Diurnal variation in sensor readings
Sensors 1 and 3 use 15 psi pressure sensors, and 2 a 6 psi sensor. The gage port to atmosphere on the 15 psi sensors is directly on the sensor body. But the gage port on the 6 psi sensor goes back through a small spaghetti tubing inside the instrument cable. There are diurnal variations in the reading of the 15 psi sensors due to different solar heating of the sensor body and measuring diaphragm. These are much worse in the 6 psi sensors because the air in the spaghetti tube can also be heated and cooled, changing the gage reference. Before the sensor tubes were installed in the larger dam, the 6 psi spaghetti tubes were cut near the sensors in an attempt to eliminate this source of error. This did not work very well. One or both of the 6 psi pressure sensors may have been damaged. This kind of sensor must be used with caution or not at all.