Tuesday, October 31, 2006

Cross-axis Sensitivity Relating to Liquid Capacitive Inclinometers

Cross-axis sensitivity or "angle orthogonal to the plane of measurement":

Cross axis sensitivity is a proportionality constant that relates the change of the inclinometer output induced by a cross acceleration/inclination input. The sensitivity can vary depending on the direction of the cross input.

For example: A liquid capacitive gravity based sensor needs to be mounted in a vertical position - if the back of the inclinometer is mounted 45° off of vertical one would expect the inclinometer to be accurate within 0.1° of the expected output when mounted vertical. Due to the unique cavity design the error is predictable without much overall change to the accuracy. The unit will still provide output along the sensor's degree range, however the error at larger cross angle inputs will be significantly higher - as much as 2 or 3 degrees - the further off vertical the sensor is mounted.

It is recommended to mount a gravity based inclinometer sensor as close to vertical as possible to maintain the integrity of the sensor's output.

Monday, October 30, 2006

What does it mean to be Intrinsically Safe?


What does it mean to be Intrinsically Safe?

The term "intrinsically safe" refers to equipment and wiring which is incapable of releasing sufficient electrical or thermal energy under normal or abnormal conditions to cause ignition of a specific hazardous atmospheric mixture in its most easily ignited concentration. This is achieved by limiting the amount of power available to the electrical equipment in the hazardous area to a level below that which will ignite the gases present. To be certified "intrinsically safe," a device or circuit must be so designed that no two simultaneous failures can cause an explosion.

In order to have a fire or explosion, fuel, oxygen and a source of ignition must be present. An intrinsically safe system assumes that fuel and oxygen are present in the atmosphere, but the system is designed such that the electrical energy or thermal energy of a particular instrument loop can never be great enough to cause ignition. Traditionally, protection from an explosion in hazardous environments has been accomplished either through the use of explosion proof conduits and enclosures (intended to contain an explosion), or via pressurization or purging (intended to isolate the explosive gas from the electrical equipment). Intrinsically safe apparatus cannot replace these methods in all applications, but in many cases can provide significant cost savings in installation and maintenance of the equipment in a hazardous area. The basic design of an intrinsic safety barrier uses diodes to limit voltage, resistors to limit current and a fuse.

Gravity Based Inclinometer Technology: Liquid Capacitive Basics

Liquid Capacitive Gravity Based Inclinometer Basics:

The sensing element of Rieker inclinometers use liquid capacitive technology, which is based on gravity. The best way to understand how they work is to think of a disc-like cavity that is half filled with a dielectric liquid. One of the sides of the cavity has an etched conductor plate that is used to form one of the conductors of a variable parallel plate capacitor. The liquid along with the other side of the cavity forms the other plate of the capacitor. In operation, the sensor is mounted so that the disc is positioned perpendicular to the ground (vertically mounted). Gravity then acts on the liquid pulling it down in the cavity forming a semi-circle. As the sensor is rotated the liquid remains in this semi-circular pattern covering a different area of the etched plate. This change in area results in a change in the capacitance. The change in capacitance is then electronically converted into an output signal that is linear with respect to in the input angle.

The liquid capacitive technology also has a very low-frequency response bandwidth's - typically 3Hz - which means that their response times are much slower than most spring mass based inclinometers, around 250mSeconds. The benefit of these small bandwidths helps to dampen spikes in the output in applications with lots of vibrations and shock. The inclinometers are made with extremely tight cavities to provide even more damping, compared to other liquid based units where the liquid is known to "slosh" around. All Rieker units are completed encapsulated providing shock and vibration protection. They contain no moving mechanical parts. Only the liquid is free to move making them extremely rugged for harsh environments. This design offers shock protection in excess of 10,000g. Some have built in filtering or come in metal boxes with additional capabilities (SB1i or SB1U).