Transdata Corporation
BDS Systems Inc. - Representative

Transdata is a manufacturer of instrumentation equipment serving the petroleum, chemical and industrial markets. Transdata specializes in tank products including displacer high level alarm switches, annunciators, tank level gauges (gaugeboard and gaugehead types), tank vents, and vent air driers etc.

The Model 6761P2 Displacer Switch (shown below) is designed for preventing tank spills. The switch assembly is installed on the top of a tank mounted on a 4" flange. This switch assembly provides two (2) points of level detection. As liquid in the tank rises and comes in contact with the displacers a switching action takes place. This switch is all mechanical, no AC or DC power is required, only the opening or closing of a switch take place as liquid raises the displacers. The switches contacts can be normally open (NO) or normally closed (NC). Models with one (1) point of level detection are available. An independent alarm switch not associated with the tank's level measuring devise is always recommended for a tank's overfill protection. This switch is a standard for tank high and high/high alarms or overfill protection.
The Model 6693 Operation Checker (shown below mounted with a displacer switch) permits an operator to check the operation of the switches without removing the switches' protective cover. This is an optional recommended accessory.
The standard materials of construction of the Model 6761 is aluminum housing and flange, stainless steel isolation tube, stainless steel cable, PVC displacers and all trim parts stainless steel.
The switches are of the encapsulated mercury type with the following ratings: 10 amps at 30 VAC, 5 amps at 115 VAC, 2.5 amps at 220 VAC. Other switches, flange sizes or threaded connections, and materials are available.

When ordering specify tank tagging, product, temperature, specific gravity, tank pressure and type of tank (cone roof, internal floating pan, etc).

Installation (Mechanical and Electrical)

  1. Mechanically install displacer switch assembly in a plum vertical position on the top of tank per instructions furnished with unit. The cable and displacer must hang in the free air space above the tank's liquid product. No obstructions are permitted. The PVC displacers are 3 1/2" in diameter. The displacers are secured on the flexable stainless steel cable according to the required switching point/s. See factory instructions for complete details.
  2. Electrically connect wiring to proper terminals in displacer switch assembly according to the switching point/s. Wiring is connected through the 3/4" conduit fitting on base of housing. Follow proper electrical codes. See factory instructions for complete details.

Operation - Basic

  1. The displacer switch is installed on the top of a tank with displacers hanging on a cable or beaded chain in the free air space attached to the float rod. The spring in the spring housing is always sightly compressed by the weight of the displacers. The spring rod and armature assembly extend up into the isolation tube out of the magnetic field of the mercury switches with magnets. The mercury switches are in a normally open (NO) or normally closed (NC) position with no alarm.
  2. The product level in the tank rises and comes in contact with displacer "B". This displacer sinks in the product but raises the displacer slightly decompressing the spring in the spring housing. This raises the floating rod and armature assembly. The magnet on the armature assembly causes the mercury switch to change position and provides an open circuit or closed circuit. Alarm point "B" is recognized. If the liquid product is removed from the displacer, the displacer again becomes heavier and the switching contacts are reversed.
  3. If the product level in the tank continues to rise and comes in contact with displacer/s "A" the mercury switch "A" changes state and a second level point is recognized.
  4. As liquid product is lowered and the displacers are again permited to hang in the free air space, the switching action is reversed. The switching action is slightly different when product is lowered. This difference is call the switches switching differential.
  5. When installed on a tank with an internal floating pan or other mechanical lifting device the switch acts in a similar manner.
  6. With the addition of an Operation Checker, an operator may manually lift the displacers simulating a rising liquid product to the displacers. Recommended option.

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BDS Systems would be happy to provide you with a quotation for any of these items.


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