How Full Is the Tube? Gen-Probe’s Differential Fluid Level Measurement System

Gen-Probe Incorporated, a specialist in automated molecular diagnostics systems, has filed a patent for a system and method that accurately measures the fluid level in a sample receptacle relative to the rim of the container – a capability critical for automated laboratory instruments that must handle thousands of sample tubes with varying fill volumes.

The Problem

Automated laboratory systems that process biological samples – blood, urine, cerebrospinal fluid and other clinical specimens – must handle sample receptacles containing widely varying volumes of fluid. Before any analytical step can proceed, the instrument needs to know how much fluid is present in each container. If the instrument tries to aspirate more liquid than is present, it will draw in air, potentially contaminating the aspiration system, giving inaccurate results and requiring additional cleaning cycles. If it aspirates too conservatively, it may fail to collect sufficient sample for the required tests.

The challenge is compounded by the variety of sample receptacle types used in clinical laboratories. Tubes vary in diameter, wall thickness and overall height, and the rim of the tube – the opening edge – is not always at a fixed height relative to the instrument’s reference point. The fluid surface within the tube varies depending on the fill volume, the presence of gel separators, and whether the tube has been centrifuged. Simply measuring distance from the instrument to the fluid surface is not sufficient; what is needed is the distance between the fluid surface and the rim, which tells the instrument exactly how much headspace exists above the fluid.

Existing fluid level detection methods include various forms of capacitance sensing and ultrasonic or optical distance measurement. However, accurately distinguishing the rim of the tube from the fluid surface in a single distance measurement pass, and computing the fluid level relative to the rim rather than relative to the instrument, has remained technically challenging, particularly when the system must process many different tube types and sizes at high throughput.

What This Invention Does

Gen-Probe’s invention uses a distance sensor to measure distances sequentially as either the sensor or the sample receptacle is moved relative to the other. As the sensor scans across the receptacle, it captures a sequence of discrete measurements – some measuring distance to the rim of the tube, some measuring distance to the fluid surface inside the tube. Because the rim projects above the fluid surface, these two measurement populations produce distinctly different distance values.

The controller processes the output signal from the sensor and uses the derivative of the sequence of measurements to identify the transitions in the output – the points where the measurement shifts from the rim region to the fluid surface region. By identifying both the rim position and the fluid surface position from the same scan sequence, the controller can calculate the fluid level relative to the rim of the receptacle, which is the practically useful quantity for instrument control.

The differential approach – using transitions in the derivative of the measurement sequence rather than absolute distances – makes the system more robust to variation in tube types and dimensions, since the rim-to-fluid differential is a property that can be extracted regardless of the absolute height of the tube or its position in the instrument. This enables the system to work reliably across the range of sample receptacle types found in a real clinical laboratory.

Key Features

Differential distance measurement. The system measures the level of fluid relative to the rim of the receptacle rather than from an absolute reference point, providing the operationally relevant measurement for automated liquid handling.

Sequential discrete measurement approach. A distance sensor obtains a sequence of measurements as it is moved relative to the receptacle, capturing both rim and fluid surface distances in a single scan pass.

Derivative-based feature identification. The controller uses the derivative of the measurement sequence to identify the rim and fluid surface positions from their characteristic transition signatures in the output signal.

Single-sensor multi-feature detection. The system identifies both the rim position and the fluid surface position from a single sensor scan, avoiding the complexity of separate sensing systems for each measurement.

Automated laboratory system integration. The invention is designed for integration into automated laboratory instruments that must rapidly and reliably characterise sample volumes across high-throughput analytical workflows.

Who Is Behind It?

Gen-Probe Incorporated is a San Diego-based molecular diagnostics company, now part of the Hologic group, specialising in nucleic acid amplification testing and automated laboratory systems for clinical and public health applications. The sole inventor named is Rolf Silbert, an engineer with expertise in automated laboratory instrument design. The application is filed through FB Rice Pty Ltd and is a divisional of an earlier filing (AU 2021259791), which was the Australian national phase entry of PCT/US2021/028719.

Why It Matters

Automated laboratory processing systems are central to the operation of modern clinical pathology departments. The accuracy and reliability with which these instruments handle samples directly affects the quality and reliability of laboratory test results – which in turn affects clinical decision-making for patients. Fluid level detection errors are a well-known source of analytical failures in automated systems, and improvements in the reliability of level sensing translate directly into improved analytical reliability and reduced instrument downtime.

The differential measurement approach described in this patent – computing fluid level relative to the rim rather than from an absolute reference – is a practical innovation that addresses the real-world variability of sample tubes in clinical use. By making the measurement more robust to the tube-to-tube variation that characterises actual laboratory practice, Gen-Probe’s system reduces the rate of aspiration errors and improves the overall analytical performance of the automated platform. For high-throughput pathology laboratories processing thousands of samples per day, even small improvements in error rates translate into meaningful reductions in repeat testing, instrument downtime and manual intervention.

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AU 2026201610 was published in the Australian Official Journal of Patents on 19 March 2026 and is open for public inspection. Patent applications represent inventions that are sought to be protected and do not necessarily reflect commercially available products.