Instrumentation Articles
Longer-form technical writing, published as it is finished and revised as field experience corrects it. Every entry carries its published and updated dates.
91 entries
4-20 mA Signals from the Instrument Side
How a 4-20 mA current loop is powered and wired at the transmitter, why the loop resistance budget matters, and how to check a loop with a meter.
9 min readUpdated Sep 5, 2026
Wet Well Level Measurement
How lift station wet well level is measured, which technology suits which well, the failure modes that flood a station, and why the high level float must stay hardwired.
8 min readUpdated Sep 5, 2026
Hydrostatic Level Measurement
Measuring level from the pressure of the liquid above a sensor: submersible transducers, bubblers, and base-mounted transmitters, the density assumption behind all of them, venting, installation, and what makes them drift.
8 min readUpdated Sep 5, 2026
Ultrasonic Level Measurement
Non-contact level by time of flight: how the sensor works, the blanking zone and beam angle, temperature compensation, the surfaces and vapors that defeat it, mounting rules, and what to do when it loses its echo.
8 min readUpdated Sep 5, 2026
Float Switches
The oldest level instrument and still the one that saves a station: how tilt and stem floats work, the wiring conventions, mounting and spacing in a wet well, classified-area ratings, and how to use them alongside a continuous instrument.
7 min readUpdated Sep 5, 2026
Pressure Transmitters
Gauge, absolute, and differential pressure transmitters: how they sense, how range, turndown, and accuracy specifications work, the process connection and its accessories, and the installation errors that show up as calibration problems.
8 min readUpdated Sep 5, 2026
HART
Digital data riding on the 4-20 mA loop: how HART signaling works, what a handheld or a HART-enabled input can read, multidrop and burst modes, and the wiring conditions that make HART fail while the current loop keeps working.
10 min readUpdated Sep 5, 2026
Differential Pressure Flow Measurement
Orifice plates, venturis, and flow nozzles: the square-root relationship, why turndown is limited, where the square root is taken, impulse line rules, and when a DP element is still the right choice against a mag meter.
10 min readUpdated Sep 5, 2026
Open Channel Flow Measurement
Flumes and weirs with a level sensor: how the head-discharge relationship works, Parshall flume and weir equations, where to put the sensor, submergence, and the errors that make a permit-reporting flow meter wrong.
11 min readUpdated Sep 5, 2026
Chlorine Residual Analyzers
Amperometric and colorimetric residual analyzers: what each measures, free versus total chlorine, pH and flow dependence, sample line design, calibration against a DPD grab sample, and using the signal for feed control.
11 min readUpdated Sep 5, 2026
pH Measurement
Glass electrodes, reference junctions, and why pH is the measurement that drifts: how the sensor works, temperature compensation, two-point buffer calibration, mounting rules, the failure modes of the reference, and using the signal for chemical feed control.
11 min readUpdated Sep 5, 2026
Dissolved Oxygen Measurement
Optical and membrane DO sensors for aeration control: how each measures, why optical has taken over, air calibration with pressure and salinity corrections, sensor placement in an aeration basin, fouling and cleaning, and the response time that limits the control loop.
11 min readUpdated Sep 5, 2026
Turbidity Measurement
Nephelometric turbidimeters for filter effluent and raw water: how 90-degree scattering is measured, the methods and the NTU, low-range versus high-range instruments, bubbles and the debubbler, formazin calibration and verification standards, and the compliance record the instrument exists to produce.
11 min readUpdated Sep 5, 2026
Surge Protection for Signal Circuits
Protecting 4-20 mA loops, discrete inputs, network cables, and radio feeds from lightning and switching surges: where surges enter, the device types and how they clamp, the grounding that makes them work, the resistance and capacitance they add, and where to put them at a remote site.
10 min readUpdated Sep 5, 2026
Loop Checks
Proving every I/O point end to end before a system goes live: what a loop check is and is not, the three-point analog check, discrete and output checks, the two-person method with the sheet that records it, what to do with a point that fails, and why a loop check is not a calibration.
9 min readUpdated Sep 5, 2026
Calibration Procedures
How a utility calibrates its instruments consistently: the as-found and as-left discipline, tolerance from the loop rather than the data sheet, the reference standard and its traceability, the five-point check with hysteresis, adjusting only when needed, intervals set from history, and the procedure written once per instrument type.
9 min readUpdated Sep 5, 2026
Calibration Documentation
The records a calibration program produces and what each is for: the calibration certificate per event, the instrument history, the reference standard file, the schedule, the tolerance and interval register, and how the records are organized so a regulator, an auditor, or the next technician can find what they need.
8 min readUpdated Sep 5, 2026
Impulse Lines
The tubing between the process tap and the pressure or DP transmitter, and everything that goes wrong in it: slope and routing for liquid and gas, elevation head and how to correct for it, air and condensate, blocking and manifold valves, freezing, plugging on dirty service, and the diaphragm seal alternative.
9 min readUpdated Sep 5, 2026
Ultrasonic Flowmeters
Transit-time and Doppler ultrasonic flow measurement: how each works, which liquids each suits, clamp-on versus wetted transducers, the pipe data the meter needs, the accuracy to expect, and the installation details that decide whether to trust the reading.
10 min readUpdated Sep 5, 2026
Flowmeter Installation
The installation rules that decide whether a flowmeter reads correctly: straight run by meter type, keeping the pipe full, orientation, velocity range, grounding for magnetic meters, isolation and access, and the commissioning checks that prove the meter.
10 min readUpdated Sep 5, 2026
Flow Troubleshooting
A method for a flow reading that is wrong, noisy, zero, or disputed: establish the truth with an independent check, separate installation from configuration from signal chain, then the specific checks for magnetic, ultrasonic, and DP meters.
10 min readUpdated Sep 5, 2026
Differential Pressure Level
Measuring level in a closed or pressurized vessel with a DP transmitter: high and low side connections, dry legs and wet legs, zero suppression and elevation, the range calculation with a worked example, remote seals, and the density dependence.
10 min readUpdated Sep 5, 2026
Level Troubleshooting
A method for a level reading that is wrong, frozen, jumping, or lost: establish the true level, check the signal chain and configuration, then the checks for ultrasonic, radar, hydrostatic, DP, and float instruments, and the failure behavior to design in.
10 min readUpdated Sep 5, 2026
Differential Pressure
The differential pressure transmitter as an instrument: what it measures, static and overrange ratings, the three-valve manifold and the order of operations that protects the cell, and the applications from filter head loss to membrane TMP, flow, and level.
9 min readUpdated Sep 5, 2026
Pressure Transmitter Installation
Where and how to install a pressure transmitter so it reads the process and not its own plumbing: tap location, mounting above or below the tap, isolation and bleed valves, line slope, pulsation, elevation, freeze protection, seals, vibration, and sun.
10 min readUpdated Sep 5, 2026
Pressure Calibration
What calibrating a pressure transmitter means: as-found and as-left, the five-point test and hysteresis, tolerance and the reference standard, sensor trim versus output trim versus reranging on a smart transmitter, gauge versus absolute, and intervals.
10 min readUpdated Sep 5, 2026
Ammonia Analyzers
Online ammonia measurement in water and wastewater: ion-selective probes, gas-sensing electrodes, and wet-chemistry analyzers, what each is good for, the interferences that decide accuracy, and the maintenance and calibration it demands.
9 min readUpdated Sep 5, 2026
Conductivity
Conductivity measurement in water systems: what it indicates, contacting and inductive sensors, cell constants and ranges, temperature compensation, the relation to total dissolved solids, uses from membrane monitoring to blending, and fouling problems.
8 min readUpdated Sep 5, 2026
Nitrate Analyzers
Online nitrate measurement: ultraviolet absorbance probes and analyzers, ion-selective electrodes, and wet chemistry, the interferences from organics, turbidity, and chloride, the uses in denitrification and source monitoring, and maintenance.
8 min readUpdated Sep 5, 2026
ORP
Oxidation-reduction potential: what the millivolt reading means and does not, the platinum and reference pair, uses in dechlorination control, disinfection indication, anoxic monitoring, and chemical treatment, checking with standards, and electrode drift.
7 min readUpdated Sep 5, 2026
Calibration Troubleshooting
When an instrument will not calibrate or stay calibrated: wrong or expired standards, slope and offset out of range, compensation errors, procedure mistakes, loop scaling that disagrees with the instrument, and deciding when to adjust, replace.
8 min readUpdated Sep 5, 2026
Digital Signals
Discrete signals between instruments and controllers: dry and wetted contacts, sinking and sourcing transistor outputs, two-wire sensor leakage, NAMUR sensors with wire-break detection, contact ratings at low current, and wiring conventions.
8 min readUpdated Sep 5, 2026
Frequency Signals
Frequency as a process signal: the instruments that produce it, how frequency differs from pulse counting, the K-factor and the conversion to flow, frequency inputs and period measurement at low rates, converters to 4-20 mA, noise, and how to verify one.
7 min readUpdated Sep 5, 2026
Pulse Signals
Pulse outputs for totalizing: flowmeters, energy meters, rain gauges, and feeders, output types, scaling in volume per pulse, counter inputs, pulse width and rate, debouncing reeds, totalizer retention and rollover, and verifying against the register.
7 min readUpdated Sep 5, 2026
Signal Isolation
Isolating analog signals: loop isolators and their variants, splitters that feed two receivers, converters, the common-mode problem isolation solves, ratings, accuracy and response costs, and when an isolated input module makes the isolator unnecessary.
8 min readUpdated Sep 5, 2026
Disinfection Control
Controlling chlorine and chloramine dosing at a water plant: the CT concept and how contact time is credited, flow-paced and residual-trimmed feed, gas, hypochlorite, and on-site generation systems, the point-of-entry residual as the compliance measurement, and the interlocks that stop a feed system from over- or under-dosing.
11 min readUpdated Sep 5, 2026
Aeration Control
Controlling the largest energy user at a wastewater plant: the DO cascade to airflow, blower staging and turndown, most-open-valve pressure control across basins, ammonia-based aeration control, the instruments each strategy depends on, and the interlocks that keep blowers out of surge.
11 min readUpdated Sep 5, 2026
Headworks Control
The first stop for raw wastewater: influent flow measurement, mechanical screens and their differential level control, grit removal, influent pumping where the plant needs it, the odor and hazardous atmosphere considerations that shape the electrical design, and what SCADA needs from the headworks.
10 min readUpdated Sep 5, 2026
Clarifier Control
Primary and secondary clarifiers from the control room: the mechanism drive and its torque protection, sludge blanket measurement and sludge withdrawal control, scum removal, the weir and the effluent turbidity, and the sludge blanket as the early warning for a secondary process losing its footing.
9 min readUpdated Sep 5, 2026
Raw Water
Controlling the front of a surface water plant: intake and screens, low-lift pumping, the raw water flow that paces the whole plant, the online quality instruments that warn of a changing source, and the alarms and ramp limits that keep the train stable.
9 min readUpdated Sep 5, 2026
Wells
Operating a wellfield as a water source: selecting and rotating wells, pump-to-waste on start, drawdown and specific capacity, wellhead chemical feed, blending for nitrate or arsenic, the telemetry a well needs, and the interlocks that protect the pump.
10 min readUpdated Sep 5, 2026
Aeration
Aeration in drinking water treatment: stripping carbon dioxide, hydrogen sulfide, radon, and volatile organics, and oxidizing iron and manganese ahead of filtration. The aerator types, what each needs from the control system, the pH shift, and the interlocks.
9 min readUpdated Sep 5, 2026
Filtration
Controlling gravity and pressure filters: effluent rate and level control, head loss and turbidity on every filter, the backwash triggers and sequence, filter-to-waste and ripening, the turbidity rules that filter monitoring answers to, and the operator view.
11 min readUpdated Sep 5, 2026
Chemical Feed
Chemical feed from the control side: the chemicals and where they go, metering pumps and dry feeders, flow pacing with residual trim, the dose arithmetic, day tanks and drawdown tests, loss-of-feed detection, interlocks, and overfeed protection.
11 min readUpdated Sep 5, 2026
Reverse Osmosis
Reverse osmosis from the controls side: what the skid contains, the instruments that matter, the numbers an operator watches, recovery, rejection, normalized permeate flow, and the trends that say when the membranes need cleaning or replacement.
10 min readUpdated Sep 5, 2026
Nanofiltration
Nanofiltration from the controls side: how it differs from reverse osmosis in pressure, recovery, and what it removes, the softening and color removal applications where it dominates, and trends that are shared with reverse osmosis and the ones that differ.
8 min readUpdated Sep 5, 2026
Membrane Control
The control system of a membrane skid: the loops for permeate flow, recovery, and dosing, the permissives and trips, the start, run, flush, and shutdown sequences, and the data the operators and the membrane supplier need from the historian.
9 min readUpdated Sep 5, 2026
Clean-in-Place
Cleaning membranes in place: the triggers from the normalized trends, the low pH clean for scale and the high pH clean for organics and biofouling, the sequence of flush, heat, recirculate, soak, and rinse, and how automation makes cleaning repeatable.
9 min readUpdated Sep 5, 2026
Concentrate Systems
Handling the concentrate a membrane plant produces: the concentrate valve and the recovery it sets, flow and quality monitoring for the permit, the disposal routes, surface discharge, sewer, and the alarms and records a concentrate permit expects.
8 min readUpdated Sep 5, 2026
Biological Treatment
The biological process from the controls side: activated sludge and its common forms, conventional, extended aeration, oxidation ditch, sequencing batch reactor, and membrane bioreactor, and the alarms that give an operator time.
9 min readUpdated Sep 5, 2026
Chemical Systems in Wastewater Treatment
The chemical feeds of a wastewater plant and their controls: coagulants for phosphorus, polymer for thickening and dewatering, hypochlorite and sulfite for disinfection and dechlorination, and the alarms that keep the plant in its permit.
9 min readUpdated Sep 5, 2026
Effluent Disinfection
Disinfecting the effluent and proving it: chlorination with contact time and residual control, dechlorination with sulfite controlled by ORP or a residual analyzer, ultraviolet systems with dose control from flow, and the records the discharge report needs.
9 min readUpdated Sep 5, 2026
Effluent
Effluent from the controls side: the flow measurement the permit is built on, the composite samplers it paces, the parameters measured online, flow, pH, dissolved oxygen, residual, turbidity, and the data integrity that makes the record defensible.
8 min readUpdated Sep 5, 2026
Residuals
Solids handling from the controls side: thickening by gravity, flotation, or drum, aerobic and anaerobic digestion with their temperature, mixing, and gas systems, dewatering by belt press, centrifuge, and the records that biosolids regulation requires.
9 min readUpdated Sep 5, 2026
How to Calibrate a Pressure Transmitter
A five-point calibration with a pressure source and a reference: isolate and vent, record as-found, decide whether to adjust, trim the sensor and the output separately, verify as-left, and record it so the next calibration means something.
9 min readUpdated Sep 5, 2026
How to Diagnose a Ground Loop
Prove that a noisy or offset analog signal is a ground loop, find the second ground, and fix it the right way: remove the extra ground, isolate the signal, or correct the shield termination. With the measurements that make the diagnosis certain.
9 min readUpdated Sep 5, 2026
How to Configure a Radar Level Transmitter
Commission a non-contact radar on a wet well or tank: mount it, set the reference and range from measured elevations, map false echoes with the vessel empty, set damping and the output, and verify against a tape at two levels before the controller uses it.
9 min readUpdated Sep 5, 2026
How to Troubleshoot a Flowmeter
Work a flowmeter that reads wrong, reads zero, or reads with the pipe still: confirm the pipe is full and the flow is real, check the installation against the meter requirements, and compare against an independent measurement before adjusting anything.
9 min readUpdated Sep 5, 2026
Transmitter Reads Wrong Value
The reading is steady, plausible, and wrong. How to prove it against a reference, then separate a scaling mismatch, a range change, a zero shift, an installation effect, and a genuine transmitter fault, with the order of checks that finds it fastest.
9 min readUpdated Sep 5, 2026
Level Reading Jumps
A level that steps, spikes, or flips to a fixed value while the water moves smoothly: the false echoes, foam, turbulence, condensation, cable faults, and controller-side causes behind each pattern, and how to tell them apart from the trend before opening a hatch.
9 min readUpdated Sep 5, 2026
Ground Loop Symptoms
How a ground loop shows itself: hum on analog signals, offsets that change when large loads switch, serial links that error, and readings that differ by location. The mechanism, the measurements that prove it, and fixes that never lift a safety ground.
9 min readUpdated Sep 5, 2026
Shield Grounded at Both Ends
A shield connected to ground at the panel and again in the field becomes a conductor between two grounds, and its current becomes noise on the pair inside. How to find the second connection, what to measure before lifting it, and when both ends are correct.
8 min readUpdated Sep 5, 2026
Radio Frequency Interference
Readings that jump when a handheld radio is keyed, inputs that flicker near a telemetry antenna, transmitters that drift beside a modem: how radio energy gets into control circuits, how to prove it with the radio itself, and the fixes.
9 min readUpdated Sep 5, 2026
Unshielded Cable in a Noisy Run
The signal was run in plain conductors through a route full of power and drive cables, and now it is noisy. What unshielded cable cannot do, how to prove the cable is the problem before pulling a new one, and the options when replacing it is not possible.
9 min readUpdated Sep 5, 2026
VFD Noise on Analog Signals
Analog readings that jump or wander when a drive runs, and speed references that make a drive hunt: how the drive output couples into signal wiring, why the input turns kilohertz noise into a slow wander, the tests that pin it on the drive, and the fixes.
10 min readUpdated Sep 5, 2026
Drive Causes Instrument Noise
When many instruments became noisy after a drive was installed, the drive installation is the place to look: the checklist for motor cable, shielding, grounding, filters, and carrier frequency that decides how much noise a drive puts into the plant.
10 min readUpdated Sep 5, 2026
Pump Loses Prime
A suction-lift pump that runs without pumping, or pumps for a while and then stops: air leaking into the suction line, a foot valve that lets the line drain, a lift too high, a vortex or an aerated inflow, or a self-priming casing that lost its charge.
8 min readUpdated Sep 5, 2026
Valve Will Not Reach Position
A valve commanded to a position that stops short: an obstruction or a seat the actuator cannot push through, travel limits or a positioner span set wrong, a stem binding, an actuator undersized or out of air, or a command signal that does not span.
8 min readUpdated Sep 5, 2026
Valve Hunting
A modulating valve that never settles: a loop tuned too hard, a positioner gain too high, stiction producing a limit cycle, a noisy command or feedback, an oversized valve working near its seat, or two loops fighting. The manual test that separates them.
8 min readUpdated Sep 5, 2026
Valve Slow to Respond
A valve that takes far longer to move than it should: a pneumatic supply restricted or leaking; a positioner tuned slow; an electric actuator whose speed is fixed; a stem binding in cold or packing; or a rate-limited output. Measuring the stroke time.
8 min readUpdated Sep 5, 2026
Position Feedback Wrong
The HMI says one position and the valve is at another: a feedback potentiometer worn or miscalibrated, a linkage that slipped, scaling that does not match, a reversed signal, limits set wrong, or noise. Comparing indicator, feedback, and process.
8 min readUpdated Sep 5, 2026
Actuator Torque Fault
An electric actuator that trips on torque: at the seat because the seating mode or setting is wrong, in mid-travel because something is in the way or the stem binds, or under flow because the differential exceeds the sizing. The checks before a reset.
8 min readUpdated Sep 5, 2026
Signal Pegged High or Low
A 4-20 mA signal stuck at a rail or a fault current: a transmitter in fault, a process truly out of range, an open or shorted loop, a wrong range, a wiring error, an input channel fault, or a scaling clamp. How the current itself tells you which.
8 min readUpdated Sep 5, 2026
Analog Does Not Match Field Indicator
The gauge on the pipe says one thing and the screen says another: a scaling or range mismatch, a drifted transmitter, a drifted gauge, a different elevation or datum, different units, or damping that lags. Deciding which is right and where the error lives.
8 min readUpdated Sep 5, 2026
Loop Powers Up but Reads Zero
The transmitter is lit and the controller shows zero, so loop current is not reaching the input: a loop wired for power but not signal, an active and passive mismatch, a missing jumper, an unconfigured channel, a blown fuse, or a transmitter sending nothing.
7 min readUpdated Sep 5, 2026
How to Read a P&ID
What a piping and instrumentation diagram shows, how ISA-5.1 tag bubbles and line types encode the control system, and how to trace a loop across sheets.
8 min readUpdated Sep 5, 2026
Site Acceptance Testing
Proving the installed system with the real field: loop checks first, then end-to-end I/O, real communications, sequences on real equipment, alarms to real recipients, and failure modes on the real network, with the record that becomes the as-built.
8 min readUpdated Sep 5, 2026
Instrument Lists
The index of every measuring and final control device on a project: what the instrument list records, how it grows from the P&ID into specification sheets and calibration records, the columns that matter at each stage, and how it becomes the asset register the utility maintains.
8 min readUpdated Sep 5, 2026
Calibration Checklist
The checklist for one calibration event, from work order to the sticker on the instrument: notify operations, loop in manual, alarms inhibited, the reference standard, as-found, adjustment, as-left, the loop check, restoration, and the record.
8 min readUpdated Sep 5, 2026
EPA Rules and Their Control System Implications
The federal environmental agency rules that shape what a water or wastewater control system must measure, record, and report: the surface water rules, residual and byproduct rules, groundwater and lead rules, discharge permits, and record retention.
10 min readUpdated Sep 5, 2026
AWWA Standards and Manuals
What the American Water Works Association publishes that a control engineer uses: the instrumentation and control manual, the risk and resilience standard behind the act assessments, the security and emergency standards, and the cybersecurity tool.
8 min readUpdated Sep 5, 2026
Calibration Is a Program, Not an Event
An instrument calibrated once at startup and again when someone complains is an instrument whose readings cannot be trusted in between. What a calibration program is, and how a small utility runs one without a full-time instrument technician.
8 min readUpdated Sep 5, 2026
Measure Before You Replace
Parts-swapping is the most expensive way to troubleshoot and the most common. A method for controls faults that starts with a measurement, splits the problem in half at every step, and ends with a cause instead of a coincidence.
8 min readUpdated Sep 5, 2026
The Chlorine Residual Loop Is Slower Than You Think
Residual control loops oscillate because the measurement arrives minutes after the dose changes, and the tuning was done as if it arrived at once. Where the delay comes from, and how to control a loop whose dead time is longer than its response.
10 min readUpdated Sep 5, 2026
When the Transmitter and the Gauge Disagree
The SCADA screen says 62 psi and the gauge on the pipe says 58, and someone has to decide which one to believe. Why two readings of the same pressure differ, and a procedure for settling the argument with a reference instead of an opinion.
9 min readUpdated Sep 5, 2026
4-20 mA Current Loops
Why the industry standardized on current instead of voltage, how a two-wire loop is powered, what the live zero buys you, and how to read a loop with a meter.
12 min readUpdated Aug 29, 2026
4-20 mA Signal Is Unstable
An analog reading that will not settle. How to separate a genuine process swing from induced noise, a ground loop, or a loop running out of voltage.
8 min readUpdated Aug 23, 2026
Ground Loops in Instrumentation
Two grounds at different potentials, one signal circuit between them, and a reading that will not sit still. How to find it and how to prevent it.
8 min readUpdated Aug 8, 2026
How to Test a 4-20 mA Loop
Prove a current loop end to end with a meter and a calibrator, without interrupting a process you were not authorized to interrupt.
8 min readUpdated Aug 6, 2026
Magnetic Flowmeters
Faraday's law applied to a pipe: how a mag meter works, the conductivity and full-pipe requirements, and why grounding decides whether it reads.
8 min readUpdated Aug 1, 2026
Radar Level Measurement
Non-contact and guided wave radar, why it handles a wet well better than most alternatives, and the installation details that decide whether it works.
9 min readUpdated Jul 9, 2026
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Content on CurryControls.com is general technical reference information provided for educational and reference purposes only. It is not engineering advice for a specific installation. All information must be independently verified against current codes, standards, manufacturer documentation, project requirements, and qualified professional judgment before it is relied upon or applied.