The 2 Most Common Industrial Temperature Sensors

Thermocouple & RTDs come in many shapes,
styles, and configurations.

This blog post will introduce the reader to the two most common industrial temperature sensors - the thermocouple and RTD (resistance temperature detector).

A thermocouple is a temperature sensor that produces a micro-voltage from a phenomena called the Seebeck Effect. In simple terms, when the junction of two different (dissimilar) metals varies in temperature from a second junction (called the reference junction), a voltage is produced. When the reference junction temperature is known and maintained, the voltage produced by the sensing junction can be measured and directly applied to the change in the sensing junctions' temperature.

Diagram of Type K thermocouple circuit

Thermocouples are widely used for industrial and commercial temperate control because they are inexpensive, sufficiently accurate for many uses, have a nearly linear temperature-to-signal output curve, come in many “types” (different metal alloys) for many different temperature ranges, and are easily interchangeable. They require no external power to work and can be used in continuous temperature measurement applications from -185 Deg. Celsius (Type T) up to 1700 Deg. Celsius (Type B).

As a general rule, industrial thermocouples can be made to withstand higher temperatures and come in a wider variety of thermocouple types. MgO thermocouples are flexible and have wider selection of measurement junction configurations. An MgO thermocouple consists of a thermocouple element encased in a metal sheath and hard-packed with magnesium oxide mineral insulation. Thermocouple sheaths are fully annealed and can be formed into different configurations (minimum bend radius is twice the outer diameter of the sheath). The measuring junction can also be sealed from the environment, reducing the potential for contamination issues.

A typical industrial thermocouple is constructed using an element inserted into a process/environment specific protection tube. In certain configurations, it may be possible to replace the element without removing the protection tube from the process. There are endless combinations of elements and tubes, and are also available separately as replacement parts.

Standard industrial temperature sensor configuration
(either thermocouple or RTD)

Common application for thermocouples are industrial processes, the plastics industry, kilns, boilers, steel making, power generation, gas turbine exhaust and diesel engines, They also have many consumer uses such as temperature sensors in thermostats and flame sensors, and for consumer cooking and heating equipment.

RTD’s (resistance temperature detectors), are temperature sensors that measure a change in
resistance as the temperature of the RTD element changes. They are normally designed as a fine wire coiled around a bobbin (made of glass or ceramic), and inserted into a protective sheath. The can also be manufactured as a thin-film element with the pure metal deposited on a ceramic base much like a circuit on a circuit board.

Coil wound RTD element
(image courtesy of Wikipedia)

The RTD wire is usually a pure metal such as platinum, nickel or copper because these metals have a predictable change in resistance as the temperature changes. RTD’s offer considerably higher accuracy and repeatability than thermocouples and can be used up to 600 Deg. Celsius. They are most often used in biomedical applications, semiconductor processing and industrial applications where accuracy is important. Because they are made of pure metals, they tend to more costly than thermocouples. RTD’s do need to be supplied an excitation voltage from the control circuitry as well.

RTDs are resistive devices, so lead wire resistance directly affects its accuracy. The error can be quite large, depending on the lead wire resistance (measured in ohms / foot).For example, an uncompensated 2-wire circuit using 30 gauge wires can have an error as high as 1.2°F per foot!Fortunately, there is a method to compensate for the lead wire resistance.

2-wire - One lead wire is connected to each end of the element. This arrangement is suitable for uses where the lead wire resistance may be considered as a constant in the circuit, or where changes in the lead wire resistance due to ambient temperature changes can be ignored.

3-wire - This is the most common of RTD configurations. One lead wire is connected to one end of the element and two lead wires are connected to the other end.The purpose of the third lead is to compensate for the lead wire resistance, thereby increasing accuracy.An instrument capable of utilizing a 3- wire RTD must be used to benefit from this configuration.

4-wire - The most accurate of the RTD configurations, this element uses two wires for each end of the element. Building on the 3-wire concept, compensation is made for the resistance in each lead wire, creating a highly-accurate temperature-measurement device for critical applications.An instrument capable of utilizing a 4-wire RTD must be used to benefit from this configuration.

For more information on temperature sensors, contact:
Power Specialties, Inc.
www.powerspecialties.com
9118 E. 72nd Terrace
Raytown, MO 64133
Toll Free: (800) 432-6550
Phone: (816) 353-6550
Fax: (816) 353-1740

Magnetostrictive Level Transmitters: How They Work

Magnetostrictive level sensor
(courtesy of MTS)

Magnetostrictive level sensors are similar to float type sensors in that they both use a permanent magnet sealed inside a float which travels up and down a rod. The difference is the use of a magnetostrictive element sealed inside the rod.

Magnetostrictive level sensors provide continuous level measurement with high accuracy, and are very effective for use in a variety of liquids. In many applications they are used in conjunction with magnetic level gages (Mag Gage) to provide visual indication along with an analog or digital output.

Magnetostrictive level transmitters charge a magnetostrictive element with electric current. When the element's electromagnetic field intersects the float's magnetic field, a mechanical pulse is created. The pulse then travels back down the element at the speed of sound. Similar to ultrasonic or radar level transmitters, the distance is measured by time of flight, which corresponds to the distance from the sensor detecting the return pulse.

Here is a short video visually demonstrating the magnetostrictive principle:

Here is a more in-depth video demonstrating how a magnetostrictive level transmitter works:

Video created by Tony Kuphaldt and Bellingham Technical College and used here under Creative Commons Attribution 3.0 Unported license.

For more information on magnetostrictive level control, contact:

Power Specialties

www.powerspecialties.com

9118 E. 72nd Terrace
Raytown, MO 64133
Toll Free: (800) 432-6550
Phone: (816) 353-6550
Fax: (816) 353-1740

Vane Actuator Damper Drives Assist in Boiler Emissions Compliance

Vane actuator style damper drive
(courtesy of Rotork Type K)

As environmental concerns increase and legislation continues to tighten standards for reduction in known air pollutants, emissions from industrial boilers faces continual scrutiny. The recent Boiler Maximum Achievable Control Technology (MACT) Rule is a prime example. MACT requires annual or semi-annual check-ups of the USA’s 14,000+ boilers and process heaters to assure compliance with emission standards.

Airflow is one of the most critical components of boiler control ensuring for safe, reliable, and efficient operation.  Poor airflow control can result in toxic gas production, inefficient burning, erratic cycling, and flame outs. More efficient combustion is key to lowering emissions, and the key to efficient combustion is optimizing the flow of combustion air and flue gas.

Dampers are used to control the quantity of air admitted to the furnace, and tighter boiler draft control significantly improves combustion. However, many of todays boilers employ old technologies, such as gear-laden, electric motor style damper drives that are not accurate or lack the required power to position the airflow dampers quickly or with repeatability.

Drop in pneumatic damper drive
(Type K)

Converting to pneumatic vane actuators is a proven method to provide high speed continuous modulation of ID (Induced draft) and FD (Forced Draft) fans, inlet guide vanes and secondary air dampers the accuracy and speed required for precise control. Replacing obsolete electric drives with fast acting pneumatic damper drives, driven by rotary vane actuators, offers significant improvements to burner management and cleaner performance.

The pneumatic vane actuator style damper drive is fabricated to deliver “drop-in-place” retrofit designs that precisely fits the application without any field engineering or additional fabrication. As part of the overall design strategy, existing connection rods, linkages and shaft accessories are be used without modification, resulting in quick change out and fast commissioning.

The performance of the vane actuator is key, providing 100% duty cycle, high torque outputs, and robust construction designed to handle hot, dirty and high-vibration environments. Vane actuators have long been known for their extreme long life and ability to withstand high cycle rates.

Tight control is provided by highly accurate, direct-mount pneumatic and electro-pneumatic positioners. A variety of optional network communication systems such as HART, Foundation Fieldbus, Profibus, and Modbus are also available.

Pneumatic vane actuator damper drives provide fast and accurate positioning, reduces overall operating air consumption, and saves cost by lowering fuel consumption. If you’re under pressure to improve boiler performance, and see improved burner management as part of the solution, upgrading your old gear-driven mechanical drives to drop-in vane actuators dampers drives will provide immediate improvements in efficiency and a substantial return on investment.

For more information on pneumatic damper drives, contact:
Power Specialties, Inc.
9118 E. 72nd Terrace
Raytown, MO 64133
Toll Free: (800) 432-6550
Phone: (816) 353-6550
Fax: (816) 353-1740

www.powerspecialties.com

Process Instrument Selection a Breeze with this Online Tool

Find process instruments fast!

Yokogawa Corporation of America, an industry recognized source for innovative process measurement and control products, has made available an easy to use product selection tool for those navigating through the company's extensive product offering. The Product Finder is a great time saver that enables a user to quickly locate product and technical information on Yokogawa products that meet the user's selected criteria.

Let's step through a quick example. You will see how this quick and easy to use tool saves time by navigating quickly to the website pages detailing products meeting your requirements.

The Product Finder is accessible through a number of links throughout Yokogawa's network of Reps. Clicking the link lands you on the start page of the Product Finder. For this example, I am going to search for a flow meter with the following characteristics:

• Mass flow measurement 
• Non-conductive liquid
• Accuracy of 1%
• Flow measurement device must have an integral transmitter
• Tri-clamp connections

Above, I declared my location as United States. The next step, shown below, is to select "Flow" as the measurement parameter. You will see in the drop down menu that there are many measurement elements that can be selected, with Yokogawa products for each.

My selection of "Flow" returns a list of all the company's flow measurement devices, of which there are many (this cropped screenshot, shown below, only shows four, but there were many more) . This is where the selector really helps you. Instead of examining several or many different models, the user can focus the search by adding more product characteristics. You can see the list of prompting questions on the left side of the page. Answering these will narrow the search results to the show only the products meeting all the criteria specified by the user.

The next image (below) shows selections of all  my sample product attributes entered on the left column. Note that there is now only a single product that matches all of my sample criteria. The whole process took less than two minutes. By clicking on the "View More Details" button below the product image, I gain access to all of the available technical, support, and product data for my selected flow measurement device.

For more help and personal assistance in Kansas, Missouri, Iowa and Nebraska, contact Power Specialties. Combine their product knowledge and expertise with your process know-how for the best solutions.

Fundamentals of pH Control in Industrial, Municipal, and Commercial Applications

Handheld pH Meter & Sensor
(courtesy of Yokogawa)

Analytical measurement and control of pH within a system is necessary for many processes. Common applications include food processing, wastewater treatment, pulp & paper production, HVAC, power generation, and chemical industries.

To maintain the desired pH level in a solution, a sensor is used to measure the pH value. If the pH is not at the desired set point, a reagent is applied to the solution. When a high alkaline level is detected in the solution, an acid is added to decrease the pH level. When a low alkaline level is detected in the solution, a base is added to increase the pH level. In both cases the corrective ingredients are called reagents.

Accurately applying the correct amount of reagent to an acid or base solution can be challenging due to the logarithmic characteristics a pH reaction in a solution. Implementing a closed-loop control system maintains the pH level within a certain range and minimizes the degree to which the solution becomes acidic or alkaline.

An example of an automatic pH level control system is a water treatment process where lime softened water is maintained at a pH of 9, using carbon dioxide as a reagent. As the untreated water (or influent) enters the tank, the pH is continuously monitored by the pH sensor. The sensor is the feedback device to the controller where the setpoint is compared to the control value. If the values are not equal, the controller sends a signal to the control valve that applies carbon dioxide to the tank. The reagent is applied to the tank at varying rates to precisely control the pH level. With the pH level at 11 detected by the sensor, the controller commands the control valve to open and introduce more carbon dioxide. As the increased carbon dioxide mixes with the influent, the pH is lowered in a controlled manner. Reaching the setpoint, the carbon dioxide flow is minimized and the process is continually monitored for variation. The effluent is the treated water that is discharged out of the tank. The process continues to provide the lime softened water at the desired pH level.

A Better Load Cell for Weighing Heated, Vibration Prone Process Vessels

Most load cells are designed to handle vertical force and cannot discern errors introduced from side loading and/or torque loading. In real world conditions, though, load cells see much more than vertical loading, and unfortunately, can output erroneous values. While they are excellent for static weighing situations, such as scales, load cells typically can’t handle the rigors of process vessel applications.

A case in point is a chemical manufacturer with an several, existing three cubic meter batching tanks. It was decided the tanks needed modifications to provide more accurate weighing of the individual ingredients. The existing load cells were experiencing errors due to thermal expansion of the vessel, and the resultant side loads from expansion. Additionally there was a problem with vibration in the plant. A better solution was needed, and whatever the solution would be, the customer made it clear the new weighing system must provide system accuracy in the range of ±0.1%.

Load cell performance problems causes.

BLH KIS sensor

An approach to deal mechanically with the thermal cycling while using the same type of load cell was discussed. It involved several mechanical modifications that required significant and costly structural changes.

Another suggestion was to evaluate a unique load cell design that was particularly tolerant against thermal expansion, vibration, and high lateral forces - the BLH Nobel KIS series.  The KIS load cell offered some obvious advantages over rebuilding the tanks supports and frame, namely time and expense. Beyond the short installation time and easy modification, the KIS also offers excellent reliability and accuracy.

BLH Instrument

The customer decided to “take the easy way out” and just replace the old, error-prone load cells with KIS load cells. Installation and start-up was very easy, taking very little time. After installation, the customer was pleasantly surprised by the high accuracy of the new KIS load cells, despite the thermal expansion of the vessel and the inherent vibration.

The video below gives an excellent view inside the KIS design and why its ideal for process vessels.

Basics of pH Measurement

pH of Common Items
(image courtesy of
Wikipedia)

pH is a numeric scale used to designate the acidity or alkalinity of an aqueous solution. Solutions with a pH below 7 are acidic and solutions with a pH higher than 7 are basic. Pure water is considered neutral and are neither an acid nor a base.

pH measurement is very important in many industries including medicine, chemistry, agriculture, food science, environmental science, chemical engineering, water & wastewater treatment, and many others.

pH electrodes and sensors are the sensing portions of a pH measurement. Various installation options including retractable, flow thru, immersion, and direct insertion. Proper pH electrode/sensor selection is critical for optimal measurement results.

2 wire transmitter (Yokogawa)

pH meters, analyzers and transmitters are used for continuous process monitoring of pH to ensure water/product quality, monitor effluent discharge, batch neutralization, pulp stock, scrubbers, cooling towers, chemical, water/wastewater treatment and many other applications.

Four Wire Analyzers: 4-wire analyzers separate the power supply from the measurement output. As a minimum we need two wires for the power supply and two wires for the current output of the analyzer.

Two Wire Analyzers: 2-wire analyzers combine the wires for current output, power supply, and digital communication.

The document below (courtesy of Yokogawa) provides a comprehensive understanding of pH/ORP measurement and how to achieve reliable results. Basic information on the principles of measuring pH/ORP, the construction of the sensing elements and their basic use in process applications are provided.

The Basics of pH Measurement from Power Specialties, Inc.

For more information, contact:

Power Specialties, Inc.

9118 E. 72nd Terrace
Raytown, MO 64133
Toll Free: (800) 432-6550
Phone: (816) 353-6550
Fax: (816) 353-1740

www.powerspecialties.com