Loop Current Step Response Method (LCSR) As such, AMS has developed new software, as well as a new procedure, to maintain high accuracy in response time testing results in spite of temperature stratification.
RTD Cross Calibration Method
Cross calibration is a test that is performed on redundant RTDs in the primary coolant system of pressurized water reactors (PWRs). The test is performed to verify the calibration consistency of RTDs to ensure that accurate steady state temperature measurements are provided to the plant control and safety systems.
The test involves a systematic intercomparison of temperature or resistance readings of redundant RTDs to identify the ones that may have a larger-than-normal deviation from the average reading of the remaining RTDs
Cross calibration tests can be performed at one or more temperature plateaus at isothermal conditions when the reactor coolant hot leg and cold temperatures are at approximately the same temperature. They can also be performed when the temperature is increasing or decreasing monotonically at a reasonably constant rate.
The advantage of performing the cross calibration under temperature ramp conditions is that it saves test time as it does not require the plant to hold at plateau conditions for the test.
Catalogs for example
Catalog for example for ceramic RTD is shown below:-
Special Features
- Custom designed for special application
- The RTD sensor is directly used which will give the feature of faster response.
- The RTD element size is available as per customer requirement.
Standard Product Details
Element type. Pt-100 RTD
Accuracy. Class B Tolerance as per IEC-751
Wire Configuration. 2 Wire System
RTD Element Dia. 2.8 mm
Sheath Material. Ceramic
Exposed Length_EL_mm. 25 mm
Lead Wire Length_X_mm. 1000 mm
Lead Wire Type. PTFE Insulated lead wires.
Selection Criterion
- Proper RTD is selected by knowing the industry and process.
- Cost considerations
- Available vendors & studying their catalogs.
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