Industrial Fluid Temperature Monitoring - An Overview
Continuing with our Instrumentation 101 blog series, let's delve next into temperature monitoring for industrial fluid applications. Virtually all automated processes are influenced by temperature, directly or indirectly, making temperature management critical for reliable process control. In practical engineering, temperature monitoring typically serves three main purposes – safety, quality, and integrity – which makes it crucial for process engineers to fully understand the many variations and nuances involved with fluid temperature sensors.
Though it may sound a bit unscientific, temperature is defined as the physical quantity that expresses a substance's hotness or coldness. All substances are made up of atoms, and these atoms vibrate with kinetic energy at all times. Higher levels of atomic vibrations translate into an object being considered hot, and lower levels of atomic vibrations are considered cold. In this way, measuring a substance's temperature is really measuring its average kinetic energy using standardized scales of degrees Celsius (°C), Fahrenheit (°F), and Kelvin (°K).
Temperature Sensor Styles and Variations
Temperature sensors come in a wide variety of flavors, both in terms of their components and their measuring technologies as we’ll explain here:
- Temperature Sensor – this generic term refers to any device that can sense and communicate temperature values, and this term can be used for any of the more specific instruments described below.
- Temperature Gauge - gauges are typically manual or non-automated devices that measure and display a readout of the temperature in a system. Gauges are the most basic types of instruments used to measure temperature, but must be visually read by an operator standing within sight of the gauge's display.
- Temperature Switch - switching instruments are used to open and close electrical circuits when the measured process values achieve their desired setpoints. Switches are available in either mechanical or solid-state designs, which can be used to turn separate devices on and off or provide an automatic status signal to a larger control system.
- Temperature Transducer - transducers are electronic devices that directly convert a physical quantity into an analog electronic signal (often 0-10 volts). Transducers are common with pressure and vacuum monitoring but not with temperature measurement, as there are more viable methods of producing a temperature signal by measuring an intermediate change in resistance or current.
Temperature Transmitter - temperature transmitters produce an analog voltage or current output that is scaled to represent the functional temperature range of a system, such as by calibrating a temperature span of 0-100°F to an electrical current range of 4-20mA.
There are several different element designs used across industrial temperature sensors, each with its own balance of technical capabilities and cost:
Thermocouple – these sensors employ a scientific principle in which exposing two connected, dissimilar conductors to heat generates an electrical voltage that correlates with temperature. While inexpensive, thermocouples are not highly accurate.
Thermistor – with thermistor sensors, an electrical current is ran through composite semiconductor materials such as manganese, copper, and nickel, and the resistance experienced in the circuit is used to calculate a temperature value. Thermistors are inexpensive and suitable for low-end, low-accuracy applications (typically up to 130°C in 10-Ohm steps).
RTD – Resistance Temperature Detectors (RTDs) operate on the same resistance/temperature relationship described above with thermistors, but with much higher accuracy. RTDs use fine, pure metal wires (usually platinum) to produce a known resistance at given temperatures which are used to calculate precise temperature values. RTDs are more costly due to their higher precision (often up to 600°C in <1 Ohm steps) as well as their need for more resilient housings to protect their sensitive internal components.
Infrared Sensors - an infrared temperature sensor infers a temperature value by measuring infrared radiation emitted from an object. Infrared sensors are not as accurate as the direct-contact sensors above, but can handle a much broader set of applications since they do not need to make contact with the measured materials. This convenience tends to make infrared sensors relatively expensive, and best suited for troubleshooting and diagnostic uses instead of real-time process control.
Inline Temperature Sensor Applications
In fluid applications involving liquid and gas medias, temperature sensors serve a critical role in monitoring conditions so that ideal parameters are maintained and unsafe scenarios are avoided. Common application examples include:
- Manual Process Monitoring
- Industrial Manufacturing such as for building materials, OEM equipment, thermoplastics, metalworks, semiconductors, and food & beverage products.
- Commercial Systems such as facility HVAC, hydronic hot water, kitchen air exhaust, food refrigeration, and fire protection services.
- Fluid Process Controls such as pasteurization, evaporation, tempering, chemical reactions, blending and batching, and sterilization.
- Safety Systems such as those found on ovens, space heaters, combustion engines, refrigeration packages, data center cooling infrastructure, and emergency medical equipment.
Key Considerations when Selecting Temperature Sensors
When selecting temperature monitoring instrumentation, there are many aspects to consider that collectively deliver a reliable solution. Every application is different, so buyers must weigh each consideration against their specific safety, quality, and integrity needs, including:
- Temperature Rating - the first decision to make when selecting temperature sensors is the instrument's rating, which should extend past the application’s operating range to include a reasonable buffer for normal system fluctuations.
- Spike and Peak Ratings - very often, fluid systems experience temperature spikes beyond their normal operating range caused by minor system upsets or turbulent startup conditions. Sensors should be purchased with temperature ratings that can tolerate these spikes without risk of failure.
- Pressure Rating - pressure and temperature are directly correlated in fluid processes, and sensors should be purchased considering the peak pressure rating that would occur at the system's peak temperature.
- Enclosure Rating - a sensor’s enclosure rating describes what level of protection its housing will provide to the internal electronics. Enclosure ratings are typically stated in either US (NEMA) or European (IP) standards. For example, a NEMA 4X / IP 66 enclosure is rated to withstand strong jets of water, dust, and corrosion.
- Accuracy and Repeatability - no instrument is perfectly accurate and will always produce a reading that has some variance to the actual process condition being measured. A temperature sensor's accuracy describes the expected deviation from the setpoint it may read (such as +/- 0.5% of span). Repeatability describes the expected variance in temperature readings that trigger successive switches (such as +/- 1°F).
- Housing Material - a temperature sensor’s housing materials are selected for compatibility with the environment and incidental media that it will be exposed to, and have direct bearing on the instrument’s overall cost and lifespan. Common options include stainless steel types 304 and 316, zinc-plated steel, powder coated steel, aluminum, bronze, and many plastics.
- Wetted Material - wetted materials are the portions of a temperature sensor that physically contact the fluid being measured, (EG the sensor’s probe sheath or thermowell, seal, and piping connection). Wetted materials must be selected for compatibility with the process fluid being monitored. Common options include stainless steel types 304 and 316, ceramic, rubbers, plastics, brass, zinc-plated steel, and silicon.
- Electrical Connection - the type of wiring connection selected with a temperature sensor directly impacts the sensor’s installation, maintenance, and reliability. Generally speaking, sensors can be permanently hardwired (this option being more reliable) or connected with a quick-disconnect wiring adaptor (more convenient).
- Fluid Connection - a sensor’s fluid connection is the physical part of the sensor that will be installed to contact the media being measured, and must be selected for compatibility with the fluid system’s design standards. Common fluid connection types include imperial pipe threads (NPT), metric pipe threads (BSP), flanged, flared (VCR, JIC), and O-ring threads (SAE). Use of a thermowell is highly recommended, which is a hollow tube that the temperature sensor's probe slides into, protecting the sensor from direct material contact and making inline replacement possible without needing to shut down the process.
- Cycle Time - a switch’s cycle time states how often the sensor can switch from an open to a closed state in a given period of time, typically read in the format of “five cycles per minute”. Cycle time requirements are driven by how often a system is expected to change states in normal operation. Selecting too low of a cycle time can result in missed switching events and premature switch failure.
- Certifications and Approvals - there are usually multiple regulatory and third-party technical requirements governing any given application, all of which serve to align minimum performance, safety, and functional expectations across devices and components. Sensors are often expected to carry UL or CSA approvals at minimum, and may be subject to additional requirements specific to the application.
As a veteran-owned small business, Whitman Controls is dedicated to supplying premium quality, reliable, technologically advanced instrumentation for use in nearly any application. Our Bristol, CT manufacturing facility embodies over 40 years of engineering, fabrication, and customer service expertise, serving both end-user and manufacturing customers nationwide through direct and distribution channels.
Our values drive us to provide the highest level of servant partnership that you can find. To discuss your applications or to learn more about our capabilities, please contact us at (800) 233-4401, via email at [email protected], or online at www.whitmancontrols.com.
