Process Instrumentation: Flow, Level, Pressure and Temperature Measurement

Quick Answer

Instrument selection is driven by what the measurement is for — control, custody transfer, or safety — because the accuracy and reliability demanded differ by an order of magnitude. Flow: Coriolis measures true mass flow directly with accuracy around ±0.1-0.2% of rate and is independent of density, temperature and profile, making it the custody transfer and dosing standard; electromagnetic meters give ±0.2-0.5% on conductive liquids above about 5 µS/cm with no obstruction and no pressure loss; ultrasonic (transit-time) suits clean liquids and large bores, and clamp-on versions need no process penetration; orifice plates remain the cheapest option at ±1-2% with a permanent pressure loss of 40-80% of the differential generated. Level: non-contacting radar at ±2-10 mm is the modern default, guided wave radar handles foam, agitation and low dielectric, and differential pressure remains the workhorse where density is stable. Pressure: transmitters at ±0.04-0.1% of span, with diaphragm seals where the fluid would plug or freeze an impulse line. Temperature: Pt100 RTDs for -200 to 600°C at ±0.1-0.3°C, thermocouples for higher temperature and faster response at ±1-2°C. For safety instrumented functions, the instrument must have a certified SIL capability with documented failure rate data; a 4-20 mA transmitter without that documentation cannot be used in a SIL 2 loop regardless of how good it is.

A fertiliser plant in South Asia installed orifice plate flow meters on its ammonia dosing lines because they were in the standard instrument list and they were cheap. The dosing accuracy requirement was ±1%.

Orifice meters measure differential pressure, from which flow is inferred via the square root of ΔP. That square-root relationship means accuracy degrades sharply at low flow: a meter accurate to ±1% of full scale at 100% flow is accurate to ±4% of reading at 25% flow and ±10% at 10% flow. The dosing rate varied between 15% and 90% of design.

The plant was overdosing ammonia by an average of about 6% because the operators had learned not to trust the low-flow readings and had added margin. On their consumption that was roughly 290,000 USD a year of ammonia going to the product as excess nitrogen or to the scrubber.

Four Coriolis meters, at about 11,000 USD each, measured mass flow directly with ±0.1% accuracy across the whole range. Payback was under three months.

The cheapest instrument is rarely the cheapest measurement. What matters is the accuracy at the actual operating conditions, over the actual turndown range, for the whole life of the plant.

Flow Measurement

Coriolis mass flow meters

Fluid flows through vibrating tubes; the Coriolis effect twists the tubes in proportion to mass flow rate.

• Accuracy: ±0.1-0.2% of rate for liquids, ±0.35-0.5% for gases
• Measures true mass flow directly, plus density, plus temperature — three measurements from one instrument
• Independent of viscosity, density, temperature, pressure, conductivity and flow profile
• Turndown: 100:1 or better
• No upstream straight run requirement in most installations — a very significant installation advantage in congested plants
• Limitations: expensive, especially above DN 100; sensitive to external vibration unless properly mounted; entrained gas degrades accuracy; pressure drop can be significant on viscous fluids; limited maximum size
• Uses: custody transfer, chemical dosing, batching, fuel measurement, LPG and refrigerant metering, anything where mass rather than volume is the quantity that matters

Where accuracy matters, Coriolis is usually the right answer and the only argument against it is price.

Electromagnetic (magmeter)

A magnetic field is applied across the pipe; conductive fluid moving through it generates a voltage proportional to velocity.

• Accuracy: ±0.2-0.5% of rate
• Requires conductivity above about 5 µS/cm (some designs to 1 µS/cm). Does not work on hydrocarbons, pure water or any non-conductive fluid — the single most common misapplication
• No obstruction, no moving parts, negligible pressure loss — full bore
• Handles slurries, sludges and abrasive fluids very well with the right liner
• Turndown: 50:1 or better
• Liners: PTFE, PFA, polyurethane, hard rubber, ceramic — selected against chemical compatibility and abrasion
• Electrodes: 316, Hastelloy, tantalum, titanium, platinum
• Installation: needs a full pipe at all times — never install at a high point or on a downward discharge where the pipe can partially drain. Grounding is essential, and poor grounding is the most common cause of unstable readings.
• Uses: water and wastewater, slurries, chemicals, pulp, mining, food

In water and wastewater, the electromagnetic meter is the default and deserves to be.

Ultrasonic

Transit-time: ultrasonic pulses sent diagonally with and against flow; the time difference gives velocity.

• Clean to moderately clean liquids, and gas with high-pressure designs
• Accuracy ±0.5-2% in-line, ±1-5% clamp-on
• Multi-path designs (4, 6 or 8 paths) reach ±0.2-0.5% and are used for gas custody transfer
• Clamp-on versions require no process penetration — invaluable for retrofit, temporary survey and hazardous fluids
• Large bores are economical, which is where ultrasonic wins decisively over Coriolis and magmeters

Doppler: reflection from particles or bubbles.

• Needs suspended solids or bubbles to work; fails on clean liquid
• Lower accuracy, ±2-5%
• Used for slurry and dirty water where clamp-on is desired

Differential pressure (orifice, venturi, nozzle, cone, wedge)

A restriction creates a pressure drop; flow is proportional to the square root of ΔP.

• Accuracy: ±1-2% of full scale for orifice plates with a good installation
• Turndown: only about 3:1 to 5:1 per transmitter because of the square-root relationship — the key limitation and the cause of the fertiliser plant's problem
• Permanent pressure loss: 40-80% of the generated differential for orifice plates, 10-20% for venturis
• Cheapest option, especially in large sizes and high pressures, and universally understood
• Requires long straight runs — typically 10-40 diameters upstream depending on the upstream fitting and the beta ratio
• Impulse lines are the weak point: plugging, freezing, gas pockets in liquid lines and liquid pockets in gas lines are the most common DP flow problems. Close-coupled transmitters and diaphragm seals mitigate this.
• Variants: venturi for low loss, flow nozzle for high velocity and steam, V-cone and wedge for dirty fluids and short straight runs

Still the right answer for large-bore steam, for high pressure, and where cost dominates — but not where turndown or low-flow accuracy matters.

Vortex shedding

A bluff body sheds vortices at a frequency proportional to velocity.

• Accuracy ±0.75-1.5% of rate
• No moving parts, good for steam, gas and clean liquids
• Turndown 20:1 on liquids, less on gas at low pressure
• Needs a minimum Reynolds number — typically above 10,000-20,000 — below which it stops reading entirely rather than reading inaccurately
• Sensitive to upstream disturbance and to pipe vibration
• Widely used for steam flow measurement

Turbine, positive displacement, thermal mass, variable area

Turbine — a rotor spun by the flow; ±0.25-1% on clean low-viscosity liquids, good turndown, but bearings wear and it is destroyed by debris.

Positive displacement — chambers filled and emptied; ±0.1-0.5%, excellent on viscous fluids where other technologies struggle, used for fuel oil, lubricants and custody transfer of viscous products.

Thermal mass — heat transfer from a heated element; measures gas mass flow directly, excellent at very low flows, calibration is gas-composition specific so a composition change shifts the reading.

Variable area (rotameter) — a float in a tapered tube; ±2-5%, local indication only in the basic form, cheap and robust. Still the right instrument for purge flows, seal flushes, sample lines and anywhere a local visual check is what is needed.

Level Measurement

Non-contacting radar

Microwave pulses from an antenna above the surface; level is derived from time of flight.

• Accuracy: ±2-10 mm; high-precision tank gauging versions to ±1 mm
• No contact with the process, no moving parts, unaffected by density, viscosity, temperature or pressure
• Frequency matters: 26 GHz and 80 GHz units have a much narrower beam than older 6 GHz units, which means far better performance in narrow vessels, around internal obstructions, and in agitated tanks. 80 GHz is now the default choice for difficult geometries.
• Limitations: foam absorbs and scatters the signal; heavy condensation or product build-up on the antenna degrades it; very low dielectric constant products (dielectric below about 1.6-2, such as some light hydrocarbons and plastic pellets) give weak reflections; turbulent surfaces reduce signal
• Uses: the general-purpose default for liquids and solids

Guided wave radar (TDR)

The microwave pulse travels along a probe — rod, cable or coaxial — immersed in the product.

• Accuracy: ±2-5 mm
• Handles foam, agitation, low dielectric and narrow spaces far better than non-contacting radar because the signal is guided rather than radiated
• Measures interface level between two immiscible liquids, which is extremely useful on separators and on oil-water interfaces
• Limitations: the probe is in the process, so it fouls, coats and in viscous or crystallising service becomes unreliable; in a long cable probe installation, lateral forces from agitation must be considered; build-up on the probe causes errors
• Uses: small tanks, foaming products, interface measurement, low dielectric liquids, agitated vessels

Differential pressure

Hydrostatic head measured by a DP transmitter; level is head divided by density.

• Robust, inexpensive, well understood
• Accuracy depends entirely on knowing the density. A density change of 2% is a level error of 2%, and in a tank with varying temperature or composition this can be large. Temperature compensation or a separate density measurement can correct it.
• Impulse lines and wet legs are the practical weak point: plugging, freezing, and wet-leg evaporation or condensation all shift the zero. Diaphragm seals with capillaries, or remote seal transmitters, eliminate most of it.
• Still the standard for boiler drum level — where three-element control needs a fast, reliable, pressure-compensated measurement — and for tanks with difficult vapour spaces where radar struggles.

Ultrasonic

Sound pulses from above the surface.

• Accuracy ±0.25-1% of range
• Cheaper than radar, non-contacting
• Affected by vapour composition, temperature gradients, foam, dust and wind — the speed of sound varies with the medium, so anything that changes the vapour space changes the reading
• Will not work in vacuum or in heavy vapour
• Uses: open channels, water and wastewater sumps, benign atmospheric tanks

Other technologies

Capacitance — measures dielectric change. Compact and cheap, but calibration is product-specific and coating causes drift. Still common for point level in solids and for interface.

Displacer and float — mechanical, proven, no power needed in some forms. Magnetic level gauges with a float and external indicator provide a local visual indication that needs no power at all, which is why they remain standard on boilers and critical vessels alongside electronic measurement.

Nucleonic (radiometric) — a sealed radioactive source and detector outside the vessel. Measures through the wall with no process penetration at all. The answer for extremely hostile service — high pressure, high temperature, abrasive, toxic, or where no nozzle can be provided. Carries licensing, handling and disposal obligations that must be accounted for.

Load cells — weigh the whole vessel. Gives true mass content independent of density, level profile and product behaviour. Excellent for batching. Requires careful mechanical design: piping restraint, thermal expansion and wind loading all transmit force into the cells and corrupt the reading.

Point level switches — capacitance, conductive, float, vibrating fork and thermal dispersion. Vibrating fork switches are very reliable in liquids and are the standard high-high level trip device.

Pressure Measurement

Pressure transmitters — piezoresistive or capacitive sensors with a diaphragm.

• Accuracy ±0.04-0.1% of span for good process transmitters
• Gauge, absolute, differential and multivariable versions
• Turndown matters: a transmitter with a wide calibration range used at a small span loses accuracy. Select the sensor range so the working span is at least 10-20% of the sensor's maximum, and preferably more.
• Static pressure effect on DP transmitters: the specification includes a zero shift per unit of static pressure, which on a high-pressure flow or level application can dominate the error budget. Check it.

Diaphragm seals (remote seals) — a sealed, fluid-filled capillary between the process and the transmitter. Required where the process would plug, freeze, crystallise, corrode the sensor, or where the process is too hot. The penalties are added temperature effect, slower response, and a fill-fluid selection that must suit both the temperature range and the consequences of a diaphragm failure.

Pressure gauges — Bourdon tube, diaphragm and capsule. Specify the range so the normal reading is between 25% and 75% of full scale, liquid filling for vibration service, and a safety pattern case with a blow-out back for hazardous service. A gauge reading at the bottom 10% of its scale is not a measurement.

Pressure switches — mechanical and electronic. For simple trips where a transmitter is not warranted, though a transmitter with an alarm in the control system is usually more diagnosable.

Installation points that cause most pressure measurement problems:

• Steam service needs a siphon (pigtail) to condense steam and protect the element
• Liquid service tapping points on the side of the line, not the bottom (sediment) or top (gas)
• Gas service tapping on the top to avoid liquid accumulation
• Isolation and a vent or drain for calibration
• Impulse lines sloped continuously, with no high or low points that trap gas or liquid
• Pulsation damping on reciprocating pump and compressor discharge

Temperature Measurement

RTD (resistance temperature detector)

Platinum resistance changes with temperature; Pt100 is the standard, Pt1000 for better resolution in lower ranges.

• Range: -200 to 600°C
• Accuracy: ±0.1-0.3°C for Class A and AA
• Stability: excellent, with low drift over years
• Output: resistance, linear and repeatable
• Wiring: 3-wire minimum; 4-wire for best accuracy, eliminating lead resistance error entirely. 2-wire RTDs should not be used except at very short lead lengths — the lead resistance appears directly as a temperature error.
• Limitations: slower response than a thermocouple, more fragile, self-heating error if excitation current is too high, limited upper temperature
• Uses: the default choice below 600°C wherever accuracy matters

Thermocouple

Two dissimilar metals produce a voltage proportional to the temperature difference between the measuring junction and the reference junction.

• Accuracy: ±1-2°C typically, ±0.5°C for special-tolerance types
• Fast response, rugged, very wide range, cheap
• Drift is the main limitation, particularly for K type at high temperature and in reducing or sulphurous atmospheres
• Cold junction compensation is required and is handled in the transmitter or input card
• Extension wire must be the matching thermocouple alloy — using copper extension wire creates a second, uncompensated junction and an error that varies with ambient temperature. This is a persistent and easily avoided field error.

Thermowells and installation

• Thermowell material selected against the process fluid and velocity; the well is a pressure boundary and must be designed as one
• Wake frequency calculation to ASME PTC 19.3 TW — a thermowell is a cantilever in a flowing stream, and if the vortex shedding frequency approaches the well's natural frequency it fails by fatigue. Broken thermowells in high-velocity lines are a recurring and avoidable failure, and the calculation is mandatory practice, not optional.
• Immersion depth must be adequate for the sensor to reach process temperature — a rule of thumb is at least 10 times the well diameter, and in any case the tip should reach the middle third of the pipe
• Tip clearance: the sensor must bottom out in the well, with spring loading, or an air gap adds lag and error
• Thermal conduction error along the well from a hot or cold flange is reduced by greater immersion and by insulating the well's external projection
• Specify a replaceable sensor in a thermowell rather than a direct-immersion sensor wherever the line cannot be shut down for sensor replacement

Non-contact

Infrared pyrometers — measure emitted radiation. No contact, very fast, for moving surfaces, furnaces, kilns and very high temperatures. Accuracy depends on knowing the emissivity of the target, which is the usual source of error, and on a clear optical path.

Thermal imaging — for survey work, hot-spot detection, refractory condition, electrical inspection and steam trap checking.

Signals, Protocols and SIL

Signals

4-20 mA analogue — still the backbone of process measurement. Two-wire loop-powered, inherently diagnosable (0 mA means a broken loop, which is distinguishable from a zero reading), noise-immune, and understood everywhere.

HART — a digital signal superimposed on the 4-20 mA loop. Gives access to configuration, diagnostics, secondary variables and device status without disturbing the analogue measurement. Essentially free with any modern transmitter and dramatically improves maintainability. If HART is available, the plant should be using it — the diagnostics it provides are the difference between a transmitter that is repaired before it fails and one that is replaced after it causes an upset.

Foundation Fieldbus and PROFIBUS PA — fully digital multidrop buses. Fewer cables, more data, control in the field device. Higher engineering effort and a commissioning discipline that must be maintained.

Modbus, PROFINET, EtherNet/IP — industrial Ethernet for higher-level integration, skid packages and remote I/O.

WirelessHART and ISA100 — for measurements where cabling is uneconomical: remote tanks, mobile equipment, temporary monitoring, condition monitoring points. Battery life and update rate are the design constraints.

Hazardous area protection

Instruments in classified areas need a protection concept:

• Ex ia / ib — intrinsic safety. Energy limited so no ignition-capable spark or surface temperature can occur. Needs barriers or galvanic isolators and a loop-by-loop energy calculation. Ex ia is suitable for Zone 0.
• Ex d — flameproof. Robust enclosure containing any internal explosion.
• Ex e — increased safety. Construction prevents arcs and hot surfaces.
• Ex ec / nA — Zone 2.
• Gas group (IIA, IIB, IIC) and temperature class (T1-T6) must match the area classification, exactly as for motors.

SIL and safety instrumented functions

Where an instrument forms part of a safety instrumented function under IEC 61508 and IEC 61511, it must have documented reliability data:

• Safe failure fraction (SFF) and probability of failure on demand (PFD)
• Hardware fault tolerance and the architectural constraints that follow
• Proof test interval — and the SIL claim is only valid if that proof testing is actually carried out at the stated interval

Certified SIL capability is a documentation property, not a quality property. A high-accuracy transmitter with no failure-rate data cannot be used in a SIL 2 loop; a simpler transmitter with a full certified assessment can. Specify SIL capability and the required proof test interval on the datasheet, and require the manufacturer's safety manual as a deliverable — the manual states the conditions under which the SIL claim holds, and those conditions are frequently ignored in practice.

Separation of control and safety — the safety instrumented function should use separate sensors from the control loop. Sharing a transmitter between control and trip creates a common-cause failure that defeats the purpose of the safety function.

Common Specification Mistakes

1. Orifice plate where turndown matters. The square-root relationship destroys low-flow accuracy; the fertiliser plant lost roughly 290,000 USD a year.

   Prevention: State the minimum, normal and maximum flow on the datasheet, and select the technology against accuracy at minimum flow, not at design flow.

2. Electromagnetic meter on a non-conductive fluid. It will not read at all on hydrocarbons or demineralised water.

   Prevention: Check fluid conductivity against the meter's minimum, and use Coriolis, ultrasonic or DP for non-conductive fluids.

3. Magmeter installed where the pipe can run partially full. Readings become meaningless.

   Prevention: Install in a rising line or at a low point with a guaranteed full bore, never at a high point or on a free discharge.

4. Non-contacting radar on a foaming or agitated product. Foam absorbs the signal and agitation scatters it.

   Prevention: Use guided wave radar for foam, agitation, low dielectric and narrow vessels, or 80 GHz non-contacting radar where the geometry is the issue rather than the product.

5. DP level measurement on a product with varying density. Every percent of density change is a percent of level error.

   Prevention: Use radar where density varies, or add temperature or density compensation, and state the density range on the datasheet.

6. Wet legs and impulse lines on services that plug or freeze. Zero drifts, and the reading is wrong in a way that is not obvious.

   Prevention: Specify diaphragm-sealed remote transmitters for plugging, freezing, crystallising and viscous services, and heat-trace impulse lines where freezing is possible.

7. No thermowell wake frequency calculation. The well fails by fatigue in a high-velocity line, which is both a measurement loss and a process leak.

   Prevention: Require a wake frequency calculation to ASME PTC 19.3 TW for every thermowell in flowing service, and accept a tapered or supported well design where the calculation demands it.

8. Copper extension wire on a thermocouple. An uncompensated junction is created and the error varies with ambient temperature.

   Prevention: Specify matching thermocouple extension wire with correct polarity throughout, and terminate on thermocouple-compatible terminals.

9. 2-wire RTD on a long cable run. Lead resistance appears directly as a temperature offset.

   Prevention: Specify 3-wire minimum, 4-wire where accuracy matters, or mount a transmitter at the sensor and transmit 4-20 mA.

10. Transmitter range far wider than the working span. Accuracy as a percentage of span becomes a large percentage of reading.

    Prevention: Select the sensor range so the calibrated span is a substantial fraction of the sensor's maximum, and check the turndown specification.

11. No SIL certification on an instrument used in a safety function. The safety integrity level claim cannot be substantiated.

    Prevention: Specify SIL capability with the required PFD and proof test interval, obtain the manufacturer's safety manual, and use separate sensors for control and for the safety function.

12. Pressure gauge sized so the normal reading is at the bottom of the scale. The reading is not usable.

    Prevention: Select gauge ranges so normal operation falls between 25% and 75% of full scale, and specify liquid filling where vibration is present.

13. No HART configuration or diagnostic use. The diagnostics are present and ignored.

    Prevention: Specify HART-capable transmitters, a HART-enabled I/O or multiplexer architecture, and include device diagnostics in the maintenance process.

Supply from Kasko Makine

Kasko Demir Çelik Makine supplies process instrumentation across all measurement types:

Flow

• Coriolis mass flow meters, DN 1 to DN 400
• Electromagnetic flow meters, DN 10 to DN 2000, with PTFE, PFA, polyurethane, rubber and ceramic liners
• Ultrasonic flow meters: in-line single and multi-path, and clamp-on
• Orifice plates, orifice flanges, restriction orifices, venturi tubes, flow nozzles, V-cone and wedge meters
• Vortex, turbine, positive displacement and thermal mass meters
• Variable area rotameters, glass and metal tube
• Flow computers and totalisers for custody transfer
• Flow straighteners and conditioning plates

Level

• Non-contacting radar at 26 GHz and 80 GHz
• Guided wave radar for foam, interface and low dielectric
• Ultrasonic level transmitters
• DP level transmitters with diaphragm seals and capillaries
• Capacitance, conductive and vibrating fork point level switches
• Magnetic level gauges with float, scale and switch options
• Displacer and float level instruments
• Load cell weighing systems for vessels and silos
• Nucleonic level systems with licensed source handling
• Sight glasses, gauge glasses and tubular level gauges

Pressure

• Pressure, absolute, differential and multivariable transmitters
• Remote seal and diaphragm seal assemblies with fill fluid selection
• Bourdon, diaphragm and capsule pressure gauges, dry and liquid filled
• Pressure switches, mechanical and electronic
• Manifolds, 2-valve, 3-valve and 5-valve
• Siphons, snubbers, pulsation dampers, needle valves and impulse tubing

Temperature

• Pt100 and Pt1000 RTD assemblies, Class A, AA and B, 3-wire and 4-wire
• Thermocouple assemblies in types K, J, T, E, N, R, S and B
• Thermowells in 316, 321, Inconel, Monel, Hastelloy, titanium and ceramic, with wake frequency calculations
• Temperature transmitters, single and dual input, HART
• Bimetal and gas-filled dial thermometers
• Infrared pyrometers and thermal imaging equipment
• Multipoint and skin thermocouples for reactors and heaters

Analytical and other

• pH, ORP, conductivity, dissolved oxygen and turbidity analysers
• Gas detection for flammable and toxic gases
• Density and viscosity meters
• Moisture and dew point analysers

Systems and accessories

• Intrinsically safe barriers and galvanic isolators
• Signal converters, isolators and surge protection
• HART multiplexers and device management
• Instrument enclosures, cabinets and heated enclosures
• Instrument tubing and fittings in 316, Monel and Alloy 625
• Impulse piping, manifolds, root valves and double block and bleed
• Instrument air filters, regulators and distribution
• Calibration equipment and portable test instruments

Engineering support

Send an instrument list with process conditions — fluid, phase, composition, conductivity where relevant, minimum, normal and maximum flow, pressure, temperature, density range, line size and material, available straight run, area classification, required accuracy, and whether the measurement is for control, custody transfer or a safety function. We will return selections with the accuracy achievable at minimum flow stated, installation requirements, and SIL capability documentation where the loop requires it.

Certification

Calibration certificates traceable to national standards, material certificates to EN 10204 3.1 for wetted parts and thermowells, ATEX and IECEx certificates for hazardous-area instruments, SIL capability assessments and manufacturer safety manuals, pressure test certificates for thermowells and seals, wake frequency calculation reports, and NACE MR0175 compliance documentation for sour service wetted parts.

Logistics

Standard transmitters, gauges, RTDs and thermocouples generally ship in 2-5 weeks. Coriolis and electromagnetic meters, radar transmitters and special materials typically 6-12 weeks. Custody transfer packages, analysers and nucleonic systems 12-20 weeks. Shipping from Istanbul by road to Europe, the Caucasus and Iraq, by sea to Gulf, African and Asian destinations, and by air for urgent instrument replacements.

Send your instrument list with process conditions and we will return selections with accuracy and installation requirements within four working days. Reach us at info@kaskomakine.com or WhatsApp +90 (537) 521 1399.

Continue Reading: Control and Equipment Series

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• Boiler and Cooling Water Treatment
• Wastewater Treatment Equipment
• Industrial Electric Motors: IE Classes and Enclosures

Frequently Asked Questions

Q: Which flow meter is most accurate?
A: Coriolis mass flow meters are the most accurate general-purpose technology, at ±0.1 to 0.2% of rate for liquids, and they measure true mass flow independently of density, viscosity, temperature and flow profile. Electromagnetic meters achieve ±0.2 to 0.5% on conductive liquids, multi-path ultrasonic meters ±0.2 to 0.5% on gas, and orifice plates around ±1 to 2% of full scale with much worse accuracy at low flow.

Q: Why do orifice plate flow meters lose accuracy at low flow?
A: Flow is proportional to the square root of the differential pressure, so a small error in differential becomes a large error in flow as the differential falls. A meter accurate to ±1% of full scale at 100% flow is roughly ±4% of reading at 25% flow and ±10% at 10% flow. Practical turndown is therefore only about 3:1 to 5:1 per transmitter, which rules orifice plates out where flow varies widely.

Q: When should guided wave radar be used instead of non-contacting radar?
A: Guided wave radar guides the microwave pulse along a probe, so it works where a radiated signal fails — in foam, in agitated vessels, in narrow tanks with internal obstructions, and on low dielectric products below about 1.6 to 2. It also measures the interface between two immiscible liquids. The trade-off is that the probe sits in the process and can foul or coat in viscous and crystallising service.

Q: What is the difference between an RTD and a thermocouple?
A: A Pt100 RTD measures -200 to 600°C at ±0.1 to 0.3°C with excellent long-term stability, making it the default where accuracy matters. A thermocouple covers much wider ranges, up to 1,700°C for type B, responds faster, costs less and is more rugged, but is accurate only to about ±1 to 2°C and drifts over time, particularly type K at high temperature or in reducing and sulphurous atmospheres.

Q: Why does a thermowell need a wake frequency calculation?
A: A thermowell is a cantilever projecting into a flowing stream, and the flow sheds vortices at a frequency that depends on velocity and well diameter. If that frequency approaches the well's natural frequency, resonance causes fatigue failure, which loses the measurement and creates a process leak. ASME PTC 19.3 TW gives the calculation method, and it should be mandatory for every thermowell in flowing service.

Q: What does SIL certification mean for an instrument?
A: SIL capability under IEC 61508 and IEC 61511 is documented reliability data — safe failure fraction, probability of failure on demand, hardware fault tolerance and the required proof test interval. It is a documentation property rather than an accuracy property, so a highly accurate transmitter without a certified assessment cannot be used in a SIL 2 loop, while a simpler certified one can. The SIL claim holds only if the stated proof testing is actually performed.

Q: Why will an electromagnetic flow meter not work on hydrocarbons?
A: An electromagnetic meter generates a voltage from conductive fluid moving through a magnetic field, so it requires fluid conductivity above about 5 µS/cm, with some designs reaching 1 µS/cm. Hydrocarbons, oils, demineralised water and most organic solvents are effectively non-conductive and produce no signal at all. Those fluids need Coriolis, ultrasonic, differential pressure, positive displacement or turbine measurement instead.