💡 Diagnosis
Adjust the controls to see how the signal chain behaves.
| Band | Current | Meaning | Status |
|---|
2-Wire (Loop Powered)
Where you see it: most pressure, level, and temperature transmitters. The transmitter regulates loop current to represent the measurement while drawing its own operating power from that same current, so its electronics must run on less than 4 mA at zero scale.
4-Wire (Self Powered)
Where you see it: pH and conductivity analyzers, magnetic flow meters, chromatographs — anything whose electronics, display, or excitation current needs more power than a 4 mA budget allows. The signal pair is independent of the power pair, so the output can be isolated.
| % of Span | Process Value | Signal | Sense Voltage | PLC Raw |
|---|
| Resolution | Counts | Signal per Count | EU per Count |
|---|
💡 How Scaling Works
The analog input module converts the loop signal to an integer. Scaling is a straight linear map from that integer to engineering units:
EU = (Raw − RawMin) / (RawMax − RawMin) × (URV − LRV) + LRV
The most common field mistake is scaling to the module’s count range while the instrument is ranged differently — the math is right and the reading is still wrong. Always confirm the transmitter LRV/URV and the module scaling agree before you trust the number on the screen.
💡 Tuning Feedback
Run a step test to evaluate the current tuning.
Select a scenario to load it and see the field diagnosis.
| Fault | 4–20 mA Signature | 0–10 V Signature | First Check |
|---|---|---|---|
| Open wire | 0 mA — hard NE 43 under-range | 0 V — indistinguishable from 0% | Loop current with a clamp or series meter |
| Shorted transmitter | Rises above 21 mA or pins to supply limit | Pins to 0 V or supply rail | Terminal voltage at the transmitter |
| EMI / noise | Rapid jitter about the true value | Larger jitter, worse over distance | Shield continuity and cable routing |
| Ground loop | Steady 50/60 Hz ripple on the signal | Same, plus a DC offset | Shield grounded at one point only |
| Zero shift | Constant offset at every point | Constant offset at every point | Re-zero at the LRV condition |
| Span error | Error grows with process value | Error grows with process value | Two-point calibration, LRV and URV |
| Lost cold-junction comp. | Reading drifts with panel temperature | Same drift | Terminal block temperature and CJC sensor |
⚠ Read This First
Everything below is paraphrased for teaching and follows the NEC (NFPA 70) 2023 article numbering. Article numbers move between editions — Class 1 circuits, for example, moved from Article 725 to Article 724 in the 2023 edition. Always work from the adopted code edition in your jurisdiction and the manufacturer’s listed instructions, which NEC 110.3(B) makes mandatory in its own right.
| Size | DC Resistance | Loop Ω per 100 ft | Drop at 20 mA per 100 ft |
|---|
Class 2 power-limited circuits — NEC Article 725. Most 24 VDC instrument loops are installed as Class 2 circuits. The classification comes from the source, not the wire: the circuit has to be supplied by a listed Class 2 power source (or listed equipment marked as a Class 2 source), and that inherent power limitation is what relaxes the wiring rules compared with light and power circuits.
Separation from power conductors — NEC 725.136. Class 2 conductors may not occupy the same cable, raceway, or enclosure as electric light, power, or Class 1 conductors, except under the specific conditions the section lists (barriers, listed composite cables, and similar arrangements). This is the code rule behind the practice you already know as “keep signal wire out of the power conduit” — the electrical noise consequence and the code requirement point the same direction.
Mixed systems in one enclosure — NEC 300.3(C)(1). Where conductors of different systems do share a raceway or enclosure, every conductor’s insulation rating must be at least the maximum voltage present. A 300 V instrument cable does not belong in a box with 480 V conductors.
Mechanical execution — NEC 725.24 and NEC 300.11. Instrument cable has to be supported by the building structure and installed in a neat and workmanlike manner. Cable draped over a pipe is a violation before it is ever a noise problem.
Intrinsically safe systems — NEC Article 504. An IS loop limits energy so that no spark or thermal effect can ignite the hazardous atmosphere, which is why 4–20 mA instrumentation dominates in classified areas. The protection lives in the whole system — barrier, field wiring, and instrument together — and the control drawing is part of the listing.
Separation — NEC 504.30. IS conductors must be separated from non-IS conductors, including by the specified spacing where they are not in separate raceways or otherwise partitioned. Running one IS pair through a non-IS wireway voids the protection concept.
Identification — NEC 504.80. IS circuits must be labeled and their raceways and cables identified. Where color coding is used for IS conductors, light blue is the identified color, permitted only where no other light blue conductors are present.
Classified location wiring — NEC Article 501 (Class I) governs the wiring methods and sealing fittings themselves. Intrinsic safety is one permitted protection technique among several; explosionproof and purged enclosures are others.
Bonding — NEC Article 250, including 250.96(A). Metal enclosures, raceways, and equipment must be bonded together to form an effective ground-fault current path. That is a safety requirement, and it is independent of anything you do with a cable shield.
Single-point shield grounding is practice, not NEC text. The NEC does not tell you which end of an instrument cable shield to land. Grounding a shield at both ends creates a circulating path when the two ground references sit at different potentials, and that is exactly the 50/60 Hz ripple this simulator injects under “ground loop.” Ground the shield at one end — conventionally the control panel — and leave the field end insulated and terminated.
Isolation as an alternative. Where two grounded devices genuinely must connect, a signal isolator galvanically breaks the loop rather than fighting it. This is the standard fix when a 4-wire analyzer with a grounded output feeds a PLC input that is also grounded.
| Standard | Covers | Why it matters here |
|---|---|---|
| ANSI/ISA-50.00.01 | 4–20 mA DC transmission signal | Defines the analog signal this whole tool is built around |
| NAMUR NE 43 | Failure signal levels on 4–20 mA | ≤3.6 mA and ≥21 mA carry diagnostic meaning, not measurement |
| IEC 60751 | Industrial platinum RTDs | Pt100 = 100 Ω at 0 °C; Callendar–Van Dusen curve |
| IEC 60584 | Thermocouple reference tables | Type K mV-versus-temperature values and tolerance classes |
| ISA-5.1 | Instrumentation symbols and tag identification | PT, TT, FT, LT loop tagging on P&IDs |
| IEC 61511 / 61508 | Safety instrumented systems | Where diagnostic coverage and fail-safe direction become design requirements |
| NFPA 79 | Electrical standard for industrial machinery | Control panel conductor colors and machine wiring practice |
💡 What This Tool Does Not Do
This simulator has no code-compliance engine. It does not evaluate conductor ampacity, box fill, overcurrent protection, area classification, SIL verification, or hazardous-location suitability, and its fault models are illustrative rather than instrument-specific. Use it to build intuition about the signal chain, then take real decisions to the adopted code, the listed instructions, and a qualified engineer.
Every automated process depends on turning a physical quantity into a number a controller can act on. Pressure becomes a current, current becomes an integer, and an integer becomes a decision. Each conversion is a place the truth can change without anything announcing it.
4 mA is 0% of span and 20 mA is 100%. The measurement never rides on zero current, which is what makes the scheme diagnosable: no current at all cannot be a valid reading, so it must be a broken loop.
Current also survives the trip down the cable. In a series loop the same current flows at every point, so conductor resistance does not change the measurement — it only eats supply voltage. That is why a current loop can run thousands of feet where a voltage signal cannot.
A 2-wire transmitter needs a minimum terminal voltage — commonly around 12 V, though it varies by instrument — to run its electronics. Everything else in the loop takes its share of the supply as I×R at the worst-case 20 mA.
Vsupply ≥ Vxmtr,min + 0.020 × (Rsense + Rwire + Rbarrier)
Rearranged, that gives the maximum total loop resistance a supply can drive. This is the calculation behind “it worked on the bench and died in the field” — the bench had three feet of wire and no IS barrier.
RTD. A platinum element whose resistance rises predictably with temperature. Pt100 means 100 Ω at 0 °C, and about 138.5 Ω at 100 °C. Accurate and stable, but slower, and it needs excitation current from the transmitter.
Lead wire matters. Because an RTD measures small resistance changes, lead resistance shows up directly as error. A 3-wire connection cancels it when the leads are reasonably balanced; a 4-wire Kelvin connection removes it outright and is the choice for laboratory-grade work.
Thermocouple. Two dissimilar metals produce a voltage that depends on the temperature difference between the measuring junction and the terminal block. Type K gives roughly 41 µV per °C near ambient — a tiny signal in an electrically hostile place.
The analog input module reports an integer. A 15-bit module gives 0–32767 counts, a 12-bit module only 0–4095. Scaling maps that integer linearly onto the instrument’s calibrated range.
Resolution is not accuracy. A 16-bit module reading a transmitter with 0.5% error still has 0.5% error — now reported to five decimal places. Resolution bounds what the module can represent; accuracy is a property of the instrument and its calibration.
A dry contact carries no source of its own — it just opens or closes, and the input card supplies the sensing voltage. Limit switches, float switches, and relay contacts all work this way.
A wet contact brings its own voltage. Feeding one into an input expecting a dry contact can read incorrectly or damage the card. Match the contact type to the input type before you land the wire.
Supervision buys diagnosis. An ordinary discrete input cannot tell an open contact from a cut wire — both are simply open. Add an end-of-line resistor and a supervised input, and the card can distinguish normal open, normal closed, open circuit, and short circuit by the resistance it measures.
Open wire: current collapses to zero. Below the NE 43 threshold, so a 4–20 mA input flags it immediately; a 0–10 V input cannot tell it from a genuine zero.
Short: current climbs past the top of the range or pins at whatever the supply can deliver. Above 21 mA is failure information, not measurement.
Noise: fast random jitter around the true value. Sources are VFDs, contactors, and unshielded runs beside power conductors.
Ground loop: a steady 50/60 Hz ripple, sometimes with an offset. Comes from two ground references at different potentials, not from the instrument.
Zero shift versus span error: a zero shift offsets every reading equally; a span error grows with the process value. Check the instrument at both ends of its range and the two are unmistakable.
Proportional acts on present error — fast, but on its own it settles with a permanent offset. Integral acts on accumulated error and removes that offset, at the cost of overshoot and windup. Derivative acts on the rate of change and damps the result, while amplifying any noise you feed it.
Dead time is the real constraint. During transport delay the controller is acting on stale information. The longer the dead time relative to the time constant, the more conservative the tuning has to be — no amount of gain fixes a loop that cannot see what it just did.
Windup happens when the output saturates and the integral keeps accumulating anyway. When the process finally responds, all that stored error has to be unwound, and the result is a large overshoot. Clamping the integral is the standard defense.