Medical equipment, especially diagnostic instruments, work with a multitude of fluids. There are liquid levels to monitor from reagent containers to waste tanks. New and innovative sensors are needed to handle these jobs. Fortunately, there are about eight categories of liquid-level sensors to choose from. But which works best and where? To answer the question, this brief review highlights the plusses and minuses of each.

Float or buoyancy switches are made from a variety of materials compatible with most fluids. The simplest and least expensive of the group use a reed switch actuated by a magnet embedded in a float that moves with the liquid level. This offers a direct, reliable, and repeatable way to monitor a tank's liquid level. Because a magnetic field actuates the mechanical reed switch, the sensor has no power requirements.

Some limitations are obvious: floats need to be inside the tank, in contact with the fluid. Volume displaced by a float sensor results in less fluid, or requirements for a larger container. Also, as diagnostic instrument footprints decrease, smaller float sensors become less effective by decreasing the buoyancy of the float. Contamination of clean or sterile liquids may be possible as well as infection from potential biohazards.

Conductivity sensors rely on the conductive nature of many fluids. At its simplest, two metallic probes extend into a tank, one with an applied low voltage and the other cut so the tip is at the actuation point. Sensor electronics can be set up for normally open or normally closed operation actuated in either a wet or dry condition. When the liquid contacts both probes, the current flows across them and a switch actuates. These sensors are successful in liquid dispensing and boiler applications.

Conductivity sensing is not recommended when contamination from a probe is unacceptable, or when conductivity of the liquid is low, or varies, or both. Saline and buffer solutions may also create bridging issues and false actuation.

Capacitance sensing can include contact or non-contact sensors. They identify the presence or absence of liquid by measuring the difference of dielectric properties between air and liquid media. The dielectric property of a substance is its ability to resist holding an electrical charge. When a change in level causes a change in the total dielectric of the capacitance system, the capacitance measurement indicates level. A container wall with stable conductive properties can be used for non-contact sensing.

Where all system materials stay absolutely consistent, capacitance sensors provide reliable results. Problems arise, however, when uniformity is not maintained. For example, changes in sensor or bottle position, bottle wall thickness, or fluid dielectric fluctuations can impair capacitance-sensor performance and recalibration becomes necessary. In other words, performance is easily impaired by changes from calibrated ideals.

Ultrasonic technology has been used in a variety of sensors: contact, non-contact, invasive, and non-invasive. Ultrasonics work by resonating a frequency and converting electric energy into acoustic energy to infer liquid level. The noncontact version bounces sound waves off the fluid's surface, measures the return time, and compares the time to the calibrated time it takes for that same frequency sound wave to travel from the top, to the bottom, and back in an empty tank.

Contact versions use sound transmissions to detect the presence of liquid or a change in state. A transmitting crystal sends sound waves to a receiving crystal. While immersed, sound waves are dispersed as opposed to the absence of fluid or air. Circuitry analyzes the signal strength and actuates a switch appropriately. Regardless of design, ultrasonic sensors are small, solid state, and highly accurate.

Even though they are highly accurate, ultrasonic sensors can be fairly expensive and require a power source for operation. They come in a variety of materials suitable for critical fluids. However, contact ultrasonic switches must be inserted into a tank or bottle, usually through a threaded fitting in the tank sidewall. In addition, the technology can deliver false positive readings from fluctuations in liquid composition, such as when it contains air pockets or bubbles, foam, or solid particles.

Piezo-resonant sensors are new on the scene. They also use ultrasonic energy. This sensor works through plastic bottle and container walls, so it's noncontacting. Piezo-resonant technology used in ExOsense sensors is exclusively patented to Gems Sensors & Controls.

ExOsense sensors adhere to the outside of plastic bottles and containers. They can be mounted anywhere on the outside to provide high, low, or any intermediate point-level-fluid sensing. Exciting a piezoelectric material makes it generate an acoustic signal as a function of the natural resonance of the material. Using a signal element ExOsense sensors generate this acoustic signal, direct it through the bottle wall and sense the reflected pulse to determine air versus liquid. Accuracy is ±1.6 mm of actual liquid surface with repeatability within 1 mm.

While this technology lends itself to many applications, it is limited to certain tanks and bottles. For instance, the container must be plastic, although it is unaffected by the color or transparency, and the wall of the container can be no thicker than a 1/4-in. In addition, high temperatures can degrade and melt the PVDF film within the sensor.

Electro-optic miniature sensors combine an optical prism, solid-state circuitry, an infrared light emitter and receiver, and transistorized switching. They are low cost, compact, liquid-level sensors. With no moving parts, these small units are ideal for a variety of point level sensing applications especially where dependability and economy are a must. These sensors are suitable for high, low, or intermediate level detection in practically any tank top, large or small. Installation is simple and quick through the tank's top, bottom, or side.

Unfortunately, their performance is hindered by reflected light, such as in a small reflective tanks, bubbles, or coating fluids. Although they protrude just a little into a container, they still require an entry through the container wall and must come into contact with fluids.

Load cells are non-invasive and need not be wired to the container. They measure container weight and infer the amount of fluid in the container as a function of change in weight. Their downside is the calibration needed to keep the system accurate. Variation in container weight or fluid density, even the position of the container on the load-cell platform may alter what the system sees as full or empty.