The selection of the right level measurement solution is based on not only the choice of measurement technology but also the application and installation requirements.
Density and Viscosity:
Viscosity and density considerations affect the selection of many liquid level switches and continuous level transmitters. High-viscosity materials can cause mechanical technology switches to stick or hang up. The forces on probes and sensors from high-density materials can cause damage.
An agitated liquid material may cause high lateral forces on a measurement probe when the viscosity or the density increases.
Measurement technologies must not only provide a correct measurement of the particular material, given its chemical nature, but they need to be able to survive as instruments. The chemical characteristics of a material may change because of the steps in the process. These characteristics, such as dielectric constant, conductivity, pH, etc., affect the different measurement technologies in different ways.
The temperature that the measurement instrument itself is exposed to must be reviewed. Cold or hot temperatures may damage, disable, or compromise the information from a measurement instrument. An optical level switch may be designed for proper operation up to 120◦F ambient with a given limit of 150◦F process temperature.
Vibrations might be present for different reasons, from engine-borne vibrations caused by pumps or agitators to vibrating silo bottoms for easier material flow in solids. These vibrations usually do not cause major problems; nevertheless, a few considerations are to be made:
- Is there a chance for the vibration to interfere with the measurement itself?
- Do the vibrations apply any unusual forces to the sensor element? If so, the instrument might be damaged over time.
- Are any strong vibrations applied to the sensor electronics? If so, a remote electronics might have to be considered. It has to be mounted at a point where no vibrations are present.
The process temperature that the measurement instrument is exposed to must also be reviewed. Naturally the measurement instrument materials of construction will dictate the maximum temperature allowed before damage occurs. The ambient temperatures may be fine for the instrument, but the process temperature may affect the level information
The impact of static pressure and pressure changes (and vacuum) on the level of information has to be taken into consideration. There are two aspects to be noted:
- The pressure rating of the instrument must at least match the process pressure in the tank. If a vacuum is present, the instrument needs a vacuum rating.
- The measuring principle must not be affected by the process pressure.
Vapour, Mist and Dust:
The space above a liquid or solids material may contain some vapour, mist or dust. This becomes important when non-contacting measurements with the laser, microwave, radar, and ultrasonic technologies are considered. The presence of enough dust and mist can attenuate the acoustic or electromagnetic signals to the point that measurement is unreliable.
Some bulk solids can have variations in moisture content that are seasonal, composition related, or from different sources. Capacitance measurements.
Measurements may often be required in environments where special regulations are in force. Installations where there is hazardous gas or dust often require a technology that is classified as meeting the requirements. National Electrical Code installation guidelines may have to be followed.