Nearly 60% of all level sensors
are point-level sensors. These types of sensors are critically
important to the profitable and safe operation of your facility,
whether they are used for high-level or low-level detection
or for some other purpose. If a high-level sensor in one of
your vessels fails what will happen? You may overfill your
vessel or damage other equipment and back-up material into
the fill line or clog filters and vents. You can have a mess
to clean up, and to explain, and repairs to make. What’s
the outcome if a low-level sensor fails? Your production line
can unexpectedly run out of material and you may incur expediting
charges to replenish your raw material inventory. If a point-level
sensor fails without warning, it isn’t going to be pretty
and you will have a headache and a costly situation on your
Given the outcome of a point-level sensor failure you’d think they would be very costly, but they’re not. The average unit price is less than $300.00 (US), according to industry research sources. This expense pales by comparison to the cost of the effects of a point-level sensor that fails without warning. So, how do we go about preventing the expense associated with this type of point-level sensor failure? What is needed is a sensor that is “safe” to use, in regards to the outcome of its failure. This means a “fail-safe” sensor. What does “fail-safe” mean? I found a definition listed by Wikipedia. The website is www.en.wikipedia.org and it is a free online encyclopedia. According to the definition offered, “fail-safe” is a term used to describe a device that will fail in such a way that it will not cause harm or will minimize the harm caused to process and personnel. Further definition indicates that a “failure” is when the device is not able to perform its intended function.
For point level sensors a failure can occur for numerous reasons, including external and internal causes. An example of an external failure is a power outage. While this may be infrequent, and you probably will have other problems to deal with besides your sensors, a power failure will mean your level sensor will not operate. Internal failures are potentially more numerous given the number of components that make up a sensor and the method of operation of each type of sensor. However, point level sensor manufacturers have long defined fail-safe in regards to the sensor output for power supply failure only. Most point level sensor literature and specifications will claim the sensor to include “fail-safe high/low output selection”, meaning that if the power to the sensor fails, the output of the sensor will go into the alarm mode.
However, this customary “fail-safe” function offered by level sensor manufacturers does not protect “against the effects of failure of the equipment”. We have a gap between protecting against the impact and results of sensor failure, and what most suppliers have been providing in regards to a “fail-safe” device. How can we meet what is needed and close the gap? By using self-validating sensors and understanding why they truly are safe, no matter what the failure mode may be.
A “self-validating” sensor is a sensor that provides both indication of a measured process variable and indication of the validity of the measured variable or intended function that the sensor performs. In our discussion here, a “self-validating” point level sensor detects the presence/absence of material at the sensing element and continuously monitors and checks it’s own ability to perform it’s sensing function. The “self-validating” point level sensor provides two outputs that ideally are monitored independent of each other. The first indicates material presence/absence, the second output indicates the health status of the sensor itself.
Self-validation is important if you want to ensure the elimination of the effect of unexpected sensor failures. While so-called fail-safe sensors will initiate an output change to the alarm state (supposedly a “safe state”) upon power system failures, ONLY a self-validating sensor will indicate the status of its ability to perform its function based on continuous internal diagnostics as well as the state of its external power supply. This is an extremely important difference to understand. Maximum protection is achieved using self-validating sensors, not “fail-safe” sensors that only take action upon power failure.