The Daily Loop โ€” Newsletter
โ† FIELD MANUAL
THE DAILY LOOP
// Instrumentation & Controls โ€” Dextrose Plant Training Series
Edition 01 4โ€“20mA Signal Fundamentals
// Signal Fundamentals
Why 4mA and Not Zero?

The 4โ€“20mA standard is one of the most elegant designs in industrial instrumentation โ€” and once you understand why it starts at 4, everything else makes sense.

If the signal started at 0mA, you couldn't tell the difference between "the process is at minimum" and "the wire is broken." Both would read zero. By starting at 4mA, the system has a live zero โ€” 4mA means "I'm alive and reading minimum." Zero means "something is wrong."

This is why every I&C technician has to know: below 3.8mA = fault condition. Always.

4mA = 0%  โ”‚  12mA = 50%  โ”‚  20mA = 100%
% of span = (mA โˆ’ 4) รท 16 ร— 100
// Signal Chain
From Process to PLC

Every 4โ€“20mA measurement follows the same path. Know this chain โ€” when something breaks, you'll know exactly where to look.

Process
Temp/Pressure
โ†’
Sensor
RTD / DP
โ†’
Transmitter
4โ€“20mA out
โ†’
Wiring
2-wire loop
โ†’
PLC Input
250ฮฉ / 1โ€“5V
// Daily Habit
Today's Non-Negotiables
  • Check one 4โ€“20mA loop with your Fluke โ€” measure mA in series
  • Convert the reading to engineering units using the formula
  • Compare to what the HMI shows โ€” should match
  • Ask one senior tech: "What's the weirdest 4โ€“20mA fault you've seen?"
  • Read one transmitter nameplate โ€” note range, type, wiring
// Fault Reference
Common 4โ€“20mA Faults
What You SeeLikely CauseFirst Check
0mA / below 3.8Open loop / no power24VDC at transmitter
4mA always (won't move)Sensing line blockedDrain/vent impulse line
Pegged at 20mA+Sensor failed highDisconnect sensor, does mA drop?
Noisy/jumpingGround loop / EMICheck shield termination
Offset (always off by X%)Calibration driftZero/span check with calibrator
"Measure a healthy system so you know what sick looks like."
// The Daily Loop Principle
THE DAILY LOOP
// Instrumentation & Controls โ€” Dextrose Plant Training Series
Edition 02 Control Valves โ€” Fisher Series
// Control Valve Fundamentals
Air-to-Open vs Air-to-Close: Why It Matters

Every control valve has a fail-safe position โ€” what it does when it loses air or power. This isn't an accident; it's engineered for your process safety.

Air-to-Open (ATO): Supply air opens the valve. Lose air โ†’ valve closes. Used on cooling water, steam, anything that should shut down on failure.

Air-to-Close (ATC): Supply air closes the valve. Lose air โ†’ valve opens. Used on applications where open is the safe state (vents, bypasses).

ATO = Fail CLOSED (spring closes on air loss)
ATC = Fail OPEN (spring opens on air loss)

Know your valve's fail position BEFORE you work on it.
// Positioner Basics
The Positioner: Your Valve's Brain

The positioner takes a 4โ€“20mA signal from the PLC and converts it to the exact air pressure needed to hold the valve plug at the commanded position. Without it, valves don't hold position accurately.

Inside: a nozzle-flapper mechanism compares the input signal to actual stem position via feedback linkage. Any error โ†’ it adjusts air pressure until they match.

Key failure: positioner hunting โ€” valve oscillates around setpoint. Usually means worn feedback linkage or air supply fluctuation.

// Daily Habit
Today's Non-Negotiables
  • Find one Fisher control valve in your area
  • Identify: ATO or ATC? How do you know?
  • Check air supply pressure at the positioner
  • Watch the valve respond to a signal change at HMI
  • Locate the manual handwheel โ€” know which way opens
// Fault Reference
Control Valve Faults
SymptomCauseCheck
Valve won't moveNo air supplyPSI at positioner
Valve stuck mid-travelStem packing tightManual override test
Hunting/oscillatingPositioner gainRecalibrate positioner
Signal changes, no movementAir tubing offTrace tubing path
Valve at wrong positionPositioner zero offZero/span calibration
"A valve that won't move is usually an air problem. Check air first."
// 90% of the time, this is true
THE DAILY LOOP
// Instrumentation & Controls โ€” Dextrose Plant Training Series
Edition 03 Limit Switches & Position Feedback
// Position Feedback
Why the PLC Doesn't Trust You

When the PLC commands a valve to open, it doesn't assume the valve actually opened. It waits for position confirmation from a limit switch before allowing the next step in the sequence.

This is why a single cam-slipped Westlock can stop an entire crystallizer cycle โ€” the valve moved, but the switch didn't confirm it, so the PLC thinks it's still closed and won't proceed.

PLC Command โ‰  Valve Movement
PLC Command + Limit Switch Confirmation = Process Advance

If the sequence stops: ask "is the confirmation signal reaching the PLC?"
// NO vs NC
The Contact That Trips Everyone Up

Normally Open (NO) contacts are open at rest โ€” no current flows until something activates them. They go HIGH when the valve reaches position.

Normally Closed (NC) contacts are closed at rest โ€” current flows until something activates them. They go LOW when triggered. Used in safety circuits because a broken wire fails safe (no current = safe).

The trap: On a crystallizer limit pack, Terminal 3 (open confirmation) may be NO or NC depending on the model. Always check the wiring diagram โ€” never assume.

// Daily Habit
Today's Non-Negotiables
  • Find the Westlock on one crystallizer valve
  • Manually stroke the valve โ€” listen for the click
  • Watch the HMI position indicator change
  • Locate the cam and verify it's contacting the switch
  • Find the wiring drawing for that valve
// Fault Reference
Limit Switch Faults
SymptomCauseCheck
Sequence won't advanceNo position confirmationHMI input status
Position wrong on HMICam slipped on stemVisually confirm cam alignment
Flickers on HMILoose wire / vibrationTug test on terminals
Always reads wrong stateNC/NO wired wrongCheck loop drawing
No change when valve movesDead switchContinuity test on switch
"No click = no confirmation. No confirmation = no next step."
// Crystallizer troubleshooting rule #1
THE DAILY LOOP
// Instrumentation & Controls โ€” Dextrose Plant Training Series
Edition 04 Troubleshooting Method โ€” I โ†’ L โ†’ O
// The Method
I โ†’ L โ†’ O: Input, Logic, Output

Every "it won't work" problem in industrial controls can be diagnosed with three questions asked in this exact order:

I โ€” Is the INPUT reaching the PLC?
Force a sensor, jumper a switch, inject a known mA signal.

L โ€” Is the LOGIC responding correctly?
Watch the HMI. Does the PLC see the input? Does it make the right decision?

O โ€” Is the OUTPUT energizing?
Does the solenoid LED come on? Is 24VDC present at the output terminal?

This method isolates: sensor problem vs PLC/logic problem vs output device problem. You'll never chase the wrong end of a circuit again.

// The Order Matters
Power โ†’ Air โ†’ Signal โ†’ Logic

Before you run Iโ†’Lโ†’O, verify the basics in this sequence:

1. Power: Is 24VDC present? Check fuses, power supplies, breakers.

2. Air: Is supply pressure adequate? Check header gauge, regulator output.

3. Physical movement: Does the actuator/valve actually move with manual override?

4. Signal integrity: Is the mA reading correct? Is the discrete signal correct?

5. Logic last: Only if everything above checks out โ€” then look at the PLC program.

// Daily Habit
Today's Non-Negotiables
  • Pick one recent "it stopped working" fault
  • Write down what the INPUT showed
  • Write down what the PLC LOGIC saw
  • Write down what the OUTPUT did
  • Identify which step in the chain failed
// Common Traps
Mistakes New Techs Make
  • Replacing a component before testing it
  • Blaming the PLC before checking field devices
  • Forgetting to check air supply first
  • Not asking "what changed recently?"
  • Fixing the symptom, not the root cause
"The surgeon finds the root cause. The parts-swapper gets lucky sometimes."
// Be the surgeon
THE DAILY LOOP
// Instrumentation & Controls โ€” Dextrose Plant Training Series
Edition 05 Calibration โ€” Fluke 754 & Rosemount 3144P
// Calibration Fundamentals
As-Found vs As-Left: Document Both

Every calibration has two sets of numbers: as-found (what you measured before touching anything) and as-left (what you set it to after). Both matter.

As-found tells you if the instrument drifted and by how much. If everything is always within spec as-found, you might be calibrating too often. If it's always out, you have a problem โ€” environment, vibration, or the instrument is failing.

As-Found โ†’ Document โ†’ Adjust โ†’ As-Left โ†’ Document

Never skip as-found. It's your evidence that the process data was good (or bad) before your visit.
// Fluke 754 Split Screen
MEASURE + SOURCE Together

The Fluke 754 split-screen lets you simultaneously source a known mA signal (simulating the PLC output) while measuring the transmitter's temperature reading.

Left side: MEASURE โ†’ TEMP โ†’ RTD โ†’ PT100 (reads actual temperature from the sensor)

Right side: SOURCE โ†’ mA โ†’ LOOP PWR (sources mA and provides 24V loop power)

At 4mA โ†’ transmitter should read LRV (e.g., 50ยฐF). At 20mA โ†’ should read URV (e.g., 300ยฐF). Any deviation = calibration adjustment needed.

// Daily Habit
Today's Non-Negotiables
  • Review one transmitter's last calibration record
  • Note the as-found error โ€” was it within tolerance?
  • Practice the 4mA โ†’ 0% and 20mA โ†’ 100% conversion
  • Find a Rosemount 3144P โ€” read the nameplate
  • Ask your mentor: what's the calibration tolerance for this transmitter?
// Calibration Reference
5-Point Linearity Check
% SpanSource (mA)Expected (ยฐF)Tolerance
0%4.0050.0ยฑ1ยฐF
25%8.00112.5ยฑ1ยฐF
50%12.00175.0ยฑ1ยฐF
75%16.00237.5ยฑ1ยฐF
100%20.00300.0ยฑ1ยฐF
"Document everything. Future-you will thank present-you."
// As-found and as-left. Always.