Industrial Web Search Buyer's Guide Sector
Devices that detect and report physical conditions such as presence, position, pressure, temperature, level, and flow for monitoring and controlling machines. This sector includes discrete sensors, process instruments, thermowells, seals, signal interfaces, and hazardous-area protection.
Overview
A working orientation to the sector before you compare specific devices: how the category divides, what actually determines whether a sensor will work, and the kinds of company you will end up talking to.
The first thing to settle is which side of the sector your need falls on, because the two halves are bought differently. Discrete sensors for factory automation, such as inductive, capacitive, photoelectric, and ultrasonic devices, detect whether something is present or where it is located, are selected from catalogs, shipped from stock, and specified by a short list of parameters: sensing distance, output type, housing, and connector. Process instruments for plants, such as pressure, temperature, level, and flow transmitters, measure how much of something there is, are configured to order against the process conditions and the mechanical interface, and are specified on a form that runs to a page.
A buyer looking for a proximity switch and a buyer looking for a pressure transmitter are in the same sector but will talk to different people and wait different lengths of time. A third family, analytical instruments, measures what a fluid contains rather than how much of it there is: pH, conductivity, dissolved oxygen, moisture, and gas composition. They are sourced from the same process instrumentation manufacturers but are selected based on sample handling, sensor chemistry, calibration standards, and consumables; this page covers them at the supplier level only. Machine vision and motion feedback devices such as encoders belong to their own sectors and are covered there.
What determines whether a sensor will work is rarely the sensing principle. It is the medium and the installation. The target material, the process fluid, the temperature at the sensing point, the washdown or weather the housing will see, and whether the location is a classified hazardous area narrow the field far more than any performance figure, and they are the details most often missing from a request for quotation. Accuracy matters, but the headline figure on a datasheet is measured at reference conditions, and the total error at yours can be several times larger, so what you actually need to decide is how much uncertainty your process can tolerate. The signal comes last: the output type or protocol has to match the control system input, and in a plant with instruments from many makers, how a device behaves when it fails matters as much as how it behaves when it works. One more boundary is worth drawing before you request quotes: the instrument is often not the whole measurement. Thermowells, diaphragm seals, safety barriers, masters, power supplies, and installation can all sit outside the instrument quote, and finding out which of them do is part of comparing quotes at all.
Four kinds of company supply this sector. Broad-line sensor manufacturers offer discrete and general-purpose devices in large catalogs and sell mainly through distributors and high-service catalog houses, which carry stock and can cross-reference one maker's part with another's. Process instrumentation manufacturers make transmitters and analytical instruments configured to order, sell through their own sales offices and manufacturers' representatives, and provide the sizing, material selection, and hazardous-area engineering that accompany them. System integrators and panel builders buy instruments on your behalf as part of a control system and take responsibility for the loop, not just the device. Calibration laboratories and service companies provide the certificates, repairs, and recalibrations that quality systems require throughout the instrument's life. Which one you need determines whether lead time is counted in days or weeks, who engineers the application, and who you call when the reading is wrong.
Sourcing Considerations
The decisions below are the ones that most often cause regret later. The detail sits in the guides at the bottom of this page.
When selecting a sensor, clearly specify what is being measured, the medium, the range, and the temperature and pressure at the sensing point. For discrete sensors, include details about the target: its material, size, surface, and detection distance. This crucial information is often omitted and essential for proper device selection, helping reduce returns due to incorrect applications.
Every sensing principle has limitations: Inductive sensors only detect metal, capacitive sensors struggle with buildup, photoelectric sensors rely on color and surface, while ultrasonic sensors can be misled by foam and angles. RTDs have a maximum temperature, and thermocouples have a minimum accuracy. Identify what disqualifies each technology from your application, eliminate those options, and then compare the remaining options based on range, accuracy, and price.
The accuracy figure on a datasheet is based on reference conditions and does not account for variations in temperature, pressure, voltage, or time. Determine the measurement uncertainty your process can tolerate, then ask suppliers for total performance under your specific conditions and compare. Repeatability and drift are often more critical than reference accuracy, especially for hard-to-reach instruments.
Washdown, outdoor exposure, vibration, corrosive atmosphere, and hazardous area classification significantly limit options. Determine the required enclosure rating for your site, specifying the system and number, as IP codes and NEMA types do not directly correspond. If in a classified hazardous area, obtain the classification from the owner before making selections, as protection methods and certification rules depend on it and cannot be modified later.
Specify the output type for your input cards, such as PNP, NPN, 4-20 mA (with or without HART), IO-Link, or network protocol, along with supply voltage, connector, and cable. For process instruments, clarify fail-safe signal levels and diagnostics to ensure devices from different makers behave consistently when failing. A correct measurement on the wrong signal means the instrument cannot connect. And where the integrated loop, not just the device, must be guaranteed, that is the system integrator route described above rather than a device purchase.
Determine whether you need a manufacturer's certificate or an accredited calibration certificate, and check whether material and hazardous area certificates must accompany the device. Consider how often the instrument needs calibration based on drift and accessibility. Choose devices that meet interchangeability standards for easy replacement and document the configuration for setup. Remember, the cost of an instrument is mainly tied to maintaining trust in its accuracy.
Glossary
The terms you will meet on a datasheet, a specification form, or a supplier quote, in plain English.
The analog current signal most process instruments use to report a measurement, where 4 milliamperes represents the bottom of the calibrated range and 20 the top. Because a healthy loop never reads zero, a broken wire is detectable, and because the signal is current rather than voltage, it is immune to cable length and resistance. It remains the default process signal and the reference against which digital alternatives are compared.
The maximum difference between the instrument's reading and the true value under stated reference conditions, usually expressed as a percentage of span or of reading. Manufacturers state it differently, and the headline figure often excludes temperature effects, static pressure effects, and drift, so two instruments with the same accuracy claim can perform very differently in service. Ask for the total performance at your conditions, meaning the combined error once temperature, static pressure, and other influence effects at your operating point are added, not the reference figure.
A document stating the measured error of a specific instrument against a reference at a set of points, with the reference traceable to a national standard. A manufacturer's certificate describes the instrument as shipped; an accredited laboratory certificate carries the uncertainty analysis some quality systems require. Specify which you need and whether it must be accredited, because they differ in cost and in what an auditor will accept.
A noncontact sensor that detects the presence of an object by the change in capacitance it causes, which allows it to sense nonmetallic materials such as plastics, liquids, and powders, often through a thin nonmetallic wall. Its sensitivity to the target's dielectric properties and to buildup on the face makes installation and adjustment more critical than with an inductive sensor.
A flexible membrane assembly fitted between the process and a pressure instrument, with a fill fluid transmitting the pressure, so the instrument never contacts the process. It protects against corrosive, viscous, hot, or sanitary media at the cost of some accuracy and temperature sensitivity, and the seal, fill fluid, and instrument have to be specified together.
The difference between two pressures measured at two points, as distinct from a single pressure referenced to atmospheric or vacuum pressure. Differential pressure transmitters are the basis of most orifice and venturi flow measurement and of many level measurements, so a buyer may need one without having a pressure problem at all.
A protection method for hazardous locations in which the enclosure is built to contain an internal explosion and cool escaping gases below the ignition temperature of the surrounding atmosphere. Known as flameproof outside North America. It allows normal power levels inside the enclosure but requires specific conduit sealing and cannot be opened while energized, which affects maintenance practice as well as purchase.
A defined output value, outside the normal measurement range, that an instrument drives when it detects its own failure, so that the control system can distinguish a fault from a real low or high reading. The standardized convention for these levels is published by NAMUR, and specifying it ensures that instruments from different makers behave the same way when they fail.
A digital communication protocol superimposed on a 4-20 mA signal, allowing configuration, diagnostics, and secondary variables to pass over the same two wires as the analog measurement. Maintained by the FieldComm Group. It is the most widely installed digital capability in process instrumentation, and a buyer gets it by specifying it, since not every 4-20 mA instrument includes it.
The formal designation of an area where flammable gases, vapors, or dusts may be present, determined by the owner and used to decide what equipment may be installed there. North America uses Classes, Divisions, and Groups under the National Electrical Code; most of the rest of the world uses Zones under the IEC system. The classification must be known before any sensor for the area can be selected: the classification describes the location, while the protection method and its certification describe the equipment permitted in it, and both halves must match.
The difference in a sensor's output or switching point when the input is rising versus falling. In a discrete sensor, the gap between the switch-on and switch-off points prevents chatter when a target sits at the edge of range. In an analog instrument, it is a component of error. Either way, it is a datasheet figure a buyer should read before comparing accuracy claims.
A noncontact sensor that detects metal objects by the change they cause in an oscillating electromagnetic field at its face. It is the most common discrete sensor in factory automation, insensitive to dirt and moisture, but it detects only metal, and its sensing distance depends on the target's size and material; nonferrous metals are detected at reduced range unless a model rated for them is chosen.
A two-digit code defined by the International Electrotechnical Commission describing how well an enclosure resists solid objects and water, with the first digit for solids and the second for liquids. It is the international counterpart of the North American NEMA enclosure type system, and the two do not convert exactly, so a specification should name the system and the rating rather than assume equivalence.
A protection method for hazardous locations that limits the electrical and thermal energy available in the field circuit to below what can ignite the specified atmosphere, using a barrier or isolator in the safe area. It permits live working and simple wiring but requires the whole loop, including barrier and cable parameters, to be engineered as a system rather than bought as a device.
A point-to-point digital interface for sensors and actuators standardized as IEC 61131-9, using standard three-wire sensor cable and connectors to carry process data, parameters, and diagnostics between a device and a master. It replaces a hardwired discrete or analog connection with a configurable one, and a buyer specifying it needs compatible masters and devices.
The two kinds of level instruments. A point-level switch reports whether material is present at a specific height and is used for alarms and pump control. A continuous-level transmitter reports the level over a range and is used for inventory and control. They use different technologies and are often both present on a single vessel; confusing one for the other is a common specification error.
A noncontact sensor that detects an object by the interruption or reflection of a light beam, available in through-beam, retroreflective, and diffuse modes with different range, reliability, and mounting requirements. It senses almost any material at longer range than proximity sensors, and the mode and the target's color and surface decide whether it will work, so these belong in the specification.
The two wiring conventions for a three-wire discrete sensor describe whether the output switches the positive supply (PNP, sourcing) or the negative supply (NPN, sinking) to the load. The control system input determines which is required; the two are not interchangeable, so the output type is a mandatory line item in any discrete sensor specification.
The mechanical interface between an instrument and the pipe, vessel, or machine it measures, such as a threaded port, a flange, or a hygienic clamp fitting. It must match what is already installed or being built, and for hygienic and high-pressure services it is governed by its own standards, so it is specified by type, size, and standard rather than left to the supplier.
The controlled laboratory conditions, defined by the manufacturer or by the test standard the datasheet cites, under which an instrument's stated accuracy was measured: a set ambient temperature, supply voltage, mounting position, and span. Every departure from them in your installation adds error to the headline figure, which is why the influence-effect lines further down the datasheet matter as much as the accuracy line.
The ratio between the largest and smallest values an instrument can measure to its stated accuracy, sometimes called turndown. A transmitter with high rangeability can be re-ranged in the field to a narrower span without loss of accuracy, allowing one model to cover several duties and reducing spares, but only if the accuracy statement holds across the turndown.
The closeness of agreement between successive readings of the same value under the same conditions. It is distinct from accuracy, and for many control and detection duties it matters more, since a control loop can correct a constant offset but not random variation. A discrete sensor's repeatability is the consistency of its switching point.
A temperature sensor that uses the predictable change in electrical resistance of a metal, usually platinum, with temperature. It is more accurate and stable than a thermocouple over moderate ranges and is interchangeable between manufacturers when built to the international standard, but it is slower, more fragile, and has a lower maximum temperature.
The time an instrument takes to reflect a change in the measured variable, often stated as the time to reach a defined fraction of the final value after a step change. It matters for fast control loops and for safety functions, and it is affected by accessories such as thermowells and diaphragm seals as much as by the instrument itself.
A rating from one to four, defined in international functional safety standards, describing the risk reduction a safety instrumented function must achieve. Instruments used in such functions need either certification to the standard or documented prior use. A SIL requirement comes from the owner's safety analysis, and asking a supplier for a SIL-rated device without such a requirement is common and usually unnecessary.
The distance at which a proximity sensor reliably detects a standard target, stated by the manufacturer as a nominal value under defined conditions. Real targets that are smaller, thinner, or made of different material reduce it, and temperature and supply voltage add tolerance, so the usable distance is less than the nominal figure and the difference must be allowed for in the mounting design.
A temperature sensor made from two dissimilar metal wires joined at the measuring point, producing a small voltage that depends on temperature. It is rugged, fast, inexpensive, and usable up to very high temperatures, but it is less accurate and less stable than an RTD and requires matching extension wire and cold-junction compensation, which are included with the purchase.
A closed tube inserted into the process into which a temperature sensor is fitted, so the sensor can be removed without opening the process. It protects the sensor and adds measurement lag, and because it is a pressure-retaining part exposed to flow, it must be specified for material, dimensions, and process connection, and may require a vibration calculation.
An instrument that converts a sensor's raw signal into a standardized, conditioned output suitable for a control system, typically 4-20 mA with or without a digital protocol. The distinction matters because a sensor alone may need separate signal conditioning, and because accuracy, rangeability, and diagnostics are properties of the transmitter rather than the sensing element.
A noncontact sensor that measures distance or detects presence by timing the echo of a sound pulse, unaffected by the target's color or transparency. It suits level and distance measurement on varied targets but is affected by temperature, air movement, foam, and soft or angled surfaces, which limit its applicability.
The parts of an instrument that contact the process fluid, including the sensing element, process connection, seals, and any fill fluid. They must be compatible with the medium at the operating temperature and, for hygienic or regulated services, with the applicable material standards. A specification that names wetted materials by grade avoids the most common cause of early instrument failure.
Standards
What each standard governs and why a buyer should care. Which ones apply depends on what you are measuring, where the device is installed, what it connects to, and who has jurisdiction.
Specification, performance, and interchangeability
Published by the International Society of Automation (ISA). The standard for instrumentation and control symbols and identification, defining the tag numbers and symbols used on piping and instrumentation diagrams to identify every measurement and control function in a process plant. It applies to any project where instruments are specified from drawings, which in process industries is nearly all of them. The tag on the drawing tells the supplier what the instrument measures and does, and a buyer who can read it can check that what was quoted matches what the drawing asked for.
Published by the International Society of Automation (ISA). Standardized specification forms for instruments, primary elements, and control valves, listing the data a supplier needs to select and quote each instrument type. It applies whenever you want a complete and consistent request for quotation, and it is the fastest way to avoid the back-and-forth that an incomplete specification causes. Completing the form for your instrument type before contacting suppliers will surface the process data you have not yet established.
Published by the International Electrotechnical Commission (IEC). Reference conditions and test procedures for evaluating the performance of industrial process measurement transmitters, with parts covering pressure, temperature, flow, and level transmitters. It supersedes the IEC 60770 series, which older specifications and datasheets still cite, and relates to the IEC 61298 series on general performance evaluation methods. It applies when you want accuracy and influence-effect figures from different manufacturers to be comparable, because a figure stated to the standard was measured consistently. If a datasheet cites the old number, ask whether the test was the same.
Published by the International Electrotechnical Commission (IEC), with a harmonized UL edition in the United States. The standard for proximity switches within the low-voltage switchgear and controlgear series, defining how inductive, capacitive, ultrasonic, and photoelectric proximity sensors are rated, tested, and marked, including the standard target and conditions behind the nominal sensing distance. It applies to the discrete sensors used in factory automation. Sensing distance and switching frequency figures are only comparable between makers when measured to this standard, and this standard forms the basis of the electrical compatibility that allows a sensor from one maker to replace another.
IEC 60751 and IEC 60584 are published by the International Electrotechnical Commission (IEC); ASTM E230 is published by ASTM International. IEC 60751 defines the resistance-versus-temperature relationship and tolerance classes for industrial platinum RTDs, and IEC 60584 defines the voltage-versus-temperature reference functions and tolerance classes for standardized thermocouple types. ASTM E230 is the North American specification for thermocouple tables and tolerances, using different class names and, in places, different limits from the IEC standard. They apply to every purchase of a temperature sensor. Specifying the sensor type, the governing standard, and the tolerance class is what makes sensors interchangeable between makers without recalibrating the loop, and the IEC and ASTM class names are not interchangeable shorthand for one another.
Enclosures, hazardous locations, and electrical safety
IEC 60529 is published by the International Electrotechnical Commission (IEC); NEMA 250 is published by the National Electrical Manufacturers Association (NEMA), with related UL enclosure standards in the United States. IEC 60529 defines the ingress protection code describing resistance to solids and water. NEMA 250 defines enclosure type numbers covering dust, water, ice, corrosion, and whether the rating applies indoors, outdoors, or in hazardous locations. They apply to every sensor and instrument housing. The two systems overlap but do not convert exactly, and a NEMA type often carries conditions, such as corrosion resistance, that no IP code implies, so name the system and the rating you require rather than accepting a stated equivalent.
IEC 60079 is published by the International Electrotechnical Commission (IEC). The ATEX Directive is issued by the European Parliament and Council of the European Union. IECEx is a certification scheme operated by the IEC. The 60079 series defines the construction and testing of electrical equipment for explosive atmospheres, including the protection methods of intrinsic safety, flameproof enclosure, increased safety, and protection by enclosure for dust. ATEX is the legal requirement for placing such equipment on the market in the European Union and European Economic Area, evidenced by CE marking. IECEx is a voluntary international scheme, accepted by regulators in many countries outside Europe, that issues publicly searchable certificates to the same standards. They apply when the sensor will be installed in a classified hazardous location outside North America, or when you buy from a supplier certified under these systems. Certification to ATEX or IECEx does not by itself satisfy North American requirements.
NFPA 70 is published by the National Fire Protection Association. Certification is issued by Nationally Recognized Testing Laboratories such as UL Solutions, FM Approvals, and CSA Group to UL, FM, and CSA standards, including the UL editions of the IEC 60079 series. The code articles define the North American system for hazardous locations, using Class, Division, and Group in the traditional system and Zones in the alternative system, and require equipment to be listed for the location. They apply to any sensor installed in a classified area in the United States, with the Canadian Electrical Code governing in Canada. Equipment must carry a listing for the specific class, division or zone, group, and temperature code of the location, and the installation requirements that go with the protection method are as binding as the device rating. Code editions are revised on a cycle, and adoption varies by jurisdiction, so confirm the applicable edition with your authority.
Published by the International Electrotechnical Commission (IEC), with harmonized UL and CSA editions in North America. IEC 61010-1 sets safety requirements for electrical equipment for measurement, control, and laboratory use, covering electric shock, fire, and mechanical hazards. IEC 61326-1 sets electromagnetic compatibility requirements for the same equipment, defining the emissions limits and the immunity to interference the instrument must demonstrate. They apply to most instruments and transmitters sold into industrial markets and are the basis of CE marking for instruments in Europe. A buyer installing instruments near variable-frequency drives, welders, or radio equipment should ask which immunity level the instrument was tested to, because the standard allows multiple levels.
Functional safety, communication, calibration, and security
Published by the International Electrotechnical Commission (IEC). IEC 61511 is adopted in the United States by the International Society of Automation as ISA-84, which the ISA standard now follows. IEC 61508 is the general functional safety standard for electrical and electronic safety systems, defining safety integrity levels and the requirements for equipment to achieve them. IEC 61511 applies those requirements to safety instrumented systems in the process industries. They apply when a sensor will be part of a safety instrumented function identified in the owner's hazard analysis, in which case it requires either IEC 61508 certification or documented justification for prior use. The SIL requirement comes from the owner's analysis rather than from the supplier, and specifying SIL-certified instruments where no safety function exists adds cost and documentation for no benefit.
Maintained by the FieldComm Group, an industry foundation. The protocol specification for digital communication superimposed on a 4-20 mA signal, allowing configuration, diagnostics, and additional variables to pass over the existing two-wire loop, together with a registration program for compliant devices. It applies to process transmitters and positioners where you want digital access without replacing the loop wiring or the analog input cards. Registration of the specific device confirms interoperability with other makers' configuration tools and host systems, so ask whether the model quoted is registered rather than whether it supports the protocol.
Standardized by the International Electrotechnical Commission (IEC) as IEC 61131-9 and managed by the IO-Link Community within PROFIBUS and PROFINET International. The specification for a single-drop digital interface between a master and a sensor or actuator over standard three-wire sensor cable, carrying process data, parameters, and diagnostics, with conformance testing of devices and masters. It applies to discrete and analog factory automation sensors where you want parameter setting, identification, and diagnostics from the control system rather than at the device. Devices and masters from different makers interoperate when both are conformance-tested, and the device description file is included with what you are buying.
Published by NAMUR, the User Association of Automation Technology in Process Industries, based in Germany. NE 43 defines the standardized current levels a 4-20 mA transmitter uses to signal its own failure, above or below the measurement range, so that a host can distinguish a device fault from a real reading. NE 107 defines four diagnostic status categories for field devices to ensure that diagnostic information from different manufacturers is presented consistently. They apply when you are building or extending a process control system and want instruments from several suppliers to behave the same way on failure. Ask whether the model quoted supports both, and state the required behavior in the specification, because where supported it is usually configurable.
Published jointly by the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC). The standard for the competence of testing and calibration laboratories, under which accreditation bodies assess and accredit laboratories for a defined scope of measurements and uncertainties. It applies when your quality system, your customer, or a regulator requires calibration certificates from an accredited laboratory rather than from a manufacturer. An accredited certificate carries a stated uncertainty and a traceability chain to national standards. Specify whether you need accredited calibration at purchase, because it is priced separately and adds lead time.
Published jointly by the International Society of Automation (ISA) and the International Electrotechnical Commission (IEC). The standard series for cybersecurity of industrial automation and control systems, with parts addressing the security development lifecycle of products and the technical security requirements of components. It applies to networked instruments, IO-Link masters, wireless sensors, and any device with a configuration interface reachable from a network, and to the procurement requirements many owners write around it. Asking whether a device was developed under a certified secure development lifecycle is a reasonable question for any connected instrument and a required one in regulated and critical infrastructure sectors.
Frequently Asked Questions
Direct answers to the questions buyers ask most often when sourcing in this sector.
A sensor is the element that responds to the physical quantity, such as a platinum resistance element for temperature or a diaphragm for pressure. A transmitter is the electronics that convert the sensor's raw signal into a standardized output a control system can use, typically 4-20 mA with or without a digital protocol, and that provide the calibrated range, linearization, compensation, and diagnostics. Many products combine both in a single housing and are loosely called sensors, but the accuracy, range, and fail-safe behavior you read in a datasheet belong to the transmitter. When you buy a bare sensor, someone has to supply the signal conditioning, and when you buy a transmitter, you need to specify the sensor type it will be matched to.
Specify what your control system can accept and what you need from the device. A 4-20 mA loop is universal, simple, and fault-detectable, and for a single measured value feeding an analog input, it remains the default in process plants. HART adds configuration, diagnostics, and extra variables over the same two wires without changing the loop, which is why it is the common choice for process transmitters. IO-Link replaces discrete and analog sensor wiring in factory automation with a point-to-point digital connection that carries parameters and diagnostics, but it requires compatible masters and a control system integration. Fieldbus and industrial Ethernet instruments suit systems already built around those networks. The wrong choice is the one your input cards, your engineering team, and your maintenance staff cannot support.
It usually refers to the reference accuracy: the maximum error under the manufacturer's reference conditions, over a defined span, excluding the effects of ambient temperature, static pressure, supply voltage, mounting, and time. Each of those effects has its own figure elsewhere on the datasheet, and the total error at your operating conditions can be several times the headline number. Accuracy is also stated on different bases, as a percentage of span, of reading, or of upper range limit, which are not comparable without conversion. Decide what measurement uncertainty your process can tolerate, then ask the supplier for the total performance at your span, your ambient range, and your pressure, and compare those figures between makers rather than the reference accuracy.
Start from the target and the environment rather than the sensor. Inductive sensors detect only metal at short range and are the most tolerant of dirt, oil, and moisture, making them the default for machine position sensing. Capacitive sensors detect almost any material, including liquids and powders, through nonmetallic walls, but they are sensitive to buildup and to changes in the target's properties. Photoelectric sensors detect most materials at longer ranges and in several beam modes, with performance depending on the target's color, surface, and transparency, as well as on ambient light. Ultrasonic sensors measure distance to varied targets regardless of color but are affected by temperature, air movement, foam, and soft or angled surfaces. Once the technology is chosen, the sensing distance, output type, housing, and connector are determined by the mounting and the control system.
Use an RTD where accuracy, stability, and interchangeability matter and the temperature is within its range, which covers most process and HVAC measurement. Use a thermocouple where the temperature is too high for an RTD, where the sensor must be very small or very fast, where it will be subjected to vibration or shock, or where the cost of many points matters more than the accuracy of each. RTDs built to the international standard are interchangeable between manufacturers within a stated tolerance class, and thermocouples are interchangeable by type and class but require matching extension wire and cold-junction compensation in the receiving instrument. In either case, specify the type, the governing standard, the tolerance class, and the temperature range over which that tolerance must hold.
They are two systems describing how well an enclosure keeps out solids and liquids, one international and one North American, and they do not translate exactly. The IP code from the International Electrotechnical Commission has two digits, the first for solids and the second for water, and says nothing about corrosion, ice, or oil. NEMA enclosure types address additional conditions and indicate whether the rating applies indoors or outdoors, so a NEMA 4X housing, for example, carries a corrosion requirement that no IP code implies. A manufacturer's stated equivalence is a convenience rather than a certification. Specify the system your site or your customer uses, and if you need both, say so.
Both are recognized protection methods for classified locations, and the choice depends on the area classification, the power the device needs, and how it will be maintained. Intrinsic safety limits the energy in the field circuit so it cannot ignite the atmosphere, permits live working and simple wiring, and suits low-power devices such as most sensors and transmitters, but the whole loop including the safety barrier and cable parameters must be engineered as a system. Explosion-proof, called flameproof outside North America, contains an internal ignition within a heavy enclosure, allows normal power levels, and requires conduit seals and de-energizing before opening. Your area classification, established by the owner, determines what is permitted, and the device must be listed or certified for that specific class, division or zone, group, and temperature code under the system that applies where it is installed.
A safety integrity level is a rating from 1 to 4, defined in functional safety standards, that describes the level of risk reduction a safety instrumented function must deliver. An instrument needs a SIL capability only if it is part of such a function, which is identified by the owner's hazard and risk analysis, not by the supplier or by the type of measurement. If your sensor is used for control, monitoring, or alarm without a formal safety function, it does not need a SIL rating, and specifying one adds cost and documentation without benefit. If it is part of a safety function, it needs either certification to the functional safety standard or a documented justification for prior use, and the safety requirements specification, not the datasheet, determines what is acceptable.
What is being measured, in what medium, over what range, with what accuracy and response time requirement. The process conditions at the measuring point: temperature, pressure, and for discrete sensors the target material, size, and distance. The environment: ambient temperature, washdown or outdoor exposure, vibration, and whether the location is a classified hazardous area and, if so, to what classification. The mechanical interface: process connection type, size, and standard, or the mounting arrangement. The electrical interface: supply voltage, output type or protocol, connector or cable, and the control system input it will feed. And the documentation: calibration certificates, material certificates, hazardous area certification, and any SIL or hygienic requirement. The items most often missing are the area classification and the process connection, both of which can entirely change the product.
Buyer's Guides
In-depth guides covering the decisions above.
Buyer's Guide
How to specify from the measurement rather than the catalog: accuracy against repeatability, output and integration, then proximity, pressure, temperature and level in turn.
Read the guideMore coming
Additional guides are added when there is something genuinely worth saying, not on a schedule. IWS is committed to providing educational content to help you find the right suppliers!
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