Flow, level & control
CHOOSING INSTRUMENTATION FOR WATER AND LIQUID PROCESS MONITORING
LEVEL MEASUREMENT AND WATER QUALITY IN PRACTICE Instrumentation engineers working across water and wastewater treatment, chemical processing, and industrial liquid storage face a recurring challenge: specifying sensors that hold up under real operating conditions, not just on a datasheet. Two of the most common — and most frequently misapplied — instrumentation categories are non-contact level transmitters and water quality probes. Get the selection wrong, and the result is nuisance alarms, unplanned maintenance, or measurements that quietly drift until a compliance report is questioned. Here, KACISE’s Gloria Honney works through two selection problems that often land on the same project: choosing between radar and ultrasonic level transmitters, and specifying water quality probes for continuous monitoring.
HOW THE TWO LEVEL TECHNOLOGIES ACTUALLY DIFFER
Both ultrasonic and radar level transmitters work on the same basic principle — send a signal towards the liquid surface, time its return, and calculate distance. What separates them in practice is the medium the signal travels through, and how that medium behaves under real plant conditions. Ultrasonic transmitters send an acoustic pulse through air. Because sound needs a stable gas path to travel through, anything that disrupts that path becomes a source of error: temperature gradients bend the acoustic signal, foam absorbs or scatters it, and dust or heavy vapour can attenuate it enough to cause dropouts. Most industrial ultrasonic transmitters build in temperature compensation, which corrects for the effect of ambient temperature on the speed of sound — a meaningful improvement, but one that only addresses part of the problem. Well-specified units typically deliver an accuracy of 0.5-1% of full scale, with IP68- rated, corrosion-resistant housings now standard for continuous duty in wet wells, sewage channels, and other aggressive environments. Radar transmitters use electromagnetic microwave signals instead of sound. Because microwaves propagate largely independently of temperature, pressure, and most vapours, radar tends to hold its accuracy in conditions that would
defeat ultrasonic measurement — foaming surfaces, steam-laden atmospheres, dusty silos. The one condition radar handles less gracefully is a liquid with a very low dielectric constant: media such as light hydrocarbons or certain oils reflect only a small fraction of the microwave signal back to the sensor, which can reduce signal strength and, in marginal installations, measurement reliability. This is a genuine limitation worth flagging to a specifying engineer, since it is less intuitive than radar’s other, better-known strengths. Compact radar units operating in the 80 GHz band have narrowed this
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gap somewhat, offering a tighter beam angle that also helps in vessels with internal obstructions or narrow mounting nozzles.
For the small number of applications where even radar struggles — surfaces with very thick, dense foam, or vessels with internal structures that scatter a free-space signal too much for a reliable reading — guided-wave radar is worth considering as a fallback. Rather than launching the signal freely into open space, guided-wave radar sends it down a probe (a rod or cable) that extends into the vessel, which concentrates the signal and largely removes
October 2026 Instrumentation Monthly
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