Continuous measurement of specific gravity (relative density) of a liquid in process Which option correctly lists instruments that can continuously indicate specific gravity in-line or on-stream?

Introduction / Context:
Specific gravity (relative density) is a critical quality parameter for acids, brines, beverages, and petroleum fractions. Plants often require continuous indication to alarm, log, and control blending operations. Knowing which instruments can deliver on-stream density readings is essential for correct selection.

Given Data / Assumptions:

• Goal: continuous (not handheld) measurement.
• Candidate instruments: hydrometer-based designs, displacement meters, contact-type electric indicators.
• We consider practical plant implementations.

Concept / Approach:
While a handheld hydrometer is a spot-check tool, “continuous indicating hydrometers” exist as float or sinker devices in slipstreams or chambers, providing an ongoing reading. Displacement meters (e.g., float-operated displacer/density cells or vibrating element density meters, historically referred to broadly) provide true continuous, in-line signals. “Contact-type electric indicators” is an ambiguous category and is not a standard name for continuous density measurement devices.

Step-by-Step Solution:

Verification / Alternative check:
Process instrumentation vendors list continuous indicating hydrometers and displacer density meters for pipelines and tanks; these are standard solutions for specific gravity monitoring.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming “hydrometer” only means handheld glass; continuous floating-sinker arrangements exist and are used industrially.

Final Answer:
both (a) and (c)