A pressure transmitter's nameplate reads 0–250 psi / 4–20 mA. What exactly is that line promising?

At 0 psi it drives exactly 4 mA, at 250 psi exactly 20 mA, and it holds a straight line in between — the two endpoints of the map the software must copy.. The published range is a contract: two endpoints plus linearity, nothing more. Everything the graphic ever shows for the point is read off that straight line, which is why the software's scale has to copy both endpoints exactly. Note that (d) is false in a useful way — the signal absolutely can leave the band, and the space outside it (under 4 mA, saturated above 20 mA) is exactly the diagnostic vocabulary the other questions in this quiz drill. Survivable overpressure (c) is a separate spec on the datasheet, not the range.

A 4–20 mA transmitter is spanned 0–200 psi (0–1379 kPa). The point's raw signal reads 16 mA. What value should the graphic show, in psi?

150 psi. 16 mA sits at (16 − 4) / (20 − 4) = 12/16 = 75% of the signal span, and 75% of 0–200 psi is 150 psi (75% of 0–1379 kPa is 1034 kPa — the same fraction on any scale). The classic slip is dividing by 20 instead of the 16 mA span: 16/20 = 80% → 160 psi, wrong by just enough to look right. Subtract the live-zero floor first, then divide by the span.

A tech commissions a duct-static point. The value tracks smoothly with the fan and looks healthy — but a hand gauge at the sensor tap reads about half what the graphic says. Here's the point config. What's wrong? point: SF-1 DUCT STATIC input: UI4 (0-10 V) xmtr: nameplate 0-2.5 in. w.c., output 0-10 V scale: 0 V → 0.0 in. w.c. 10 V → 5.0 in. w.c. symptom: graphic reads ≈ 2× the hand gauge

The software span (0–5) doesn't match the transmitter's published range (0–2.5): the sensor means 10 V as 2.5 in. w.c., the software reads it as 5.0 — so every reading is exactly double, plausibly, forever.. The two endpoints are the whole contract, and here the software broke it: the sensor's 10 V means 2.5, the scale says 5.0. Nothing alarms, the value moves with the process, and the loop will happily control to a number that's twice the truth. The field tell is the ratio: calibration drift is small and uneven, but a reading wrong by an exactly round factor (2×, 10×) is a span mismatch. Fix the scale to the nameplate, then re-check against the gauge.

A graphic shows duct static at 1.8 in. w.c. (450 Pa). You discover the software point is scaled 0–5 in. w.c., but the transducer nameplate says 0–2.5 in. w.c. (both sides are 0–10 V). What is the duct really at, in in. w.c.?

0.9 in. w.c.. Work back to the wire, then forward through the right range. The software shows 1.8 on a 0–5 scale, so the signal is 1.8 / 5 = 36% of span → 3.6 V. The transducer drives 3.6 V when it measures 36% of its range: 0.36 × 2.5 = 0.9 in. w.c. (225 Pa). Same wire, same volts — the graphic is reading the sensor's honest answer through the wrong ruler, and every value comes out exactly double.

A 4–20 mA input reading 0 mA and a 0–10 V input reading 0 V tell you the same thing.

False. They could not be more different. On a 4–20 mA point, 0 mA is below the live zero — no real measurement can produce it, so it can only mean a broken loop, lost power, or an open terminal: the value itself testifies to the fault. On a 0–10 V point, 0 V is a legal value — the bottom of the range — so the same reading is ambiguous between "truly at range-min" and "broken wire," and the graphic cannot tell you which. That asymmetry is the entire diagnostic payoff of the live zero.

A chilled-water pressure point on the graphic reads −62.5 psi. The point is a 4–20 mA transmitter scaled 0–250 psi. What is that number telling you?

The input is at 0 mA — the full 4 mA below the live zero, which scales to −25% of range. The loop is broken, unpowered, or open; it isn't measuring anything.. Scale 0 mA through the line and you get (0 − 4) / 16 = −25% of span → −62.5 psi: the b in the point's y = mx + b, and a number no real gauge could ever read. That impossibility is the feature — a dead 4–20 loop announces itself with a value that can't be mistaken for data. Calibration drift (a) is small and plausible-looking, not a quarter of the range negative; a vacuum (c) doesn't reach −62.5 psi; an inverted span (d) would read high, not impossible-low.

You suspect a 4–20 mA pressure loop is dead, but the graphic just shows 0.0 psi — the bottom of the range — with no fault flag. This front end clamps under-range values at range-min. How do you confirm from the workstation?

Open the point's detail and read the raw signal: the clamped display hides the negative, but a raw value near 0 mA — below the ~3.8 mA under-range threshold — can only be a fault.. Scaled values are a presentation — they can clamp, round, or default, and this front end clamps. The raw signal is the wire's own testimony: anything below ~3.8 mA sits under the live zero where no real measurement lives, so 0 mA raw settles it without leaving the chair. (A flat trend (a) is consistent with a dead loop but also with a dead-calm process; and note the same trick is weaker on a 0–10 V point, where 0 V raw is still ambiguous — the live-zero advantage restated.)

A 0–10 V input can't distinguish a broken wire from a genuine zero — both read 0 V. So how do you ever catch a dead 0–10 V sensor from the graphic?

Process knowledge: when range-min is a value the real process can't plausibly produce — 0% RH outdoors, zero flow with the fan proven on — a flatlined range-min is a fault wearing a legal costume.. The signal can't tell you, but the physics can: outdoor air is never at 0% RH, a proven-on fan never moves zero air, an occupied space is never at 0 ppm CO2. A 0–10 V point pinned at range-min for hours is a suspicion the graphic can raise even though only a meter can confirm it — which is the half-truth in (a). Choice (d) is the 4–20 mA story, not the 0–10 V one: with no live zero there's no below-range space for a fault to land in. This ambiguity is exactly why critical points earn live-zero signals.

A duct-static trend climbs during a fan ramp, then flatlines at exactly 2.5 in. w.c. (625 Pa) — the top of the sensor's 0–2.5 range — and holds there, perfectly smooth, for hours. What is the trend actually telling you?

The static is at least 2.5 — the sensor is railed at full scale, and the real value is somewhere above it, unmeasured.. A railed reading carries exactly one bit: the truth is at least this much. It's a ceiling, not a measurement. The tells are all there — a round number, sitting exactly at range max, suspiciously smooth (real processes wander; ceilings don't). Note (c) misses the point in a dangerous way: the sensor hasn't failed, it's answering honestly at the edge of what it can say — and whatever pushed the duct past its range is still out there, above the flatline.

You're selecting a static-pressure transducer for a duct that will control to 1.5 in. w.c. (375 Pa). Which range serves the graphic best?

0–2.5 in. w.c. — the operating point lives mid-span, with resolution to read it and headroom above it to see an abnormal excursion.. Both extremes fail, in opposite directions. Range-topped at the setpoint (a), the sensor rails during exactly the events you most need to see — every abnormal excursion vanishes over the ceiling. Oversized (b), normal operation lives in the bottom 15% of the scale, where an accuracy spec written as a percent of full scale becomes a big slice of the reading. Mid-span (c) buys both resolution and headroom. And (d) confuses correct with useful: scaling faithfully maps whatever the sensor can say — it can't add precision or extend the ceiling.

Practice · Analog Sensing

Analog Sensing Quiz Signals

Read the Analog Sensing lesson first, then test yourself — ten questions on the reading side of analog signals: what the published range promises, scaling a raw mA value to engineering units, the span-mismatch that reads plausibly wrong forever, what a live zero buys you at the graphic, and why a value flatlined at the top of its range is a ceiling, not a measurement. Each reveal links back to the section it came from.

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