Mystery black water sensor readings?

[Bel]

Hi from New Zealand.

Our 54 ft Taiwanese trader has had new blackwater tank and sensors fitted when the heads were replaced about 18 months ago with the previous owner but it keeps giving us strange readings, and saying the tank is full meaning the heads are unable to be flushed.

So we left the boat last week and it was reading 35% full, and came back 4 days later abd it’s reading 98% full (and not allowing any flushing).

And a couple of weeks ago we emptied the tanks completely, and within an hour its reading 25% full but nothing had been used.?

We have had the tank cleaned, sensor checked and calibrated just before Christmas so I am at a loss to explain whats going on.

Any advice would be appreciated as this is causing us grief.

 

[Comodave]

What is the holding tank made out of? Is there a calibration mode on the alarm? I just put a Scad holding tank monitor on our holding tank. But it isn’t hooked to the head. I plan on just checking the level of the contents and pumping the tank when it is getting full.

 

[Bel]

Our tank is stainless steel, and the sensor alarm doesn’t seem have a calibration mode on the panel - we are wondering if seawater might be leaking back in? But this seems an unlikely cause

 

[Comodave]

Too bad it is metal. The non contact sensors don’t work on metal tanks as far as I know. Do you have any clean out ports on the top? Maybe pull any port cover and use a rod to actually measure the tank levels over a couple of days to see if the contents are actually rising or is it a sensor error. If the contents are really rising then I would turn off the through hull seacock and see if it still rises. First thing is to find out if you really have a problem with more contents or is it a sensor issue. Metal tanks for sewage can be an issue since urine is very corrosive so keep an eye on it. Plastic or fiberglass is preferred.

 

[RT Firefly]

Greetings,
Mr. B. No seawater leakback. No additions of effluent to the tank. So your "hard" system is static as far as you know.


I am in NO way electrically skilled but a WAG is that something is drifting or floating.


The resistance of the electronics seems to be changing giving you erroneous readings. I have no idea if this is even possible somehow and can offer no solutions unless you have loose/weak connections, a faulty sensor or gauge some sort of poltergeist aboard.

Is there an exorcist (kaiwhakahere) nearby?

 

[C lectric]

Who made the holding tank sensor system?

There are a couple systems, SCAD is one, that use no contact strips placed on the tank exterior. But the tank must be plastic or FRP or the sensor cannot sense the level.

One way around that though would be to install a PVC pipe, shed 40 , in two holes , one at or near the bottom, the other near the top.
Then the sensor strips are installed on the PVC pipe exterior. The sewage level will rise in the pipe and the sensor can read that through the PVC.


So the major question now is whose level system do you have?



And keep an eye on the SS steel tank. No metal is ideal for sewage.

 

[HeadMistress]

There are a couple systems, SCAD is one, that use no contact strips on the tank exterior. But the tank must be plastic or FRP or the sensor cannot sense the level.
One way around that though would be to install a PVC pipe, shed 40 , in two holes , one at or near the bottom, the other near the top.
Then the sensor strips are installed on the PVC pipe exterior.

You're actually suggesting putting holes in a vertical wall of a tank???

The advantage to systems like SCAD whose senders attach to the outside of the tank is that the tank contents never come in contact with the sensors, preventing the waste from building up on them, which happens in systems like the Dometic/SeaLand Tankwatch that use internal sensors. SCAD and most other mfrs of external sensor systems also make a version for use in metal tanks that encapsulates the sensors IN a PVC tube inserted in the TOP of the tank.. Installing the sensors on the OUTside of the tube defeats the entire purpose of using a tank monitor that uses sensors that never come in contact with tank contents. You might want to spend a little time on the SCAD site learning how these system are correctly installed and how both the external and internal sensor systems work in it and all other similar systems.

--Peggie

 

[Sababa]

Are you sure you don’t have seawater siphoning back through the macerator pump? If your through hull is not fully closed, the joker valves in the line can be prone to failing and allowing the tank to be filled with water. I know because I have a keyed electrical lockout on my macerator pump installed by the PO that allows me to run with my through hull open while still complying with USCG regs. I continued to do this after I replaced the valves the first time, but got wise when the replacements failed too in short order. Now I run with the through hull closed and crawl down in the bilge to open it when I need to pump, though I still don’t need to zip tie it because of the keyed switch.

 

[Bmarler]

It could also just be water filling the tank through the toilet itself. If there’s not a siphon breaker installed on the raw water inlet, or it’s malfunctioning, sea water can slowly drain through the toilet into the tank. My aft head does this if I leave the valves open. We’ve shut the raw water intake to it and don’t use it now. We’ll change it to a new one sometime and switch to freshwater flush like the fwd head.

 

[HeadMistress]

There should be a vented loop/siphon breaker in the intake on all sea water toilets. Where to put it depends on the toilet. If it's a manual toilet or an electric toilet that has a connecting line between the pump and the bowl, that's where it belongs...it'll require replacing the short piece of hose the mfr used to connect them with two longer lines that'll let you put the loop at least 6-8" above the bowl. If it's an electric toilet that does NOT have that connecting line, you'll need an electric solenoid valve in the in the intake line.

The installation instructions for most toilets include an illustration showing the correct placement. If you don't have them, you should be able to download them from the mfr's website.

--Peggie

 

[kchace]

Hi from New Zealand.

Our 54 ft Taiwanese trader has had new blackwater tank and sensors fitted when the heads were replaced about 18 months ago with the previous owner but it keeps giving us strange readings, and saying the tank is full meaning the heads are unable to be flushed.

So we left the boat last week and it was reading 35% full, and came back 4 days later abd it’s reading 98% full (and not allowing any flushing).

And a couple of weeks ago we emptied the tanks completely, and within an hour its reading 25% full but nothing had been used.?

We have had the tank cleaned, sensor checked and calibrated just before Christmas so I am at a loss to explain whats going on.

Any advice would be appreciated as this is causing us grief.

So the main question is - IS IT filling or just saying its filling? If its filling by itself, then this can be serious and could lead to flooding. In that case, a proper vented loop needs to be installed. If its merely saying its filling, then the sensor could be defective, improperly installed or calibrated.

Ken

 

[C lectric]

Yup,
I goofed.


Look up SCAD but ignore the rest of my suggestion.

 

[HeadMistress]

You're not the only person who's ever activated his/her mouth/keyboard before putting brain in gear.

 

[Comodave]

The Scad sensors need X number of square inches to work. My holding tank is about 12” tall so I had to double up on each sensor strip to get the right number of square inches. Mine had to be 3” wide instead of just the 2” strips. I don’t see how you could fit 2 sensors in or on a PVC tube.

 

[NWSeadog]

Dave, What were your symptoms that caused you to increase the width of sensor strip? My strips are about 12" long and it seems to calibrate fine but the system seems erratic over time. I'm thinking that is caused by the build-up of "gunk" on the inside of the tank walls.

 

[Comodave]

Dave, What were your symptoms that caused you to increase the width of sensor strip? My strips are about 12" long and it seems to calibrate fine but the system seems erratic over time. I'm thinking that is caused by the build-up of "gunk" on the inside of the tank walls.

The directions said to. They had a chart with the height of the tank to sensor size. I believe, but not absolutely sure, that my tank was about 12” tall so they said to make the sensor 3” wide and gave a specific spacing between the sensors. Don’t remember exactly what spacing.

 

[NWSeadog]

Found it. If anyone is interested in the actual installation and troubleshooting instructions they can be found here https://www.scadtech.com/TMHelp#foill_placement

Looks like the break point from 2" to 3" is at a strip length of 10".

Probably not my problem. I'll continue on down the road of cleaning my tank walls.

 

[Comodave]

Here is a thread OC Diver did regarding cleaning his holding tank. I did mine last summer with his procedure. Worked well.


https://www.trawlerforum.com/forums/s31/holding-tank-cleaning-easy-way-46357.html

 

[NWSeadog]

Thanks for the reminder. I'm currently in the cleaning process by adding a bacteria along with NoFlex to the nearly empty tank (1/4 full). After no use I went below after several weeks and the tank walls look clean but the SCAD gauge is reading full. I now need to pump and flush the tank to see if the gauge is now working. My next step would be the OC Diver cleaning followed by a flush and recalibration of the SCAD.

 

[bkcorwin]

Echo what sababa said above. In a silverton a few years ago the macerator failed and let raw water come back up into the tank. Came out the top inspection hatch and filled a bilge. Good times.

 

[HeadMistress]

The Scad sensors need X number of square inches to work. My holding tank is about 12” tall so I had to double up on each sensor strip to get the right number of square inches. Mine had to be 3” wide instead of just the 2” strips. I don’t see how you could fit 2 sensors in or on a PVC tube.

You're ignoring the decimal points, Dave. If you re-read the instruction again--carefully this time...I think you'll see that the external sensor size is actually
2.2 x 2.2 x 0.3 inches (56 x 56 x 7.6 mm)

The sensors in pipe for use in metal tanks are installed in the pipe at SCAD based on tank depth.
Scad Tank Monitors

--Peggie

 

[Comodave]

I checked and my tank is actually 10” tall. Scad installation manual says 3” wide sensor spaced 1.5 to 1.75” apart. It shows taking the foil sensor and overlapping them to get the correct width. I tried to copy the chart but it didn’t work well. For a 8” tall tank they wanted 3.5” wide sensor. I copied the below from the manual.


External Sensor Foil Placement Guide

The performance of the external sensor can be optimized by placing the foil strips according to the table below. The external sensors are provided with two inch width aluminum foil tape. When foil height is ten inches or less, the width should be increased. Foil width can be increased by placing additional foil tape toward the outside edges of the two inch strips overlapping by 1/2 inch as in the figure below. For example, an additional two inch strip would result in an overall foil width of 3.5 inches. Measurements in inches.

 

[Bmarler]

The Scad sensors need X number of square inches to work. My holding tank is about 12” tall so I had to double up on each sensor strip to get the right number of square inches. Mine had to be 3” wide instead of just the 2” strips. I don’t see how you could fit 2 sensors in or on a PVC tube.

You're ignoring the decimal points, Dave. If you re-read the instruction again--carefully this time...I think you'll see that the external sensor size is actually
2.2 x 2.2 x 0.3 inches (56 x 56 x 7.6 mm)

The sensors in pipe for use in metal tanks are installed in the pipe at SCAD based on tank depth.
Scad Tank Monitors

--Peggie

It’s the display that’s 2.2 square, not the sensor. My tank is shallow like Dave’s, I had to overlap the foil just like he did in order to get the required square inches of sensor foil.
Hard to say how they do the pipe sensor without seeing one in person, but I’m sure they have it figured out.

 

[Bacchus]

It’s the display that’s 2.2 square, not the sensor. My tank is shallow like Dave’s, I had to overlap the foil just like he did in order to get the required square inches of sensor foil.

Hard to say how they do the pipe sensor without seeing one in person, but I’m sure they have it figured out.

Exactly... the linked dimensioned diagram clearly shows that.
The tape is foil and likely only thousands of an inch thick.