Liquid’s Temperature Measurement Using an RTD
Measuring Air & Gas Temperatures Using RTD Sensors
Measuring Surface Temperatures
Introducing Resistance-Temperature-Detectors (RTDs)
These happen to be the temperature sensors that tend to consist of a resistor, which alters the resistance value when there are changes in the temperature. These are being used for several years now for measuring temperatures in labs & other industrial processes. These have formed a good reputation for repeatability, accuracy, & stability. Most of the elements of an RTD contain a finely twisted wire that is wound around a glass core or a ceramic. The element is generally very delicate; therefore, it’s usually located in a sheathed protective probe. The RTD element happens to be built of pure materials for withstanding varied temperatures. Material’s resistance changes predictably when there are changes in the temperature; that predicted change is usually utilized for determining the temperatures.
The Origin
Sir Humphrey Davy saw that metals resistivity showed considerable temperature dependence, and this discovery happened in the same year in which Seebeck discovered thermoelectricity. After 50 years, Sir William Siemens proposed using platinum as the resistance ‘s element.
Advantages of Using RTDs
The resistance temperature detector happens to be the most accurate temperature sensor. In addition to good accuracy, it also offers great stability & excellent repeatability. Such an RTD is even relatively immune to electric noises and are thus suited for temperature measurement in different industrial environments, specifically near generators, motors, & other such high voltage equipment.
RTD Probe’s Construction
Such probes are the assemblies consisting of one element, one sheath, one lead wire & one terminal/connection. Once the element has been chosen, the wiring & packaging needs are to be decided. Sensors can be connected in many ways & there are unlimited sensors or sensor structures to make the selection.
Wiring Arrangement
For measuring temperatures, the element of the RTD should be joined to a monitoring/control device. Now, as the measurement of the temperature is entirely based upon the resistance of the element, any other resistance that has been added to that circuit (resistance of wires, connections, etc.) can lead to an error in the measurement. Any other wiring than the two-wire configuration allows control/monitoring of the devices to account for unwanted wire resistance & other such resistances inside that circuit. Sensors using a three-wire design happen to be quite common designs utilised for industrial processes as well as monitoring/control applications. The resistance of the wires is considered, provided that all the lead wires are having similar resistance; else errors may occur.
Materials of the Wires
While determining the materials of the lead wire, the correct wires must be selected based on the temperature & environment to which that sensor would be exposed during operation. Temperature is by far the most crucial factor in wire selection, but physical properties like abrasion resistance & water submersion properties could be significant too. The 3 widely popular structures are given as under:
- PVC insulated probe provides the temperature range of -40 degrees Celsius to 105 degrees Celsius. It also has nice wear resistance. It is also suitable for water immersion.
- PFA insulated RTD probe provides the temperature range of -267 degrees Celsius to 260 degrees Celsius. It also provides supercool wear resistance. It is even perfect for water submersion applications.
- Although an RTD probe with fiberglass insulation tends to provide a higher temperature range from -73 to 482 degrees Celsius, its performance is considered less effective when consumed or immersed in water.
The Termination
An RTD probe might be stopped via a connection end, or through a quick disconnect, or by a terminal block or even by an extension wire. There are other finishing styles too.
The Configuration
As the RTD element, the wire layout & the wire structuring have been chosen, the physical structuring of the sensor must be acknowledged. The configuration of the final sensor largely depends on the type of application needed. Varied sensor configurations are needed for measuring the temperatures of liquids, surfaces, or gas streams.
Widely Popular RTD’s Resistance Materials
- Platinum (highly accurate & widely popular)
- Nickel
- Copper
- Balco (very rare)
- Tungsten (quite rare)
RTD standards
There happen to be 2 standards for a platinum RTD. These are 1) the European standard (which is even called IEC standard or the DIN) and 2) the American standard. The European one has been considered the global platinum RTD standard. This particular standard, DIN/IEC 60751 (or only IEC751), needs the RTD for having an electric resistance of 100.00 Ω at 0 degrees Celsius, & also the temperature-coefficient-of-resistance (i.e., the TCR) of 0.00385 Ω/Ω/degree Celsius between 0 degree Celsius & 100 degree Celsius. The amalgamation of temperature coefficient & resistance tolerance tends to determine the resistance & temperature characteristics of such an RTD sensor. The greater is the tolerance of the element, the more the sensor deviates from the general curve & greater will be the variability from 1 sensor to another (which is also called interchangeability). This is significant for those users who require the replacement of sensors & need to curtail any interchangeability errors.
Selecting the Most Ideal RTD
- RTD Element: This is RTD’s simplest form. This contains one piece of wire that is wrapped around a glass core or a ceramic. Owing to their small size, such an element is usually utilised where space is quite limited.
- RTD Surface Element: This happens to be a special RTD element type. This is made pretty thin, which ensures nice contact for measuring temperatures on a flat surface.
- RTD Probe: This happens to be a highly robust RTD form. This contains an RTD element, which has been mounted in a metal tube, which is also called sheath that safeguards the element from the environment.
FAQs
- Why should one be using an RTD rather than a thermistor sensor or a thermocouple? All types of temperature sensors have specific conditions for which they are ideal. As far as RTDs are concerned, with a wide temperature range up to even 850 degrees Celsius, they could be utilised in most of the industrial processes, even in the high-temperature ones. When an RTD is made with the metals like platinum, these become highly stable, and then they aren’t damaged much by oxidation/corrosion. Other materials including copper, nickel-iron alloy, or just nickel too could be utilised for RTDs. But such materials aren’t used generally as they are having low temperature ability, and they aren’t quite stable also or have repeatable capability as that is there in platinum. So, RTDs provide many benefits, some of which are as follows:
- Lengthy stability
- Nice interchangeability
- Fine accuracy (even better than a thermocouple)
- Vast temperature range (from around -200 degree Celsius to 850 degrees Celsius)
Liquid’s Temperature Measurement Using an RTD
Sensors that are probe-type are most commonly utilised to measure liquids. These could be quite simple ones having connection heads along with transmitters. Out of these, a widely popular sensor is the quick-disconnect one. It could be utilised as is, with compression fitting for installing flexibly. While doing the measurement of temperature in very harsh environmental conditions like that in plating baths or like in high-pressure systems, these sensors could be covered materials such as PFA Teflon® or contained in a protective housing of thermowell for protecting it against any harsh conditions.
Measuring Air & Gas Temperatures Using RTD Sensors
Measuring air & gas flow is quite a challenge in itself as the rate at which temperature transfers from liquids to sensors tends to be quite slow in the case of liquids. This is why, sensors especially made to be used in air/gas place the sensing element in close proximity to the environment. Such sensors tend to allow the sensing element to be in almost direct contact with the air flow. Such a design is quite popular for measuring temperature of the air in labs, clean rooms, & other such places because the housing design includes elements, which allow air to flow past the element. If such a situation needs some more protection for the sensors, an alternative is to utilise a structure quite like the RTD-860. Such a model has a small-diameter sensor having a mounting flange. The set reacts a little slower to alterations in the air flow, however, it provides better protection for the sensors.
Measuring Surface Temperatures
Measuring surface temperature could possibly be a very intricate thing to do properly & accurately. There’re several styles to make the selection, as per how anyone wishes to mount the sensors, how sensitive the sensor should be to any temperature alterations, and whether or not the installation would be permanent. Some of these are quite speedy & highly accurate responding surface RTDs.
RTD Glossary
- RTD: RTD is the abbreviation for resistance-temperature-detector/device. These indicators work on the principle that the electrical resistance of the conductor alters with the temperature.
- RTD Element: Part of an RTD sensor that could be built mostly of nickel, platinum, or copper material.
- RTD Probe: This happens to be an assembly consisting of an element, a sheath or housing, a wire & a terminal/connection.
- Platinum RTDs: Even called Pt RTDs, these are usually the most linear, most stable, most repeatable and highly accurate of all of the other RTDs.
- Thin Film RTD: These consist of a thin layer of base metal, which has been embedded in a ceramic substrate & cut for achieving the wished resistance.
- Class A RTD: Highest tolerance & accuracy of RTD elements, Class A (IEC-751), alpha = 0.00385
- Class B RTD: The most common of the RTD element tolerance & accuracy, Class B (IEC-751), alpha = 0.00385
- Aplha .00385 curve: The European curve corresponds to the tolerance of the “0.1% DIN” standard & conforms to the DIN 3760-standard.
- Sheath: The casing, a tube with a closed end, secures the element, and protects it against moisture & the medium to be measured. This also gives protection as well as stability to the transition wires from the sensitive element wires.
Buy an RTD in India – Contact Heatcon
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