SECTION IV. RELAY-TYPE STEP-RESISTANCE GAGE FOR SALT AND FRESH WATER 



1. Theory of Operation of a Relay- Type Step-Resistance Gage 



The CERC relay-type step-resistance gage is designed for opera- 

 tion where water salinity is expected to vary widely. This variation may 

 approach that of fresh water or that of sea water with little change in 

 gage operation. This gage holds calibration longer than other staff gages. 



The gage operates on the principle of water completing a circuit 

 consisting of a power supply, a relay coil, and a switch (the switch is 

 the water path) in series (see Figure 25 on the following page). 



The gage uses 125 relays for a 25-foot gage, each relay closing when 

 its associated water contact is submerged. Only one power supply is 

 required to operate all the relays. 



In order that the relays will operate in both fresh and salt water, 

 it is necessary to modify the basic circuit in Figure 25 to the circuit 

 in Figure 26. 



Electrolytic action in the water path makes it necessary to use 

 alternating current in the gage circuit. However, when an a.c. relay is 

 used, excessive relay chatter shortens relay life. This limitation makes 

 it necessary to select a d.c. -operated relay, and subsequent selection of 

 suitable rectifiers and filters for converting the a.c. gage-circuit 

 potential to d.c. for relay operation. 



The basic relay circuit requires approximately 18 volts for operation 

 in fresh water. When the same voltage is applied to the gage circuit in 

 salt water, the voltage across the relay coil exceeds the coil voltage 

 rating. To overcome the relay-coil overload, a 28-volt .07 ampere pilot 

 lamp is installed to provide relay protection in salt water. In addition, 

 this lamp will have a lower resistance value when not fully excited, thereby 

 providing a correspondingly higher voltage to the relay coil when used in 

 fresh water. 



The relays are connected to the copper contacts on the epoxy gage 

 sections so that when the bottom contact on the gage is submerged, relay 

 No. 125 is first to close, and when all contacts are successively submerged, 

 relay No. 1 will be the last to close. 



The a.c. power-supply voltage to the relay circuit is adjustable by 

 changing a jumper wire on a terminal strip on the power-supply chassis. 

 The voltage should be adjusted to the minimum value that will provide 

 positive relay closure at the location of the gage. For sea water, 9 to 

 12 volts should be adequate; for most fresh water locations, 18 to 24 

 volts should be adequate. 



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