Since SG = density (of sample) / density (of reference), the units cancel, leaving just a number. True SG is a ratio of densities (mass/volume), while apparent SG is a ratio of weights (mass*gravity / volume). SG calculations are done by comparing equal volumes of the sample and reference substance. Temperature and pressure are factored into specific gravity calculations because these two values will affect the volume of a substance. Density = Mass / Volume or mass per unit volume.
In other words, make sure that the SG of the float < SG of the liquid.Ī float’s SG rating is especially important for applications involving various types of oil or alcohol, both of which are less dense than water (i.e. Regarding float switches the question is, “ Will the float work in a given liquid?” For example, the SG of the gasoline in your car is about 0.74, so even if a float will work in a tank of water it may sink if placed in a tank of gasoline. Thus, if water is used as the reference, substances with specific gravities greater than 1 will sink, because they are more dense than water, while substances with specific gravities less than 1 will float. By this definition the SG of the reference substance will always be 1. Specific gravity (SG) is a ratio comparing the density of one substance to the density of a reference substance, which is usually water or air. Some float switches can be used in tanks containing boiling water, while other switches have floats designed for tanks containing liquid nitrogen. For example, the float switches we carry are single-pole, double-throw (SPDT) with one N.O. However, SPST is not the only configuration possible.
(See the picture above for an example of a N.O. sump pump) when the liquid level gets too high. If you want to empty a tank, then you will need a switch with N.O. If you want to fill a tank, then you will need a normally-closed switch to start a pump motor when the liquid level is low and to stop the pump once the tank is full. SPST float switches that are normally-closed will conduct a current when the float is at the bottom of a tank, while normally-open switches will not. contact, the “normal” position is when the float is sitting, “at rest,” at the bottom of the tank, and the float actuates the contacts as it rises. Normally-Open (N.O.) and Normally-Closed (N.C.) Contactsįor single-pole, single-throw (SPST) switches with one N.O. You will need to find switches with an enclosure (and wire insulation) that can handle being submerged in a given liquid. in aquariums or swimming pools) to heavy-duty sewage applications. The types of liquid can vary widely from water level control (e.g. This should always be the first thing to specify as it will determine other factors like the enclosure type and specific gravity rating of the float. Please see our electrical switch overview for general information, descriptions of various types of switches, and explanations of commonly used terms like pole, throw, N.O. Some common uses are: powering a relay that controls a pump motor that will either fill or empty a tank, controlling liquid level indicator lights, or signalling alarm systems if the level of a liquid gets too high or too low. The core function of float switches is to detect the level of a liquid in a container, so they can be used to power any device related to that core function. Otherwise, you will get undesired closing of the reed contacts and shorten the operating life of your switch. NOTE: You will want to isolate reed switches from magnetic fields generated by nearby equipment, such as motors or electromechanical relays. Other switches will use reed contacts that are actuated by magnets. Many float switches use a tipping-point mechanism that produces a snap-action, so that the contacts will not move until the float reaches specific points in its range of motion. For example, a float switch being used to keep an aquarium tank topped off may have a differential of less than 1 inch, while other switches being used in applications where a tank is being completely filled or emptied may have differentials as large as the height of the tank. The differential is the distance the float has to travel in order for the contacts to change from one state to another (e.g. The levels at which the switch contacts will open or close depend on where you set up the float in your tank and the differential distance. A float switch is a type of sensor that detects the level of liquid in a container and actuates a set of contacts as the liquid level rises or falls.