Signals and loops
4–20 mA
The standard analog current signal for process transmitters: 4 mA at the lower range value, 20 mA at the upper. The live zero at 4 mA tells a real zero from a broken wire and leaves enough current to power a two-wire transmitter. See the 4–20 mA converter.
Live zero
A signal whose zero is not zero current or voltage, such as 4 mA in a 4–20 mA loop. A dead loop (0 mA) is therefore recognisable as a fault.
LRV and URV
Lower and upper range value: the process values a transmitter is calibrated to at 4 mA and 20 mA. The difference between them is the span.
NAMUR NE 43
A NAMUR recommendation that defines how a 4–20 mA signal reports failures: currents up to 3.6 mA and from 21 mA signal a fault, 3.8 to 20.5 mA is a valid measurement including slight under- and over-range.
HART
A digital protocol superimposed on the 4–20 mA current as a small frequency-shift signal. It needs a loop resistance of typically at least 250 Ω for the handheld or modem to read the signal.
Two-wire transmitter
A transmitter powered by the loop it signals on. It needs a minimum terminal voltage at the highest loop current; the loop power calculator checks the budget.
Load resistance (burden)
The total series resistance in a current loop: input resistors, barriers and the cable. Every ohm takes voltage away from the transmitter.
Square-root output
A transmitter output proportional to the square root of the measured differential pressure, used for flow measurement with orifice plates and similar primary elements.
Temperature
RTD
Resistance temperature detector: a sensor whose resistance rises with temperature, usually platinum. See the PT100 / PT1000 calculator.
PT100 and PT1000
Platinum RTDs with 100 Ω and 1000 Ω at 0 °C. Both follow the same IEC 60751 curve; a PT1000 is less affected by lead resistance and self-heating.
Callendar–Van Dusen equation
The equation IEC 60751 uses for the resistance of an industrial platinum RTD: R(t) = R₀ [1 + A·t + B·t² + C·(t − 100)·t³], with the C term only below 0 °C.
Tolerance class (IEC 60751)
The permitted deviation of an RTD element from the standard curve: class AA ±(0.10 + 0.0017·|t|) °C, A ±(0.15 + 0.002·|t|) °C, B ±(0.30 + 0.005·|t|) °C.
2-, 3- and 4-wire connection
Ways to connect an RTD. Two wires add the full lead resistance to the reading, three wires cancel it if the leads are equal, four wires remove it. See the RTD lead-wire error calculator.
Self-heating
The temperature rise of a sensor caused by its own measuring current. It makes RTDs and thermistors read high; small currents keep it negligible.
Thermocouple
Two dissimilar metals joined at the measuring point; they produce a voltage that depends on the temperature difference to the reference junction. See the thermocouple calculator and the thermocouple types.
Cold junction compensation
Correcting a thermocouple reading for the temperature of the reference (cold) junction, usually the instrument terminals. The correct method adds voltages, not temperatures.
Seebeck coefficient
The sensitivity of a thermocouple, in µV/°C: the slope of its voltage-temperature curve at a given temperature.
Extension and compensating cable
Cable that continues a thermocouple to the instrument. Extension cable uses the thermocouple alloys; compensating cable uses cheaper alloys that match over a limited temperature range.
ITS-90
The International Temperature Scale of 1990. NIST publishes thermocouple reference functions on this scale, adopted in IEC 60584-1.
NTC thermistor
A resistor whose resistance falls steeply with rising temperature. See the NTC thermistor calculator.
Beta value (β, B)
The constant of the NTC Beta model, in kelvin, stated for a pair of temperatures such as B25/85. It describes the curve well near those temperatures only.
Steinhart–Hart equation
1/T = A + B·ln R + C·(ln R)³: an NTC model fitted exactly to three calibration points, accurate over a wide range.
Pressure and transmitters
Gauge, absolute and differential pressure
Pressure relative to the local atmosphere, to vacuum, or between two process points. Units such as bar or psi do not say which; suffixes like barg and bara do. See the pressure unit converter.
URL (upper range limit)
The highest range a pressure transmitter’s sensor can be set to. Temperature effect and stability are usually specified as a percentage of the URL.
Turndown
The ratio of URL to calibrated span. A high turndown makes the URL-based errors large compared with the span.
Reference accuracy
The error of a transmitter at constant laboratory conditions, usually a percentage of span. In service, temperature effect and drift add to it.
Total probable error (TPE)
The combined error of reference accuracy, temperature effect and long-term stability, taken as the root-sum-square. See the pressure transmitter accuracy calculator.
Load cells
Rated output (mV/V)
The signal of a load cell per volt of excitation at full rated load. A 2 mV/V cell at 10 V gives 20 mV at capacity. See the load cell output calculator.
Excitation
The supply voltage across a load cell’s bridge. The output is proportional to it, so six-wire connections sense it at the cell.
Junction (summing) box
The box that connects several load cells in parallel and averages their outputs into one signal.
Dead load and live load
The weight of the platform or vessel (dead load) and of the product being weighed (live load). Both use up capacity; only the live load is weighed.
Division (e)
The smallest weight step a scale displays. Indicators need a minimum signal per division, typically 0.5 to 1 µV.
Threads
G (BSPP) and R (BSPT)
British pipe threads with a 55° flank: G is parallel and seals on a gasket or O-ring (ISO 228-1), R is tapered 1:16 and seals on the thread (ISO 7-1). See the thread identifier and thread dimensions.
NPT
American taper pipe thread with a 60° flank (ASME B1.20.1), sealing on the thread with sealant. Not interchangeable with R threads.
PG
Steel conduit threads to DIN 40430 with an 80° flank, used for older cable glands; largely replaced by metric M threads.