Turndown Ratio Calculator

This professional turndown ratio calculator determines optimal instrument range selection and effective operating range for maximum accuracy and performance. Calculate rangeability, determine appropriate instrument sizing, and verify operating point optimization per ISA standards for process control and custody transfer applications.

Key Features: Turndown ratio calculation, minimum measurable span, accuracy verification, operating point optimization, instrument capability assessment, and compliance verification across all industries.

Turndown Ratio Analysis Results

Analysis & Recommendations

Understanding Turndown Ratio

What is Turndown Ratio?

Turndown ratio (rangeability) is the ratio of maximum span to minimum span an instrument can measure: Turndown = Max Span / Min Span. For example, a 20:1 turndown meter can measure flows from 5% to 100% of its capacity while maintaining specified accuracy. Higher turndown provides more flexibility without requiring multiple instrument sizes. Critical for selecting instruments that match wide process variations while maintaining measurement accuracy.

Accuracy Degradation at Low Flows

Measurement accuracy typically degrades significantly below 20-30% of instrument span. At low signal levels, hysteresis effects dominate, non-linearity becomes pronounced, and sensor noise represents larger percentage errors. For example, ±0.5% span accuracy (±5 PSI on 0-1000 PSI) becomes 10% error at 50 PSI—unacceptable for precision work. Proper selection ensures operating points remain in the accurate region of the instrument's characteristic curve. For wide-range applications, consider dual instruments: small-range meter for normal operations (better accuracy), large-range meter for emergency conditions.

Optimal Range Selection

Select instrument span such that: (1) Normal operating point falls between 40-60% of range, (2) Maximum foreseeable value (including upsets) remains below 95% of range, (3) Minimum value stays above 20% of range where accuracy is acceptable. This positioning keeps measurements in the sensitive, accurate region while maintaining safety margins. Always calibrate at actual operating points (not just 0-100%), verify accuracy across your specific range, and document all calibration results.

Standards & References