Using water or an ultrasonic coupling gel is a necessary part of operating a Portalevel® unit. The sensor works by transmitting a high‑energy ultrasonic pulse that must pass through the cylinder wall before returning to the sensor as an echo.

If there is any air gap between the sensor and the cylinder surface, the ultrasonic signal will be reflected away at the surface and will not penetrate the material. This prevents the returning echoes from being detected and can result in inaccurate or failed measurements.

Applying a thin layer of coupling gel or a small amount of water removes these air gaps and creates effective acoustic contact between the sensor and the cylinder. This ensures consistent signal transmission and reliable coupling throughout the measurement process.

SPA stands for Special Power Application. It is a feature designed to assist with measurements in difficult conditions where standard ultrasonic settings may not provide a strong enough signal.

Certain cylinders—especially those that are large, heavily corroded, or affected by surface degradation—can interfere with ultrasonic signal transmission even when correct coupling has been achieved. Similar challenges may arise if the liquid inside the cylinder significantly absorbs or weakens the ultrasound, causing the returning echo to be too faint for reliable detection.

In such cases, activating the SPA mode increases the ultrasonic output power of the Portalevel®. This enhanced signal strength improves echo detection, allowing accurate measurements to be taken in situations where normal operating power may be insufficient.

The numerical display on the Portalevel® INTRINSICALLY SAFE shows the strength of the ultrasonic echoes returning to the sensor. Stronger echoes appear as higher readings on the display.

During use, the instrument is calibrated in an area where the signal strength is high, causing the on‑screen bar graph to fully fill. Once calibrated, the operator slowly moves the sensor toward areas where the signal becomes weaker. As the echo strength decreases, the bar graph reduces and eventually disappears.

The point at which the bar graph is no longer visible corresponds to the change in ultrasonic response, indicating the position of the liquid level inside the cylinder.

For some liquids under pressure, accurate liquid level measurement is only possible when the temperature remains below a specific limit known as the critical temperature. Above this temperature—provided the pressure is also sufficiently high—the substance no longer behaves as a conventional liquid. Instead, it enters a supercritical state, where the distinction between liquid and gas disappears.

When a fluid is in this supercritical condition, there is no clearly defined liquid surface. As a result, it becomes impossible to identify a liquid level using ultrasonic measurement techniques.

In most cases, this limitation is not an issue because many industrial liquids have critical temperatures well above normal operating ranges. For example, NOVEC™ 1230 has a critical temperature exceeding 150 °C.

CO₂ and FE‑13™, however, are exceptions. Their critical temperatures are relatively low—approximately 31 °C for CO₂ and 26 °C for FE‑13™. If either agent is stored in a cylinder at temperatures above these thresholds, the contents may enter a supercritical state, preventing successful liquid level measurement.

To carry out a valid test, the cylinder temperature must be reduced below the relevant critical temperature. This can be achieved using methods such as air‑conditioning, running cool water over the cylinder, or applying ice packs around the exterior until the temperature is sufficiently lowered.