Topic
Analog
34 guides across the knowledge base.
PLC I/O Systems
Discrete and analog input and output modules, sinking and sourcing, relay and transistor outputs, wiring conventions, and the module diagnostics that find field faults from the panel.
9 min readUpdated Sep 5, 2026
Function Block Diagram
The graphical data-flow language of IEC 61131-3: how blocks, pins, and wires work, execution order, where FBD beats ladder and structured text, building reusable equipment blocks, and the habits that keep a diagram readable.
9 min readUpdated Sep 5, 2026
Analog Signal Problems
The controller side of a bad analog value: raw counts against the module range, channel diagnostics, scaling that does not match the transmitter, wiring by device type, noise and filtering, resolution, and a symptom table that points at the check.
8 min readUpdated Sep 5, 2026
Raw Counts
What an analog input module actually hands the program: the count range, where 4 mA and 20 mA fall in it, how under-range and over-range show up, and why the module manual is the only source that counts.
7 min readUpdated Sep 5, 2026
Filtering Analog Inputs
How to damp a noisy analog input in the controller without hiding a real process change: the first-order filter, the moving average, the median, where each belongs, and what never to filter.
8 min readUpdated Sep 5, 2026
Deadband and Hysteresis
Why a single setpoint chatters, how a deadband turns it into a start point and a stop point, how wide to make it, and where deadbands belong in alarms, level control, and controller outputs.
7 min readUpdated Sep 5, 2026
Signal Validation
Catching a failed analog input in logic before it runs a pump on a dead transmitter: range checks, module status bits, frozen-value detection, rate checks, and what the program does with a bad value.
8 min readUpdated Sep 5, 2026
Engineering Units
Choosing and documenting the units a tag carries: where the unit is decided, how it flows from transmitter range to controller scaling to screen and historian, the conventions used in water and wastewater, and the mismatches that put a wrong number on screen.
7 min readUpdated Sep 5, 2026
Historian Data Collection
Deciding what the historian stores and how often: polled versus exception collection, the deadband per tag, rates for control loops, levels, totals, and compliance values, timestamps at the source, quality codes, calculated tags, and the tag list review that keeps a historian from filling with noise and missing what matters.
9 min readUpdated Sep 5, 2026
Historian Compression
How historians store years of data on a disk: exception deadbands at the collector, swinging door compression at the archive, what each discards and what it keeps, the settings that turn a trend into a staircase, lossless alternatives, and how to check that the compressed data still tells the truth.
8 min readUpdated Sep 5, 2026
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
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
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
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
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
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
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
Common-Mode Noise
Noise that rides on both conductors of a signal pair together, relative to ground: where it comes from, why a differential input rejects it only up to a limit, how to tell it from normal-mode noise with a scope, and the fixes that work.
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
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
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
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
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
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
PID Control for Pumps and Valves
What each term actually does, how to tune a loop on a real pump station, and the handful of mistakes that cause most of the oscillation people blame on tuning.
11 min readUpdated Aug 19, 2026
How to Scale a 4-20 mA Input in a PLC
Turn raw analog counts into engineering units, verify at three points, and add the validity checks that stop a failed transmitter from producing a believable number.
7 min readUpdated Aug 12, 2026
Analog Scaling: Raw Counts to Engineering Units
The arithmetic that turns an analog card reading into feet, psi, or gallons per minute — plus the range mismatches that silently corrupt it.
8 min readUpdated Jul 16, 2026
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