Industrial Temperature Sensors: Basics of Thermocouples

Industrial thermocouples
(courtesy of Applied Sensor Technology)

Thermocouples are the most widely used industrial temperature sensor found in industrial processes today. They are rugged, relatively inexpensive to manufacture, and provide fairly good accuracy.

Thermocouples operate on the "Seebeck Effect", which is the phenomena whereby two dissimilar metal conductors (wires), joined at two points, with one point kept at a known constant temperature, produce a measurable voltage difference between the two conductors.

Thermocouple types - such a type J, type K, type R, and type S - refer to the alloy combinations used for the conductors and are based on standardized color designations. 

Thermocouples are used widely in industrial processes in industries such as power generation, primary metals, pulp and paper, petro-chemical, and OEM equipment. They can be fabricated in protective wells, and can be housed in general purpose, water-tight, or explosion-proof housings.

The following video provides a basic visual understanding of thermocouple wire, how a T/C junction is determined, and also discusses thermocouple connectors, polarity and some aspects of construction (such as grounded vs. ungrounded vs. open tip).

Pressure and Temperature Transmitters/Switches - Safety Right Out of the Box

UEC Safety Transmitter

Many process safety experts are looking for sustainable ways to help their personnel improve their safety critical loops, do it in the most cost-effective way possible, and with a minimum of complexity. The problem is the traditional approaches to deploying a full blown safety system are expensive and very complex, and still may not deliver the needed risk reduction for some safety critical systems and loops.

In the sensor subsystem for example, United Electric’s certified safety transmitter for pressure or temperature has opened up a new, less costly, less complex path for designers, I&C engineers, and maintenance personnel. It has something very unique. In addition to a 4-20 mA output, is has an embedded programmable high-capacity relay which exida has certified as a safety variable output. Now you have a device that provides designers the option of a hard wired trip in less than 100 milliseconds, with a tenth of a percent repeatability, while still providing the monitoring functions of a traditional continuous analog output.

For equipment under control, like pumps and compressors that require protection, or processes where rapid excursions can initiate dangerous events, this unique pressure and temperature transmitter, (certified for use in SIL2 safety instrumented functions, with SIL3 capability)  is addressing process safety time constraints, coupling issues with PLC and DCS’s, and adding diversity to the safety instrumented function.

The safety transmitter has a safe area fraction of 98.6% with breakthrough, automatic, self diagnostics and is one-third the cost of typical certified process transmitters.

Series One Safety Transmitter, Pressure and Temperature Switch from Ives Equipment

An Introduction to Industrial Pressure, Differential Pressure, and Temperature Switches

Pressure switch with large diaphragm

Most industrial applications require the monitoring of pressure and temperature of a process. Pressure and temperature measurement can be accomplished either by transmitters, gauges or by switches.
This post will provide a quick introduction of industrial electromechanical pressure switches and temperature switches.

An industrial pressure and temperature switch is made up of the three main components: 1) the sensor, 2) the housing and 3) the switching element.

The correct combination of each component assures proper application of the device for its intended use.

Sensor

The sensor is located above the pressure port and process connection. For pressure and differential pressure switches, there are several varieties of pressure sensors to choose.  The most common types of pressure sensors are:

Metal Bellows - an accordion-like device that provides linear expansion and contraction based upon the application of pressure or vacuum. Bellows are excellent sensors because they provide good overall pressure range and are fairly sensitive to small changes in pressure.

Piston - A rod and o-ring combination that moves linearly in direct response to applied pressure. Piston sensors are normally only applied to only very high pressure ranges. They have very small surface areas and wide deadbands (the change in pressure required to change the position of the switch output).

Pressure switch with piston sensor

Diaphragm - A thin, elastomer or metallic membrane, often with a rolled lip that allows for greater movement. The diaphragm has a large surface area and provides the most sensitivity to pressure change, making it ideal for low to mid-range pressure sensing.

Housing

Housings are classified and selected based on the atmosphere in which they’ll be used. Housing ratings are classified by several national and international agencies such as NEMA and CENELEC. Very generally put, housings can be rated as general purpose, dust & water resistant, water tight, corrosion resistant and hazardous (explosive) environments. Proper selection of the housing is important to the operation and life expectancy of the device. In hazardous environments, proper selection is absolutely critical. If unsure about the housing classification, consultation with an applications expert is required.

Switching Element

The switching element refers to the signaling device inside the enclosure that responds to the movement of the sensor. It can be either electrical or pneumatic, and provides an on-off signal (as opposed to an analog, or proportional signal produced by transmitters).

Differential pressure switch

The switching element is most times a “micro” type single pole, double throw (SPDT) electrical switch. These microswitches come in many configurations and electrical ratings, such as double pole, double throw (DPDT), 120/240 VAC, 12VDC, 24VDC, and hermetically sealed.

For the switching element and the sensor, it is very important to know the cycling rate (number of on vs. off times over a period of time) the instrument will see. Since both of these elements are mechanical, they will eventually wear out and need to be replaced. Switches are an economical and strong performing choice for low to medium cycle rates. For extremely high cycle rates, the use of solid state transmitters are a better choice.

Temperature switch

Temperature Switches

An electromechanical temperature switch (sometimes called a thermostat) is, for the most part, a piston type pressure switch connected to an oil filled capillary and bulb sensing element. The thermal expansion of the oil inside the bulb and capillary creates the pressure and linear movement upon the piston sensor of the switch. The bulb and capillary elements can be supplied in copper or stainless steel, and at various lengths.

There are many more details to selecting and applying electromechanical pressure and temperature switches. This post is only intended to provide a very general introduction. It is always suggested to discuss your application with a qualified applications engineer so that you are assured to get the longest lasting, most economical and safest instrument possible.

Basics of Differential Flow Devices

Orifice plate flow meter
(courtesy of Siemens)

The differential flow meter is the most common device for measuring fluid flow through pipes. Flow rates and pressure differential of fluids, such as gases vapors and liquids, are explored using the orifice plate flow meter in the video below.

The differential flow meter, whether Venturi tube, flow nozzle, or orifice plate style, is an in line instrument that is installed between two pipe flanges.

The orifice plate flow meter is comprised the circular metal disc with a specific hole diameter that reduces the fluid flow in the pipe. Pressure taps are added on each side at the orifice plate to measure the pressure differential.

According to the Laws of Conservation of Energy, the fluid entering the pipe must equal the mass leaving the pipe during the same period of time. The velocity of the fluid leaving the orifice is greater than the velocity of the fluid entering the orifice. Applying Bernoulli's principle, the increased fluid velocity results in a decrease in pressure.

As the fluid flow rate increases through the pipe, back pressure on the incoming side increases due to the restriction of flow created by the orifice plate.

The pressure of the fluid at the downstream side at the orifice plate is less than the incoming side due to the accelerated flow.

With a known differential pressure and velocity of the fluid, the volume metric flow rate can be determined. The flow rate “Q”, of a fluid through an orifice plate increases in proportion to the square root the pressure difference on each side multiplied by the K factor. For example if the differential pressure increases by 14 PSI with the K factor of one, the flow rate is increased by 3.74.

A Clean in Place (CIP) Mag Flow Meter with Pasteurized Milk Ordinance (PMO) Approvals

Magmeter with PMO approval
(courtesy of SIEMENS)

Dairies have been hampered by a limited selection of flowmeters to process raw ingredients and maximize productivity. Faced with limited choices for instrumentation, the Food and Beverage Industry is always interested in new products and certification. In this case, the product is a "Clean in Place" (CIP) electromagnetic flow meter (Magmeter) with Pasteurized Milk Ordinance (PMO) approvals.

Clean-in-place (CIP) is a method of cleaning the interior surfaces of process equipment, pipes, vessels, and fittings, without disassembly. This is an invaluable technology in the dairy, brewery, beverage, processed foods, cosmetics, and pharmaceutical industries by providing a cleaning process which is faster, far less labor-intensive, more consistent, and with less chemical exposure to workers.

The Pasteurized Milk Ordinance, is published by the Food and Drug Administration further defines minimum standards and requirements for Grade A milk production and processing.

A Magmeter is an excellent flowmeter choice for dairy use because it is unaffected by suspended solids, viscosity, and temperature challenges typically found in food and beverage applications. Additionally, magnetic flowmeters provide:

• Ease of installation with Tri-clamp fittings.
• Stainless steel, obstruction less flow performance meets all sanitary requirements and is 3A certified.
• Suitable for CIP and SIP cleaning.
• IP67 I NEMA 4X rating that is upgradeable to IP68 /NEMA 6P.

Carrying a PMO approval provides the dairy with confidence and assurance that the magnetic flowmeters have been tested and approved precisely for use in their plant.

For more information, see the document below:

A Clean in Place (CIP) Mag Flow Meter with Pasteurized Milk Ordinance (PMO) Approvals from Ives Equipment

Remote Sensing of Gases Directly in the Process

Electric Power Plant

Industrial operations, whether for direct process control or emissions compliance monitoring, have a need for accurate, reliable measurement of specific gas concentrations within a flowing medium. Tunable diode laser spectroscopy, packaged for industrial use, provides a number of substantially positive attributes.

• Rapid measurement.
• Can be focused on a specific component of interest.
• Multi-channel operation provides analysis of several components.
• In situ installation can provide direct measurements within a stack, pipe, or duct without sample handling or conditioning.
• Can measure NH3, HF, HCl, H2O, CO, CO2.
• Internal reference cell provides long term stability.
• Some models have continuous automatic calibration.

Siemens manufactures a line of tunable diode laser gas analyzers for industrial applications. In the company's own words, here is a basic description of how it works.

As a tunable diode laser-based technology this in-situ device enables high-performance measurements. The sensors (transmitter and receiver) are meant to be mounted directly on the process with no need of sampling systems. Laser light is sent from the transmitter, passing through the process gas, arriving at the detector on the receiver side. The measurements are carried out on-line with a very short response time permitting fast and effective cost-savings in process control. The laser characteristics allow single-line spectroscopy free of interferences. Since the band width of the laser light is extremely narrow, only the gas component of interest will interact with it. Other process influences, such as dust and temperature, are easily eliminated due to the excellent inherent compensation capabilities of this technique.

There is application assistance and more detailed information available from knowledgeable sales engineers in all localities. Combine your process mastery with their product application resources to meet the challenges posed by modern industrial process operation.

Sitrans Tunable Diode Laser Gas Analyzer from Ives Equipment

Basic Programming for the SITRANS F M Electromagnetic Flowmeter

The video below explains the basics of programming the SITRANS F M electromagnetic flowmeter.

The SITRANS F M MAG 5000 & 6000 are microprocessor-based transmitters engineered for high performance, easy installation, commissioning and maintenance. These transmitters are truly robust, cost-effective and suitable for all-round applications. The MAG 5000 has a measuring accuracy of ± 0.4% of the flow rate (incl. sensor), while the MAG 6000 has a measuring accuracy of ± 0.2% of the flow rate and can be fitted with optional plug-in communication modules.

Other Members of the SITRANS F M MAG Family:

SITRANS F M MAG 1100 is a wafer design sensor in stainless steel with highly resistant liners and electrodes and is designed for the general industry environment. The flangeless wafer design meets all flange standards. The SITRANS F M MAG 1100 is used in all industries where the corrosion-resistant stainless steel housing and the highly resistant liner and electrodes fit even the most extreme process media.

The SITRANS F M MAG 1100 F sensor is especially designed for the food & beverage and pharmaceutical industries and is available with hygienic and flexible process connections. It meets all sanitary requirements and is 3A certified and EHEDG approved.

The SITRANS F M  MAG 5100 W  with its patented liners of hard rubber NBR or ebonite and EPDM is a sensor for all water applications such as ground water, drinking water, cooling water, waste water, sewage or sludge applications.

SITRANS F M MAG 3100 is an electromagnetic flow sensor with a large variety of liners, electrode material and with grounding electrodes as standard, all this ensures a perfect fit for almost every flow application.
Also, measuring electrodes which are capable of withstanding the most extreme processes and various liners are available.

The SITRANS F M  MAG 3100 P sensor  is designed to meet the most common specifications within the chemical and process industries. It has PTFE or PFA liners and Hastelloy electrodes being the ideal combination. The fully welded construction provides a ruggedness that fits almost every flow application.

The SITRANS F M MAG 6000 I and the SITRANS F M MAG 6000 I Ex de transmitters have an measuring accuracy of + 0.2% of the flow rate and are designed to meet the demands of the process industry. Both versions are based on a microprocessor with a built-in alphanumeric display.