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. 

Thermal cutoff switch

Thermal cutoff switch 

       Thermal switch is an electromechanical device which opens and closes the contacts to control the flow of electrical current in response to temperature change. The term “Thermal Cutoff Switch” generally refers to how the switch is used, ie. It cuts off the current to critical machinery when a temperature limit is exceeded preventing potential burn out or failure.  The applications of, and need for electromechanical thermal switch devices are broad and cover a huge diversity of industrial applications. In their most basic form, they can be found on home appliances such as dryers for over temp protection.  Thermal cutoff switches are also used widely in sophisticated industrial equipment as well as commercial jetliners. There are a number of different technologies used to implement a thermal cutoff switch that rely on expanding elements to provide the movement to open or close contacts, including vapor filled, rod and tube, Bimetal, and  bimetallic disc to name a few.

Applications

        Thermal switches are specified as cutoff switches because they represent a straightforward approach to shutting down a system if a critical temperature is reached. The simplicity of an electromechanical thermal switch is what makes this approach so desirable to designers, as they are passive devices which require no power, and will reliably change state at the specified set point. 

Applications include:

a) Plastics extruder barrel overtemp detection.
b) Brake overtemp indication
c) Engine cooling fan control d) Clutch overtemp in escalators
e) Bleed air overtemp indication on aircraft environmental control systems f) Window defrost overtemp on military vehicles
g) Overtemp in refinery process
h) Avionics overtemp on aircraft avionics
i) Gas shut-off  flame detection on railroad switch de-icing
j) Flame detection in aircraft engines.

Analyzing and Processing the Malfunction of Control Valve

The control channel of the valve is installed as shown in Fig. 1, DCS output
4–20 mA control command that send to the positioner though local CR box. The
positioner and position sensor form a closed control loop. The positioner receiving
setpoint from DCS. At the same time, it acquires valve position feedback signal
from sensor, then after internal PID operation output signal to drive the actuator
control valve actuation.
Positioner electronic module consist of a multiplexer, an A/D converter, a D/A
converter, a temperature sensors, a Hall-effect position sensor (or a slide rheostat), a
pressure sensor, a microprocessor and a power distribution management circuit. The
diagram of functional block.

Pressure Transducer

Pressure Transducer

A pressure transducer, often called a pressure transmitter, is a transducer that converts pressure into an analog electrical signal. Although there are various types of pressure transducers, one of the most common is the strain-gage base transducer.

The conversion of pressure into an electrical signal is achieved by the physical deformation of strain gages which are bonded into the diaphragm of the pressure transducer and wired into a wheatstone bridge configuration. Pressure applied to the pressure transducer produces a deflection of the diaphragm which introduces strain to the gages. The strain will produce an electrical resistance change proportional to the pressure. 

The Electrical Output of Pressure Transducers

Pressure transducers are generally available with three types of electrical output; millivolt, amplified voltage and 4-20mA. 

Mass Flow Measurement

MASS FLOWMETER

Why is Mass Flow Measurement important within process industries and what are the strengths of Coriolis Flow Meters and Controllers?

Measurement of the flow of a fluid, either liquid or gas, is commonly a critical parameter in many processes. In most operations this can be linked to the basic “recipe” of the process – knowing that the right fluid is at the right place and the right time. Equally, it can be linked to asset management, keeping the fluid in motion or even simple tank balancing. Some applications, however, require the ability to conduct accurate flow measurements to such an extent that they influence product quality, Health & Safety, and ultimately can make the difference between making a profit or running at a loss. 

In other cases, the inaccurate measurement of flow, or even the failure to take such measurements, can cause serious or even disastrous results. With most liquid and gas flow measurement instruments, the flow rate is determined inferentially by measuring the fluid velocity or the change in kinetic energy. Velocity depends on the pressure differential that is forcing the fluid through a pipe or conduit. Because the pipe’s cross-sectional area is known and remains constant, the average velocity is an indication of the flow rate. The basic relationship for determining the liquid’s flow rate in such cases is:

Q = V x A 

Where 

Q = fluid flow through the pipe 

V = average velocity of the flow 

A = cross-sectional area of the pipe 

Other factors that affect the liquid flow rate include the liquid’s viscosity and density, and the friction of the liquid in contact with the pipe.

With the many variations of flowmeter technology available it can be very hard for an operator to make a decision on which technology is right for the application. Industry experts claim that a majority of flowmeters in the field is selected incorrectly. An important and perhaps overlooked question, is what the instrument is supposed to do versus what it is able to do? When selecting a flowmeter technological improvements can sometimes get overlooked through historical knowledge of what has been possible in the past – in a way, experience working against you. 

Direct mass flow measurement is an important development across the industry as it eliminates inaccuracies caused by the physical properties of the fluid, not least being the difference between mass and volumetric flow. Mass is not affected by changing temperature and pressure. This alone makes it important method of fluid flow measurement. Volumetric flow remains valid, in terms of accuracy, provided that the process conditions and calibration reference conditions are adhered to. Volumetric measuring devices, such as variable area meters and turbine flow meters, are unable to distinguish temperature or pressure changes. 

One method of Mass Flow measurement employs the phenomenon of the Coriolis force.  This force is a deflection of moving objects when they are viewed in a rotating reference frame. The Coriolis force is proportional to the rotation rate and the centrifugal force is proportional to its square. 

This long understood principle is all around us in the physical world; the flow of water down the sink, the Earth’s rotation and its effect on the weather. The principle, and mathematical formula developed back in the 1800’s, was further developed during the 1970’s and then applied to the measurement of fluid flow. The operating principle is basic but very effective. A tube, or tubes, with a known mass is energized by a fixed vibration. When a fluid passes through the tube(s) the mass will change, the tube(s) will twist and the inlet and outlet sections will result in a phase shift. This phase shift can be measured and a linear output derived proportional to flow. As this principle simply measures whatever is within the tube it can be directly applied to any fluid flowing through it, liquid or gas. Furthermore, in parallel with the phase shift in frequency between inlet and outlet it is also possible to measure the actual change in frequency. This change in frequency is in direct proportion to the density of the fluid – and a further signal output can be derived. Having measured both the mass flow rate and the density it is, interestingly, therefore, possible to derive the volume flow rate.

The Coriolis principle, applied as a mass flow meter, therefore, has its place within the fluid measurement and control within the traditional Process Industry. Perhaps more importantly though, the additional features of the technology allow for an extension of the accuracy and precision into other, more non-traditional, applications.  

Take, for example, filling and dosing applications across a great many industries and the replacement of both weighing scales and the gravimetric method. Traditionally, the dosage of mass/volume was achieved by using a shut-off valve with a weighing scale/balance. The weighing scale is located under a valve outlet nozzle and, after a zeroing procedure once the vessel being filled is in position, the valve will open. The weighing scale will send a signal to the PLC or control unit and, once the batch has been reached, the valve will close. Multiple dosing, building up a recipe, is achieved by moving the vessel to the next dosing point in the line and repeating the process. The alternative solution of simultaneous mass flow dosing/filling significantly reduces the amount of time needed, and the loss of volatiles, whilst increasing productivity, quality and repeatability. 

Another example of process improvement has been seen within the field of specialist chemicals. The customer was unaware that low to ultra-low flow control was possible with a Coriolis instrument resulting in the raw ingredient being mixed with water to create a carrier volume. This higher volume was then metered and dosed into the main product flow. The process added cost to the production method and, as the dilution step added variability to the concentration of the additive, product quality was often compromised with a resulting additional cost of re-work. Furthermore, the final process step saw the bulk material being heated and stirred to evaporate the added water to reduce volume and increase concentration. The energy requirement to do so was significant and the operational stock-holding was high. Further complications were added by the need for the “dosing system” to handle multiple additive doses with stringent cleaning needed between batches resulting in yet more wastage and high additional cost.    

By understanding the extended capabilities of Coriolis instruments it was possible to establish that the concentrated raw ingredient could be added via a highly accurate low flow Coriolis Flow Meter directly coupled and controlling a precision pump. This solution ensured that the costly addition and removal of the water could be eliminated and that very close tolerances on the dosage rate, and hence final product quality, could be maintained. The inclusion of multiple synchronous injection points eliminated the costly clean-down process and the reduction of working process volume also reduced the stock holding inventory further reducing operational costs. Reproducible product quality has been increased, productivity has been increased, wastage has been reduced, energy consumption has been reduced and operational costs have also been dramatically reduced. 

Although currently configured for control via the client DCS the Coriolis flow meter can, if needed, be “paired” with the main process line flow meter to act in master/slave mode. Standard on-board firmware can be utilized to immediately match the required dosage rate to any variability within the main flow line. This facility eliminates any time lag in process response and further enhances the very tight tolerances on product quality. A host of secondary benefits has also been utilized within the solution. The density of the concentrated natural raw ingredient is measured, recorded and trended thereby allowing tracking of the natural innate variability and further fine-tuning of the control process. The pump steering signal is utilized for condition monitoring and as a preventative maintenance tool. This, together with dry-running protection, will ensure less emergency break-down and catastrophic down-time. 

A further example illustrating where Coriolis flow technology can benefit the customer has been seen with the dosing of performance chemicals within the Oil & Gas Industry. The traditional method of chemical injection, a piston pump with check valves on the inlet and outlet, is tried and tested and works well for quite long periods of time.  However, on occasion the check valves can foul and begin to “pass”.  Also, out-gassing or entrained air can cause an air-lock within the piston chamber that is simply compressed/decompressed in situ rather than pumped. In each of these cases the pump appears to be still working but there is no actual transfer of chemical into the pipeline. The only way to verify actual flow has been via a graduated gauge and a stop-watch; an empirical measurement but time consuming.  

Another issue with the traditional method of injection is actually changing the flow rate. This can only be done manually by changing the stroke length of the piston – a process that is “trial and error” and only verifiable using the graduated gauge as above. Fine tuning of injection rates, for example to compensate for day/night changes in temperature across a field, is virtually impossible as the labour required to do so is prohibitive. This results in the injection rate being set for worst case thereby resulting in overdosing during normal conditions – a very expensive waste.   

Modern communications networks now allow for technology to arrive at diffuse production fields. The Coriolis flow system can be installed at each injection point and real-time monitoring, control and logging of injection rates can be achieved.  This allows for remote checking of flow rates, remote instantaneous re-setting of those flow rates, on-board auto-alarm for status checking (for example, empty tank alarm and pump protection shut down), density change alarm, single point totalisation, multi-point (total field) totalisation for cost per barrel calculations and pump steering signal monitoring as a guide to preventative maintenance. In short, a very powerful tool within field management.

With these applications it can be seen that Coriolis Flow Technology can be a benefit to the user especially when the extended product capabilities are employed. Process improvement, cost reduction, real-time measurement and greater accuracy can all be achieved.