Resistance Temperature Detectors (RTD)

What is an RTD?
       Resistance Temperature Detectors (RTDs) are temperature sensors that contain a resistor that changes resistance value as its temperature changes. They have been used for many years to measure temperature in laboratory and industrial processes, and have developed a reputation for accuracy, repeatability, and stability.

Why use an RTD instead of a thermocouple or thermistor sensor?
        Each type of temperature sensor has a particular set of conditions for which it is best suited.
RTDs offer several advantages:
 • A wide temperature range (approximately -200 to 850°C)
 • Good accuracy (better than thermocouples)
 • Good interchangeability
 • Long-term stability

        With a temperature range up to 850°C, RTDs can be used in all but the highest-temperature industrial processes. When made using metals such as platinum, they are very stable and are not affected by corrosion or oxidation.
        Other materials such as nickel, copper, and nickel-iron alloy have also been used for RTDs. However, these materials are not commonly used since they have lower temperature capabilities and are not as stable or repeatable as platinum.

RTD standards
        There are two standards for platinum RTDs: the European standard (also known as the DIN or IEC standard) and the American standard.
        The European standard, also known as the DIN or IEC standard, is considered the world-wide standard for platinum RTDs. This standard, DIN/IEC 60751 (or simply IEC751), requires the RTD to have an electrical resistance of 100.00 Ω at 0°C and a temperature coefficient of resistance (TCR) of 0.00385 Ω/Ω/°C between 0 and 100°C.
        There are four resistance tolerances for Thin Film RTDs specified in IEC60751:
Class C = ±(0.6 + 0.1*t)°C or 100.00 ±0.24 Ω at 0°C (-50 to 600°C)
Class B = ±(0.3 + 0.005*t)°C or 100.00 ±0.12 Ω at 0°C (-50 to 500°C)
Class A = ±(0.15 + 0.002*t)°C or 100.0 ±0.06 Ω at 0°C (-30 to 350°C)
Class AA (Formerly 1 ⁄3B) = ±(0.1 + 0.0017*t)°C or 100.0 ±0.04 Ω at 0°C (0 to 150°C)
        The combination of resistance tolerance and temperature coefficient define the resistance vs. temperature characteristics for the RTD sensor. The larger the element tolerance, the more the sensor will deviate from a generalized curve, and the more variation there will be from sensor to sensor (interchangeability). This is important to users who need to change or replace sensors and want to minimize interchangeability errors.

        The following interchangeability table shows how the tolerance and temperature coefficient affect the indicated temperature of the sensor in degrees Celsius:


       The American standard, used mostly in North America, has a resistance of 100.00 ±0.10 Ω at 0°C and a temperature coefficient of resistance (TCR) of 0.00392 Ω/Ω/°C nominal (between 0 and 100°C). Section Z also includes a resistance vs. temperature curve from -100 to 457°C, with resistance values given every one degree Celsius.

        RTD elements can also be purchased with resistances of 200, 500, 1000, and 2000 Ω at 0°C. These RTDs have the same temperature coefficients as previously described, but because of their higher resistances at 0°C, they provide more resistance change per degree, allowing for greater resolution.

RTD Element Construction Platinum 
         RTD elements are available in two types of constructions: thin film and wire wound.

Thin Film
        Thin-film RTD elements are produced by depositing a thin layer of platinum onto a substrate. A pattern is then created that provides an electrical circuit that is trimmed to provide a specific resistance. Lead wires are then attached and the element coated to protect the plat
inum film and wire connections.

Wire Wound
        RTD elements also come in wire-wound constructions. There are two types of wire-wound elements: those with coils of wire packaged inside a ceramic or glass tube (the most commonly used wire-wound construction), and those wound around a glass or ceramic core and covered with additional glass or ceramic material (used in more specialized applications).

Probe Construction 
       Once the RTD element is selected, the wiring and packaging requirements need to be determined. There are a number of ways to wire the sensors, along with an unlimited number of probe or sensor constructions to choose from.

Wiring Arrangement
       In order to measure temperature, the RTD element must be connected to some sort of monitoring or control equipment. Since the temperature measurement is based on the element resistance, any other resistance (lead wire resistance, connections, etc.) added to the circuit will result in measurement error. The four basic wiring methods are shown below.

Except for the 2-wire configuration, each of the above wiring arrangements allows the monitoring or control equipment to factor out the unwanted lead wire resistance and other resistances that occur in the circuit. Sensors using the 3-wire construction are the most common design, found in industrial process and monitoring applications. The lead wire resistance is factored out as long as all of the lead wires have the same resistance; otherwise, errors can result.

Sensors using the 4-wire construction are found in laboratories and other applications where very precise measurements are needed. The fourth wire allows the measuring equipment to factor out all of the lead wire and other unwanted resistance from the measurement circuit. In the 2-wire with loop construction, the sensor resistance measurement includes the lead wire resistance. The loop resistance is then measured and subtracted for the sensor resistance. The 2-wire construction is typically used only with high resistance sensors, when lead lengths will be very short, or when tight measurement accuracy is not required.

Wire Materials 
       When specifying the lead wire materials, care should be taken to select the right lead wires for the temperature and environment the sensor will be exposed to in service. When selecting lead wires, temperature is by far the primary consideration, however, physical properties such as abrasion resistance and water submersion characteristics can also be important. Below is a table listing the capabilities of the three most popular constructions:

Configuration
Once the RTD element, wire arrangement, and wire construction are selected, the physical construction of the sensor needs to be considered. The final sensor configuration will depend upon the application. Measuring the temperature of a liquid, a surface, or a gas stream requires different sensor configurations.
Liquid Measurements
      Probe-type sensor styles are normally used for measuring liquids. They can be as simple as our general purpose PR-10 and PR-11 constructions, or as involved as our PR-12, 14, 18, or 19—with connection heads and transmitters. A popular choice is the quick-disconnect sensor. This can be used as is, with compression fittings for flexible installation, or with our PRS plastic handle for a handheld probe.
      When measuring the temperature of harsh environments such as plating baths or highly pressurized systems, sensors can be coated with a material like PFA Teflon®, or they can be housed in a thermowell to protect the sensor from extreme conditions. Speak to our application engineers if you have any special measurement challenges.

Air and Gas Stream Measurements
        Air and gas stream measurements are a challenge because the rate of transfer of temperature from the fluid to the sensor is slower than for liquids. Therefore, sens
ors specifically designed for use in air or gas place the sensing element as close to the media as possible. With a housing design containing slots that allow the air to flow past the element, this construction is very popular in measuring air temperature in laboratories, clean rooms, and other locations. When the situation requires a little more protection for the sensor, an option is to use a design similar to the RTD-860. This design has a small diameter probe with a flange for mounting. The configuration will be a little slower to respond to changes in the air stream, but it will provide improved protection for the sensor.

Surface Temperature Measurements
       Surface measurements can be one of the most difficult to make accurately. There are a wide variety of styles to choose from, depending on how you want to attach the sensor, how sensitive to changes in temperature the sensor has to be, and whether the installation will be permanent. The most accurate and fastest-responding surface RTD is our SA1-RTD sensor.
When applied to a surface, it becomes virtually a part of the surface it is measuring. Surface sensors can also be bolted, screwed, glued, or cemented into place. The RTD-830 has a pre-machined hole in the housing to allow for easy installation with a #4 screw. The RTD- 850 has a housing with threaded tip that allows it to be installed into a standard #8-32 threaded hole. This RTD is handy for measuring the temperature of heat sinks or structures where screw holes may already exist.

Orifice Meter

The orifice plate flow meter is commonly used in clean liquid, gas, and steam services. It is available for all pipe sizes but it is very cost-effective for measuring flows in larger ones (over 6" diameter). The orifice plate is also approved by many organizations for custody transfer of liquids and gases. 

The orifice plate calculations used nowadays still differ from one another, although various organizations are working to adopt an universally accepted orifice flow equation. Orifice plate sizing programs usually allow the user to select the flow equation desired.

The orifice plate meter can be made of any material, although stainless steel is the most common. The thickness of the plate used ( 1/8-1/2") is a function of the line size, the process temperature, the pressure, and the differential pressure. The traditional orifice flow meter is a thin circular plate (with a tab for handling and for data), inserted into the pipeline between the two flanges of an orifice union. This method of installation is cost-effective, but it calls for a process shutdown whenever the plate is removed for maintenance or inspection. In contrast, an orifice fitting allows the orifice to be removed from the process without depressurizing the line and shutting down flow. In such fittings, the universal orifice plate, a circular plate with no tab, is used. 



The concentric orifice plate flow meter has a sharp (square-edged) concentric bore that provides an almost pure line contact between the plate and the fluid, with negligible friction drag at the boundary. The beta (or diameter) ratios of concentric orifice plates range from 0.25 to 0.75. The maximum velocity and minimum static pressure occurs at some 0.35 to 0.85 pipe diameters downstream from the orifice plate. That point is called the vena contracta. Measuring the differential pressure at a location close to the orifice plate minimizes the effect of pipe roughness, since friction has an effect on the fluid and the pipe wall. 

Flange taps are predominantly used in the United States and are located 1 inch from the orifice plate's surfaces (Figure 2-3). They are not recommended for use on pipelines under 2 inches in diameter. Corner taps are predominant in Europe for all sizes of pipe, and are used in the United States for pipes under 2 inches (Figure 2-3). With corner taps, the relatively small clearances represent a potential maintenance problem. Vena contracta taps (which are close to the radius taps, Figure 2-4) are located one pipe diameter upstream from the plate, and downstream at the point of vena contracta. This location varies (with beta ratio and Reynolds number) from 0.35D to 0.8D. 

The vena contracta taps provide the maximum pressure differential, but also the most noise. Additionally, if the plate is changed, it may require a change in the tap location. Also, in small pipes, the vena contracta might lie under a flange. Therefore, vena contracta taps normally are used only in pipe sizes exceeding six inches. 

Radius taps are similar to vena contracta taps, except the downstream tap is fixed at 0.5D from the orifice plate (Figure 2-3). Pipe taps are located 2.5 pipe diameters upstream and 8 diameters downstream from the orifice (Figure 2-3). They detect the smallest pressure difference and, because of the tap distance from the orifice, the effects of pipe roughness, dimensional inconsistencies, and, therefore, measurement errors are the greatest.

Orifice Types & Selection

The concentric orifice plate is recommended for clean liquids, gases, and steam flows when Reynolds numbers range from 20,000 to 10⁷ in pipes under six inches. Because the basic orifice flow equations assume that flow velocities are well below sonic, a different theoretical and computational approach is required if sonic velocities are expected. The minimum recommended Reynolds number for flow through an orifice (Figure 1-5) varies with the beta ratio of the orifice and with the pipe size. In larger size pipes, the minimum Reynolds number also rises.



Because of this minimum Reynolds number consideration, square-edged orifices are seldom used on viscous fluids. Quadrant-edged and conical orifice plates (Figure 2-5) are recommended when the Reynolds number is under 10,000. Flange taps, corner, and radius taps can all be used with quadrant-edged orifices, but only corner taps should be used with a conical orifice. 

Concentric orifice plates can be provided with drain holes to prevent buildup of entrained liquids in gas streams, or with vent holes for venting entrained gases from liquids (Figure 2-4A). The unmeasured flow passing through the vent or drain hole is usually less than 1% of the total flow if the hole diameter is less than 10% of the orifice bore. The effectiveness of vent/drain holes is limited, however, because they often plug up.

Concentric orifice plates meter are not recommended for multi-phase fluids in horizontal lines because the secondary phase can build up around the upstream edge of the plate. In extreme cases, this can clog the opening, or it can change the flow pattern, creating measurement error. Eccentric and segmental orifice plates are better suited for such applications. Concentric orifices are still preferred for multi-phase flows in vertical lines because accumulation of material is less likely and the sizing data for these plates is more reliable. 

The eccentric orifice flow meter is similar to the concentric except that the opening is offset from the pipe's centerline. The opening of the segmental orifice (Figure 2-4C) is a segment of a circle. If the secondary phase is a gas, the opening of an eccentric orifice will be located towards the top of the pipe. If the secondary phase is a liquid in a gas or a slurry in a liquid stream, the opening should be at the bottom of the pipe. The drainage area of the segmental orifice is greater than that of the eccentric orifice, and, therefore, it is preferred in applications with high proportions of the secondary phase. 

These plates meter are usually used in pipe sizes exceeding four inches in diameter, and must be carefully installed to make sure that no portion of the flange or gasket interferes with the opening. Flange taps are used with both types of plates, and are located in the quadrant opposite the opening for the eccentric orifice, in line with the maximum dam height for the segmental orifice.

For the measurement of low flow rates, a d/p cell with an integral orifice may be the best choice. In this design, the total process flow passes through the d/p cell, eliminating the need for lead lines. These are proprietary devices with little published data on their performance; their flow coefficients are based on actual laboratory calibrations. They are recommended for clean, single-phase fluids only because even small amounts of build-up will create significant measurement errors or will clog the unit.

Restriction orifices are installed to remove excess pressure and usually operate at sonic velocities with very small beta ratios. The pressure drop across a single restriction orifice should not exceed 500 psid because of plugging or galling. In multi-element restriction orifice installations, the plates are placed approximately one pipe diameter from one another in order to prevent pressure recovery between the plates.

Orifice Performance

Although it is a simple device, the orifice plate flow meter is, in principle, a precision instrument. Under ideal conditions, the inaccuracy of an orifice plate can be in the range of 0.75-1.5% AR. Orifice plates are, however, quite sensitive to a variety of error-inducing conditions. Precision in the bore calculations, the quality of the installation, and the condition of the plate itself determine total performance. Installation factors include tap location and condition, condition of the process pipe, adequacy of straight pipe runs, gasket interference, misalignment of pipe and orifice bores, and lead line design. Other adverse conditions include the dulling of the sharp edge or nicks caused by corrosion or erosion, warpage of the plate due to waterhammer and dirt, and grease or secondary phase deposits on either orifice surface. Any of the above conditions can change the orifice discharge coefficient by as much as 10%. In combination, these problems can be even more worrisome and the net effect unpredictable. Therefore, under average operating conditions, a typical orifice installation can be expected to have an overall inaccuracy in the range of 2 to 5% AR.

The typical custody-transfer grade orifice plate meter is more accurate because it can be calibrated in a testing laboratory and is provided with honed pipe sections, flow straighteners, senior orifice fittings, and temperature controlled enclosures.