Two-wire loops in hazardous areas
Most field instruments in oil and gas are two-wire transmitters on a 4–20 mA loop with HART, installed where a flammable atmosphere can occur. That combination sets the recurring engineering questions: is the instrument certified for the zone, does the loop have enough voltage after barriers and cable, and is the measurement still accurate at the range it is actually calibrated for.
Pressure accuracy at turndown
A pressure transmitter is bought with an upper range limit and then calibrated to the span the process needs. Reference accuracy is stated as a percentage of that calibrated span, but temperature effect and long-term stability are usually stated against the upper range limit. A transmitter with a 400 bar limit calibrated for 0 to 100 bar is at a turndown of 4:1. With 0.075 % reference accuracy, 0.1 % of the upper range limit per 10 K and 0.1 % per year, a 30 K ambient swing gives a total probable error of about 1.27 bar, or 1.27 % of span, seventeen times the headline figure. The transmitter accuracy calculator shows which term dominates.
Specifications mix psi and bar: 1500 psi is 103.42 bar. The pressure converter converts without deciding gauge or absolute for you; that has to come from the datasheet.
Loop power through barriers
A transmitter needs its minimum terminal voltage at the highest current it can drive, which is the upscale alarm current near 22 mA, not 20 mA. An isolator or zener barrier adds series resistance, sometimes 300 Ω or more. With a 24 V supply, 550 Ω of barrier and input resistance and 500 m of 1.5 mm² cable, a transmitter needing 10.5 V has a margin of only about 1.2 V. The loop power calculator shows the margin and how it shrinks in a hot cable tray. An intrinsically safe loop also has to satisfy the entity parameters of the barrier and the device, which is a separate check that this tool does not make.
Reading an Ex marking
A marking such as II 2G Ex db IIC T4 Gb states the equipment category, the type of protection, the gas group, the temperature class and the equipment protection level. The Ex marking decoder breaks it down and compares it with a zone and gas group. It explains the marking; the certificate and its conditions of use decide what may be installed.
Temperature, flow and level
Process temperature is measured in thermowells with PT100 sensors and, at higher temperatures, type K or N thermocouples. Type K gives 22.776 mV at 550 °C; the thermocouple calculator converts with cold-junction compensation.
Differential-pressure flowmeters produce a pressure difference proportional to the square of flow. If the transmitter output is linear in differential pressure, 12 mA, the middle of the signal, is 70.7 % of full-scale flow, not 50 %. The 4–20 mA converter applies the square root; if the transmitter already extracts it, the signal is linear and must not be rooted again.
Hydrostatic level depends on density: 10 m of a crude at 870 kg/m³ is 0.853 bar, against 0.979 bar for water. The level calculator gives the calibration range for a given density, and the reading shifts whenever the density does.
Outside the scope of this site
Custody-transfer flow metering, gas detection, safety instrumented systems and intrinsic-safety entity calculations are not covered here.
Updated October 11, 2026