SCAD TM2 Sensor Not Detecting Tank Level on Plastic Holding Tank

[skunther]

I’m troubleshooting a SCAD TM2 external capacitance sensor on the holding tank of my 1993 Hunter Legend 35.5 and would appreciate any ideas from those with experience.

The original aluminum holding tank was replaced sometime ago, probanly around 2003 with an unknown plastic tank. It measures approximately 24” × 24” × 12” tall (30 gallons).

The SCAD foil strips were installed exactly per the instructions:

• 11” long with correct 1.5” spacing and 1/2” margins at top and bottom of 12” tall tank.
• Smooth, flat installation with good adhesion
• New, properly functioning TM2 monitor (I replaced the original after discovering it had a T2 channel issue, and verified it is correctly displaying voltage receibed from the holding tank sensor)

Here’s what I’ve measured:

• Tank nearly empty: ~1.0 V (SCAD diagnostic)
• Tank nearly full: ~1.0 V
• Shorting the foil strips together: ~4.0 V
• Pressing a silicone bag full of water against the outside of the strips: ~2.2 V

Additional observations:

• Swapping signal wires between T1 and T2 on the new TM2 gives identical readings, so the monitor channels appear to be working correctly.
• SCAD told me they typically expect about 0.2 V empty and 1.5 V full, and that jumping the sensors should produce a 4v signal from the sensor (it did)

What puzzles me is that the sensor appears to be working properly as it clearly responds (4 V when strips are shorted, 2.2 V with an external water bag), yet the actual tank changes almost not at all between empty and nearly full.

Has anyone seen this with a replacement plastic holding tank? Could the tank construction (internal baffle, thickened wall, molded rib, double-wall, etc.) prevent the sensor from seeing the liquid even though the sensor itself appears to function normally? Any other ideas?

 

[dLj]

Do you know how thick the tank wall is? Have you tried moving the sensor to a different location on the tank?

Sure seems like the sensor itself is working....

dj

 

[jssailem]

1. thickened wall,
2. molded rib,
3. double-wall,

For the externally attached SCAD sensor, each of these conditions could impact the ability of the sensor to function as desired.

Even an accumulation of fecal material on the plastic wall behind the sensor could contribute to false readings.

 

[skunther]

I managed to spray off some of the interior wall where the sensor strips are located with a garden hose and spray nozzle, and the sensor signal voltage range improved. A lot of buildup remains though. I’d like to more effectively clean the rest of that wall. Does anyone know of good methods for cleaning off the interior of a holding tank? Are any of the additives effective or are there better methods? I have a 1800psi pressure washer - what about using that with a 90° gutter cleaner and 40° tip to disperse the pressure? (I am hesitant on this idea, but if it weren’t to damage the tank, it seems it would be a pretty effective and quick method).

I should note - it seems their interior sensor might be a better way to go as it doesn’t require regular cleaning of buildup on tank walls to maintain sensor accuracy. The sensor is easily removed and reinstalled for cleaning. The external sensor is appealing, but it seems it can still be effectively fouled by buildup on the interior of the tank.

I wonder if @Peggie Hall HeadMistress might weigh in on this point? I actually picked up the external sensor due to her recommendation.