Topic
Instrumentation
Field instruments and their signals: temperature, level and flow measurement, loop-powered transmitters, HART, and intrinsic safety in hazardous areas.
5 articles
HART: digital data on the same wires as the 4-20 mA
HART superimposes a low level frequency shift keyed digital signal on a standard 4 to 20 mA current loop. The analogue value keeps working exactly as before, and the digital layer carries device identity, configuration, diagnostics and up to three additional process variables. Most installed HART devices are never read digitally, because doing so needs a HART capable input card or a multiplexer.
7 min readChoosing a level measurement that will still work in a year
Guided wave radar suits most liquids including foam and varying density. Non contact radar suits corrosives and where nothing may touch the product. Ultrasonic is cheap and fails on foam, vapour and dust. Hydrostatic is simple and reads wrongly when density changes. Load cells measure mass rather than level and are unaffected by the product's properties entirely.
8 min readFlow meters: which technology, and what it will not measure
Electromagnetic for conductive liquids, which covers most water based processes, with no pressure drop and no straight run problem. Coriolis where mass flow or density is genuinely needed and the budget allows. Vortex for steam. Ultrasonic clamp-on for retrofit without breaking the line. Differential pressure where it already exists. Conductivity, viscosity and available straight run rule most options out before cost does.
8 min readLoop powered transmitters, and the voltage budget nobody calculates
A two wire transmitter draws its power from the same loop it modulates, regulating its own consumption between 4 and 20 mA to represent the measurement. That works only if the supply voltage exceeds the transmitter's minimum operating voltage plus the drop across every resistance in the loop at 20 mA. Adding an indicator or a barrier without redoing that sum is the commonest reason a loop works at low readings and fails at high ones.
6 min readIntrinsic safety, zones, and what Ex ia actually promises
Intrinsic safety limits the electrical energy available in a hazardous area so that no spark or hot surface can ignite the atmosphere, even under fault conditions. It is achieved with barriers or isolators that clamp voltage and current. Unlike flameproof enclosures, which contain an explosion, intrinsic safety prevents one, which is why an intrinsically safe loop can be worked on live.
8 min read