rsdoran said:
This woke me in the middle of the night, electricity is not "consumed" per se. In other words the current that flows is based on the resistance in the ckt, so the electronics in that sensor is basically a varying resitor that changes due to its interaction.
I am not sure I am saying this properly but current is electron flow, the electrons just move from hole to hole, they do not get used.
So if there is current flowing a potential difference will develop across the device, the device will change resistance values because of its interaction with whatever it is "sensing".
Where consumption comes in is in the form of "watts"; which is the result of voltage and current. The primary form of wattage is heat.
Maybe Keithkyll or someone can express this better.
I think you expressed it quite well, as a matter of fact I've been trying to explain it to Florida Power and Light for the past hour and a half!!!!
THOSE idiots think I "used" $300.00 worth of electricity last month. (THAT wakes ME up in the middle of the night)
Of course I'm just kidding, But once again it is the "terminology" that causes the confusion. While electrons are not "used up", ENERGY is. (actually lost, or converted, but gone none the less) As rsdoran stated, energy is measured in "watts" ( or in the case of the sensor's LED for example, MICRO-watts.) It is the function of the sensor to "limit" the potential current flow in the loop to the correct MA to express what is being sensed.
The total resistance that results in the total MA in the loop INCLUDES the resistance caused by the voltage drop required by the components that need "energy" to function. Thus "energy" is consumed (converted to light, heat, etc).
These "loop powered" sensors can QUALIFY as loop powered because they use so little energy, that is: they are so efficient. Very little power is required to "operate" them. Very little energy is "lost" to heat, or converted to light.
As far as the "wire size" part of your question goes: If you notice the FACTORY wire on a 2 wire loop powered sensor is usually VERY small, 22gauge or so. As long as there is only ONE loop being fed by the wire, and it is properly FUSED, and LENGTH is not such that it becomes a factor, you can run the entire loop with this same size wire. I go larger sometimes just for durability.
Stationmaster