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IMI Sensors – Industrial Monitoring Instrumentation You Must Know

Industrial monitoring instrumentation (IMI) sensors play a crucial role in measuring and controlling processes across various industries. This article provides an overview of key IMI sensor types, applications, selection considerations, and frequently asked questions.

Temperature Sensors

Temperature measurement and control is critical for product quality, system performance, and safety. Common temperature sensor types include:

Thermocouples

Thermocouples contain two conductors made of different metals fused at one end. This junction produces a small voltage relative to temperature:

  • Inexpensive compared to other sensors
  • Wide temperature ranges up to 2300°C
  • Low-cost options for fairly accurate temperature measurement
  • Available in different types—K, J, T, E, R, S, B, N

Resistance Temperature Detectors (RTDs)

RTDs contain a wire coil or film made of metals like platinum, copper or nickel. As temperature increases, electrical resistance increases linearly:

  • High accuracy with tight tolerance capability
  • Better repeatability and stability than thermocouples
  • Range capabilities from -200 °C to 850 °C
  • Common RTD types—Pt100, Pt1000, Ni120, Cu10

Thermistors

Thermistors contain semiconductor ceramics that alter resistance according to temperature fluctuation:

  • Inexpensive
  • Fast response times
  • Common as inrush current limiters and self-regulating heating elements
  • Can have high non-linearity requiring linearization

Table comparing different temperature sensor attributes:

SensorAccuracyRepeatabilityPriceResponse Time
ThermocoupleMediumMedium$Medium
RTDHighHigh$$Slow
ThermistorMedium/LowMedium$Fast

Infrared Thermometers

Infrared (IR) thermometers measure surface temperature from heat radiated without contact:

  • Non-contact temperature measurement
  • Capable of measuring moving surfaces
  • Prone to interference from external thermal influences
  • Typically offer fixed spot or line scanning

Integrated Circuit Sensors

Miniaturized integrated circuit sensors combine amplification, linearization, and temperature signal conditioning:

  • Built-in cold junction compensation for thermocouples
  • Linearized sensor output
  • Small physical size
  • Analog or digital output

Pressure Sensors

Pressure sensors measure and convert pressure forces into standardized electrical signals. Common types include:

Piezoelectric Pressure Sensors

Piezoelectric crystals produce proportional electrical charges under applied pressure:

  • For dynamic pressure measurement
  • Extreme precision and speed capabilities
  • Can only measure positive pressure change
  • Rugged and mechanical shock resistant construction
  • Bandwidths over 100 kHz

Capacitive Pressure Sensors

Use capacitive plates or diaphragms to detect pressure via dielectric medium deformation and capacitance change:

  • High sensitivty and accuracy potential
  • Expensive compared to other technologies
  • Capable of static and dynamic measurement
  • Vulnerable to high temperatures and shock/vibration damage

Strain Gauge Pressure Sensors

Detect pressure through deformation of strain gauges:

  • Most common pressure sensing technology
  • Low cost compared to others
  • Various pressure range capabilities
  • Difficult to protect from corrosion and moisture ingress

Piezoresistive Pressure Sensors

Piezoresistive elements alter conductive properties when mechanically stressed:

  • Low cost force and pressure sensing
  • Fast dynamic response times up to microseconds
  • Temperature sensitive requiring compensation
  • Vulnerable to damage from moisture and debris

Table comparing pressure sensor types:

SensorAccuracyBandwidthPriceRobustness
PiezoelectricHighestUp to 100 kHz$$$Excellent
CapacitiveHighUp to 10 kHz$$$Medium
Strain GaugeMediumUp to 1 kHz$Medium
PiezoresistiveMediumUp to 20+ kHz$$Low

Level Sensors

Level sensors measure and monitor liquids and other materials within a container or vessel. Models include:

Float Switches

A float containing magnets rises/falls with liquid level to activate a reed switch:

  • Inexpensive
  • Simple setup
  • Prone to scale buildup and debris fouling

Capacitive Level

Detects liquid level changes through capacitance measurement instead of physical contact:

  • No moving parts
  • Resistant to vibrations and pressure fluctuations
  • Difficult mounting through existing vessel tops

Differential Pressure

Infers level based on measured differences in hydrostatic pressure:

  • Simple electronic design
  • Low cost compared to other technologies
  • Requires impulse piping to liquid vessel
  • Subject to clogging

Ultrasonic Level

Uses reflected sound waves to determine liquid level and volume:

  • Works on various liquid types
  • Unaffected by electrical conductivity
  • Top-mounted for easy access and visibility
  • Expensive compared to other options

Guided Wave Radar

Propagates low-frequency radar wave through probe into liquid:

  • High accuracy and precision measurement
  • Capable in extreme pressure/temp applications
  • Difficult calibration and temperature compensation
  • Generally the most expensive solution

Table showing key attributes of level sensor technologies:

SensorAccuracyLocationMaintenancePrice
Float SwitchMediumInside vesselHigh$
CapacitiveHighOutside vesselLow$$
Differential PressureMediumOutside vesselMedium$$
UltrasonicHighAbove vesselLow$$$
Guided Wave RadarHighestInside vesselMedium$$$$

Flow Sensors

Flow sensors analyze, measure, and monitor flowing liquids, air, steam or gases. Instruments include:

Differential Pressure

Measure flow rate based on energy loss across an orifice plate:

  • Low capital cost and simple operation
  • Works across diverse fluids and pipe scales
  • Orifice plates prone to fouling and clogging

Magnetic Flow Meters

Use electromagnetic principles to measure conductive fluid velocities:

  • High accuracy and excellent repeatability
  • No moving parts or orifice constrictions
  • Only works on electrically conductive liquids

Coriolis Flow Meters

Precisely track mass flow using resonance frequencies in vibrating flow tubes:

  • Direct mass flow measurement capabilities
  • High accuracy and rangeability
  • Generally one of the most expensive technologies

Ultrasonic Flow Meters

Leverage Doppler shifts in ultrasonic waves reflecting through liquid:

  • High accuracy and repeatability
  • Bi-directional flow measurement
  • Requires particulates for wave reflection
  • Typically costly compared to other options

Vortex Flow Meters

Generate and measure fluid vortices for volumetric flow indication:

  • No moving parts or obstructions
  • Good accuracy across wide velocity ranges
  • Performance impacted by low flow velocities
  • Unable to measure mass flow

Comparison of key flow sensor attributes:

SensorAccuracyRangeabilityPriceRobustness
Differential PressureMedium10:1 typical$Excellent
MagneticHigh100:1+$$$Very Good
CoriolisHighest100:1+$$$$Limited
UltrasonicHigh50:1+$$$Good
VortexGood20:1+$$Very Good

Gas Detection Sensors

Gas monitoring is vital for safety, emissions control, and process optimization. Instruments include:

Infrared Gas Sensors

Leverage infrared light absorption characteristics of target gases:

  • Available for most common toxic gases
  • Typically very sensitive and stable
  • Can have interference issues in dirty environments

Electrochemical Gas Sensors

Use electrode reactions to quantitatively indicate target gases:

  • Offer ppm sensitivity levels
  • Simple and inexpensive
  • Prone to poisoning/contamination failure

Metal Oxide Semiconductor (MOS) Sensors

Alter conductive states based on reactions with oxidizing/reducing gases:

  • Respond to wide variety of reactive gases
  • Operate well at high temperatures
  • Lack specificity between gas types
  • Lifespans limited by operational duty cycles

Photoionization Detectors (PID)

Ionize chemical gases using ultraviolet light for detection:

  • Respond to diverse VOCs
  • Sub-ppm sensitivity
  • Unable to differentiate specific compounds
  • Require periodic UV lamp replacement

Catalytic Bead Sensors

Detect combustible gases via combustion on electrically heated pellistors:

  • Wide rangeability can reach 100% LEL
  • Low purchase and operating costs
  • Lack specificity between gas types

Table summarizing instrumentation for gas detection:

SensorRangeAccuracyPriceSelectivity
Infrared0-100% LELVery High$$Excellent
Electrochemicalppm levelsHigh$Good
Metal Oxideppm levelsMedium$Poor
Photoionizationppb-ppmHigh$$Medium
Catalytic Bead0-100% LELMedium$Poor

Process Analytical Instruments

Process analyzers provide real-time quantification of liquid/gas chemistries:

PH Instrumentation

PH measurement is critical for monitoring water quality, reactor processes, and biological processes. Options include:

  • Electrochemical electrodes – common for inline industrial measurement
  • Litmus paper – inexpensive spot sampling
  • Digital benchtop/handheld meters – laboratory analysis

PH control instrumentation includes chemical metering pumps, solenoid valves, and databases of automated dosing control algorithms.

Conductivity Measurement

Conductivity cells quantify ion concentrations within process liquids. This can identify:

  • Impurities in water feeds
  • Chemical reaction progression
  • Alkalinity and other water chemistry

Typical instrumentation incorporates contactless toroidal sensors or diagnostic tables for identifying specific chemical components based on conductivity.

Gas Chromatographs

Gas chromatographs utilize separation columns and compositional analysis to identify gas mixtures. Applications involve:

  • Natural gas, fuel, and feedstock purity
  • Quantifying levels of contaminants
  • Monitoring chemical reaction progress

Chromatographs offer excellent analytical precision but require skilled personnel for calibration and data interpretation.

Spectrophotometers

Spectrophotometric analyzers pass specific wavelengths of light through samples to quantify absorption characteristics. This identifies:

  • Chemical reaction completion
  • Presence of unsaturated organics and compounds
  • Component concentrations in mixtures

IMI Sensor Selection Considerations

Proper IMI sensor selection, installation, wiring, and maintenance ensures optimal performance. Considerations include:

Measurement Properties

  • Required engineering units and process ranges
  • Necessary accuracy and precision
  • Expected process variable dynamics

Environmental Factors

  • Process media types and conditions
  • Ambient temps, pressures, vibration
  • Exposure to moisture, debris, corrosion

Infrastructure Integration

  • Electrical outputs, communication interfacing
  • Physical footprint limitations
  • Mounting provisions, remote capabilities

Operational Realities

  • Maintenance access and intervals
  • Recalibration and replacement needs
  • Operator skill level and preferences

Budget Constraints

  • Initial capital outlay acceptable
  • Recurring ownership costs over lifetime
  • Installation, civil, and secondary infrastructure

Frequently Asked Questions

What are the different kinds of transmitters for IMI sensors?

Common transmitter outputs include 4-20 mA analog, HART digital communication, WirelessHART, Foundation Fieldbus, Profibus PA, and modular plug-in universal designs. This provides connectivity to control systems like PLCs and DCS platforms.

How do you install RTD temperature sensors?

RTD sensors require precise immersion depth control and thermal grease/compound to ensure proper heat transfer contact. 3 or 4-wire configurations compensate for lead resistance. Match sheath materials to environmental media compatibility.

How do I prevent corrosion failure in pressure sensors?

Choose compatible wetted parts materials for process fluids. Use chemical seals/diaphragms. Control fluid velocity/turbulence at sensor connections. Avoid fluid ingress through proper mounting orientation. Consider corrosion resistant coatings and surface treatments.

What flow sensors work with dirty liquids?

Differential pressure instruments tolerate considerable solids contamination when properly designed. Magnetic flow meters lack flow obstructions enabling particulate passage. Coriolis meters dynamically flex flow tubes to prevent accumulation when measuring slurries.

How often should gas detectors be calibrated?

Electrochemical and infrared sensor performance drifts over 6-12 month intervals depending on environmental factors and pollutant exposure. Recommended gas detector calibration frequency ranges from monthly to annually. Always adhere to manufacturer guidance for recalibration and replacement intervals.

 

 

 

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