OTT Hydromet C31 Portable Current Meter

Overview

The OTT Hydromet C31 Portable Current Meter is a mechanically robust, field-deployable instrument engineered for accurate point-velocity measurement in open-channel flow environments. It operates on the fundamental principle of rotational hydrodynamics: water velocity imparts torque to a calibrated propeller, inducing rotation proportional to flow speed. The resulting rotational frequency is converted into discrete electrical pulses via a magnetic pickup system, enabling precise digital conversion of mechanical motion into quantifiable velocity data. Designed for compliance with established hydrometric standards—including ISO 748:2021 (Measurement of liquid flow in open channels—Velocity-area method) and ASTM D5388-22 (Standard Practice for Measuring Open-Channel Flow with Current Meters)—the C31 delivers traceable, repeatable measurements suitable for regulatory reporting, hydraulic modeling input, and long-term monitoring programs.

Key Features

• Low-threshold mechanical design ensures reliable start-up at velocities as low as 0.025 m/s—critical for shallow streams, irrigation ditches, and low-flow calibration scenarios.
• Stainless-steel housing and aluminum-alloy propeller provide corrosion resistance and dimensional stability across freshwater, brackish, and mildly aggressive environments.
• Reinforced main shaft assembly minimizes wobble and bearing wear, preserving angular accuracy over thousands of measurement cycles.
• Modular construction enables rapid field replacement of propellers (part no. 10.001.077.5.1) and connection hubs without recalibration.
• Dual-frequency pulse output (50 Hz / 100 Hz) supports compatibility with legacy analog loggers and modern digital data acquisition systems.
• Integrated timing electronics offer sub-second resolution (0.1 s) and high pulse fidelity (±0.5 pulse), meeting GLP-aligned data integrity requirements for environmental monitoring networks.

Sample Compatibility & Compliance

The C31 is validated for use in natural rivers, artificial canals, wastewater outfalls, and stormwater conveyances where flow depth exceeds 10 cm and suspended solids concentration remains below 500 mg/L. Its compact cylindrical profile (Φ35 mm × 310 mm) allows deployment from bridges, wading rods, or boat-mounted poles without significant flow disturbance. All components comply with RoHS 2011/65/EU directives. Calibration certificates traceable to NIST-standard current flumes are available upon request. Instrument configuration and measurement protocols align with USGS Techniques and Methods Book 3, Chapter A22 (Field Manual for Measurement of Discharge Using Current Meters) and EN ISO/IEC 17025:2017 requirements for testing laboratories.

Software & Data Management

While the C31 operates independently as a standalone meter, it interfaces seamlessly with OTT’s MZ400 Digital Counter (part no. 12.440.005.9.0), which provides real-time velocity display, automatic averaging, and timestamped pulse logging. The MZ400 supports configurable measurement durations (5–120 s in 5-s steps), selectable integration modes (pulse count, time-gated, or continuous), and audit-trail-enabled memory storage compliant with FDA 21 CFR Part 11 principles when paired with OTT’s certified firmware version 3.2+. Exported CSV files include metadata fields for operator ID, location coordinates (via optional GPS module), temperature, and measurement uncertainty—facilitating direct ingestion into hydrological databases such as HEC-RAS, QGIS, or EPA’s STORET platform.

Applications

• Discharge estimation in ungauged watersheds using the velocity-area method per ISO 748.
• Validation of acoustic Doppler velocimeter (ADV) and electromagnetic flow sensor deployments.
• Regulatory compliance monitoring for NPDES permits and EU Water Framework Directive (WFD) status assessments.
• Calibration and verification of flume and weir installations in municipal wastewater treatment plants.
• Academic field hydrology instruction and student-led stream gauging campaigns.
• Post-flood debris flow velocity reconstruction in mountainous catchments.

FAQ

What is the minimum measurable velocity, and how is low-flow performance verified?
The C31 achieves a verified start-up threshold of 0.025 m/s under laminar flow conditions, confirmed through repeatable bench testing in ISO-certified flume facilities. Its low-inertia propeller and hardened ceramic bearings minimize static friction effects.
Is the instrument suitable for saline or estuarine environments?
Yes—the stainless-steel body and anodized aluminum propeller resist pitting corrosion up to 15 ppt salinity; however, post-deployment freshwater rinsing is recommended for extended service life.
Does the C31 require annual recalibration?
Per ISO 748 Annex B, field current meters should undergo verification every 12 months or after 200 hours of cumulative operation. OTT Hydromet offers accredited recalibration services with full uncertainty budgets.
Can the MZ400 counter store multiple measurement sessions offline?
Yes—the MZ400 retains up to 1,000 timestamped velocity records in non-volatile memory, each tagged with duration, pulse count, calculated velocity, and operator-defined site ID.
Is there a NIST-traceable calibration certificate included with purchase?
A factory calibration report (traceable to NIST SRM 8550) is provided standard; full ISO/IEC 17025-accredited certification is available as an optional add-on.