A B737-800 FLT CREW NOTICED AN INCREASING FUEL TOTALIZER SHORTLY AFTER TAKEOFF. WITH THEIR FUEL QUANTITY IN DOUBT; THEY DECLARED AN EMERGENCY AND RETURNED TO LAND AT THEIR DEPARTURE AIRPORT.

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RIGHT AFTER TKOF; THE FUEL TOTALIZER BEGAN INCREASING TO OVER 1700 LBS OVER DEP FUEL. THE CTR TANK WAS CAUSING THE ERRONEOUS READING AND REMAINED AT THAT HIGHER VALUE FOR THE REMAINDER OF THE FLT. WE XCHKED THE FUEL BURNED WITH THE FLT PLAN AND CONFIRMED THE ERROR WAS INDEED STILL THERE. DISPATCH AND MAINT CTL WERE ADVISED AND HAD NO SUGGESTIONS AS THERE IS NO PROC IN THE BOOK FOR THIS CONDITION. I HAD CONCERNS THAT I DIDN'T REALLY KNOW MY DEP FUEL ASSUMING THE GAUGE WAS INACCURATE AT THE GATE I COULD NOT CONFIRM I HAD THE NEEDED FUEL TO CONTINUE TO ZZZ1. I CONFIRMED THE TOTAL GALLONS ADDED IN ZZZ AT OVER 3000 GALLONS; SO I KNEW WE HAD ENOUGH FUEL TO SAFELY PROCEED BACK TO ZZZ. I ALSO HAD SUSPICIONS THERE COULD HAVE BEEN WATER CONTAMINATION CAUSING THE ERRONEOUS READINGS. I ELECTED TO RETURN TO ZZZ AND DECLARE AN EMER AS A PRECAUTION IN THE EVENT WE HAD ANY ENG ABNORMALITIES. WE ALSO HAD TO LAND OVERWT AND CHOSE THE LONGEST RWY AT ZZZ. APCH AND LNDG WERE UNEVENTFUL AND MAINT TOWED THE ACFT TO THE MAINT HANGAR. NEW PROCS NEEDED TO XCHK ACTUAL FUEL ONBOARD BEFORE AND AFTER FLTS AND FUELING.