The short answer
Engineering Units
An engineering unit is the physical unit a scaled value represents, and it has to be the same at every point where the value is used: the transmitter range, the controller scaling, the tag on the screen, the alarm setpoints, the historian, and the reports. The controller is the right place to convert raw counts into engineering units once, in a floating point tag, with the unit written into the tag description and the I/O list; everything downstream then displays and stores that value without converting again. Most unit problems are not conversions done wrong but conversions done twice, done in one place and not another, or done against a range that someone changed. The discipline is a single documented unit per tag, chosen from the plant conventions, applied in the controller, and carried unchanged to every consumer.
Key points
- Convert to engineering units once, in the controller, into a floating point tag; every consumer uses that value.
- The unit is part of the tag definition: in the description, the I/O list, the screen, and the historian.
- Pick units from the plant conventions and keep them: gallons per minute or million gallons per day, feet or psi, not both for the same kind of tag.
- A transmitter range change is a unit change until the controller scaling is updated to match.
- Alarm setpoints, totalizers, and reports inherit the unit; a wrong unit at the tag is wrong everywhere.
Where the unit is decided
The unit starts at the instrument: a transmitter is ranged in a unit, and its 4 to 20 milliamps represents that range. The controller scales the raw count to the same range in the same unit, and from that point the value is a number with a meaning. The screen shows the unit next to the number, the alarm setpoints are entered in it, the historian stores it, and the report prints it. Each of those places can hold its own idea of the unit, which is why the unit has to be written down where every one of them can be checked against it: the tag description in the controller and the instrument entry in the I/O list.
| Place | What it holds | Must match |
|---|---|---|
| Transmitter | Lower and upper range values in a unit | The design range on the instrument list |
| Controller scaling | Raw minimum and maximum to engineering minimum and maximum | The transmitter range and unit exactly |
| Controller tag | The scaled floating point value; the unit in the description | The I/O list |
| Screen | Value, format, and the unit label | The controller tag unit |
| Alarm setpoints | Numbers entered by engineers and operators | The controller tag unit |
| Historian | The value and a unit attribute | The controller tag unit |
| Reports and totals | Sums, averages, and conversions for compliance | The historian unit; conversions documented |
Conventions in water and wastewater
| Measurement | Usual units | Notes |
|---|---|---|
| Flow, pumps and pipes | gpm | Instantaneous rate on screens and in control logic |
| Flow, plant and reporting | MGD | Daily and regulatory reporting; 1 MGD = 694.4 gpm |
| Level, wells and tanks | ft | Above a documented datum, usually the tank floor or the wet well invert |
| Pressure | psi | Distribution and pump discharge; convert to head only in hydraulic calculations |
| Head | ft | 2.31 feet of water per psi for fresh water |
| Chemical residuals and concentrations | mg/L | Chlorine, fluoride, phosphorus, ammonia |
| Turbidity | NTU | Filter effluent and raw water |
| pH | pH | Unitless on a 0 to 14 scale |
| Dissolved oxygen | mg/L or percent saturation | Choose one for control; document it |
| Speed | percent or Hz | Drive reference; percent is friendlier on screens |
| Temperature | degrees F or C | One plant, one convention |
| Volume | gal or MG | Totalizers; roll over documented |
| Chemical feed | gph or lb/day | Match the pump and the dosing calculation |
MGD = gpm × 1440 ÷ 1,000,000
- MGD = million gallons per day
- gpm = gallons per minute
- 1440 = minutes per day
head (ft) = psi × 2.31 ÷ SG
- head = feet of liquid column
- psi = gauge pressure
- SG = specific gravity, 1.0 for fresh water
Convert once
The controller holds the raw count and produces one scaled value in engineering units, in a floating point tag, using the range from the instrument list. That tag is the value. Screens format it and label it but do not rescale it; the historian stores it as it is; reports convert it for presentation, with the conversion written into the report and not into the tag. A second scaling in the screen tag, a percent tag derived from an engineering tag and then displayed as if it were engineering, or a historian that stores the raw count and scales on retrieval are the ways the same measurement ends up with three values.
Documenting it
- The I/O list carries the range and unit for every analog point, and it is the reference the controller scaling is checked against.
- The tag description in the controller states the unit: Wet well level, ft above invert.
- Screen labels show the unit next to every value; a number without a unit is a guess.
- Historian tag attributes carry the unit so that reports and exports are labeled.
- Totalizer tags state the unit and the rollover value.
- Derived values state what they were derived from and how, especially conversions between rate and daily volume.
Mistakes that recur
- Gallons per minute shown as million gallons per day
- A rate tag labeled with a daily unit, or a daily total divided incorrectly. The report and the screen disagree by a factor of 694.
- Pressure and head confused
- A tank level in feet compared with a pressure setpoint in psi; a difference of 2.31 that looks like a calibration drift.
- Gauge and absolute
- A transmitter ranged in absolute pressure scaled as gauge; a constant offset of 14.7 psi at sea level.
- Percent of range as engineering
- A tag scaled 0 to 100 percent displayed with an engineering label; correct at the ends and wrong everywhere between if the range does not start at zero.
- Level datum
- A transmitter zeroed at its own elevation while the screen implies the tank floor; the level reads high or low by the mounting height.
- Temperature scale
- A Celsius transmitter on a Fahrenheit plant; alarms set in the wrong scale.
Frequently asked questions
- Should the controller work in percent and let the screen convert?
- No. Percent hides the range and the unit, and every consumer then needs the range to make sense of the number. The controller scales to engineering units once; a percent value can be derived for a bar graph if a screen wants one.
- Where do I convert gallons per minute to million gallons per day?
- In the report or the daily totalizer calculation, documented as a conversion, not in the flow tag. The flow tag stays in the unit the instrument and the control logic use.
- What data type should an engineering unit tag be?
- A 32-bit floating point value. Integers scaled by ten or a hundred were a memory-saving habit on old controllers and now cause more errors than they save; a float carries the value and its precision without a hidden scale factor.
- The transmitter is ranged in feet and the operators think in gallons. Which does the tag carry?
- The measured unit, feet. A volume in gallons is a calculation from the tank geometry, done in the controller into a second tag with its own description. Both are real values; neither is the other relabeled.
Related topics
- Analog Scaling: Raw Counts to Engineering UnitsThe arithmetic that turns an analog card reading into feet, psi, or gallons per minute — plus the range mismatches that silently corrupt it.
- Raw CountsWhat 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.
- How to Scale a 4-20 mA Input in a PLCTurn 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.
- I/O ListsThe document that ties the field to the controller: what every I/O list column means, how tags, terminals, and module channels are assigned, spare capacity rules, how the list drives the drawings and the program, and how to keep it true after commissioning.
- Analog Does Not Match Field IndicatorThe 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.
- Reporting from the HistorianTurning history into the reports a utility must produce: the monthly operating report, compliance reports for the regulator, daily operator summaries, pump run and energy reports, the calculations behind daily minimums, maximums, and totals, handling bad quality and gaps, and building reports once so they run every month without an engineer.
Direct contact
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