Holding Tank Monitor

After 3 years sailing Silver Lining, including some extended trips, I finally got around to installing something we’ve always wanted – a holding tank monitor. Normally, I would have not considered an internal, float-style system, but the WEMA kit received high marks on a recent Practical Sailor magazine test. So went it went on sale this summer, I figured the time (and price) was right. 
I purchased the gauge (HTG-WW) and 12” sending unit (SHS-12) online (www.wemausa.com) for about $83. WEMA recommends that the sensor be sized so that its bottom is no lower than the ¼ full point of your tank. This is to keep it out of the “sludge” and minimize the chance of fouling. Although the sensor can fairly easily be removed for cleaning, I chose to locate it as close to the tank vent fitting as possible. The reason being is that I usually introduce fresh water to the holding tank via the vent line while pumping out. I believe that helps to flush out any solids. Positioning the sensor close to that fresh water spray may help to keep it clean also. 

I elected to mount the gauge in the head compartment, next to the shower sump rocker switch. Rather than run a new, dedicated 12vDC line back to the circuit breaker panel, I chose to tap into the line supplying power to the shower sump switch by rewiring the disconnect so that the 2 wires from the gauge were added (one for power, the other selects which color backlighting you desire – red or yellow). The 100 mA load for this system is a negligible addition to the 4.4 amp draw of the shower sump pump. For the 12vDC negative, I added a terminal strip with a lead back to the main negative buss found behind the circuit breaker panel. To that terminal block I connected the negative lead from the gauge and the ground from the sending unit. The third connection you may note on the block is the negative from my washdown pump. 
With this set-up, I find that the needle on the gauge begins to move at about 6 gallons of fluid. Total install time was about 4 hours.
 

   

    

- Karl Mielenhausen, Silver Lining, #690