Fresh water tank outlet

[Ratty's Retreat]

Hello All.

 

The outlet of my stainless water tank is about 3" above the base. Is it likely to have an internal dip tube?

 

I am fitting a pressure type water gauge and need to know approx where the minimum level s.

 

Thanks

 

John.

[RobinJ]

The outlet of my stainless water tank is about 3" above the base. Is it likely to have an internal dip tube?

 

I am fitting a pressure type water gauge and need to know approx where the minimum level s.

Certaining on stell tanks it is a straight fitting, allowing space to accomodate a stop tap and also allow for de-furring the tube.

I imagine stainless will be the same!

[grahame r]

Is it one of these?

 

http://www.hendersons.co.uk/boattank/HP_Install_inline_graphics_gauge.pdf

 

We've got one and it always reads empty when its actually about quarter full. I've been looking at the installation literature and think there may be 2 possible causes: 1) the sender is mounted too high, which I guess is what you're worried about or 2) the breather pipe (to the overflow skin fitting) is holding water and is causing a decrease in pressure in the tank. I'll be investigating this further in the next day or so and will let you know of any amazing discoveries.

[RobinJ]

We've got one and it always reads empty when its actually about quarter full. I've been looking at the installation literature and think there may be 2 possible causes: 1) the sender is mounted too high, which I guess is what you're worried about or 2) the breather pipe (to the overflow skin fitting) is holding water and is causing a decrease in pressure in the tank. I'll be investigating this further in the next day or so and will let you know of any amazing discoveries.

They are not 100% accurate, but the sensor needs to be located as close to the bottom level of the tank as possible, even adjusting the zero to calibrate it will result in an offset and false reading.

It does not matter if the pipe has to go up and come down again, as it is the relative height that determines the pressure!

[Ratty's Retreat]

Is it one of these?

 

http://www.hendersons.co.uk/boattank/HP_Install_inline_graphics_gauge.pdf

 

We've got one and it always reads empty when its actually about quarter full. I've been looking at the installation literature and think there may be 2 possible causes: 1) the sender is mounted too high, which I guess is what you're worried about or 2) the breather pipe (to the overflow skin fitting) is holding water and is causing a decrease in pressure in the tank. I'll be investigating this further in the next day or so and will let you know of any amazing discoveries.

 

Yes that's the one. As I see it, if the tank has a straight fitting the sender should be by the outlet. If there is a dip tube then it should be fitted near the bottom.

 

John.

[grahame r]

Yes that's the one. As I see it, if the tank has a straight fitting the sender should be by the outlet. If there is a dip tube then it should be fitted near the bottom.

 

John.

 

Yes, that's what it says in the destructions but as I see it if you fit the sender to the bottom of a vertical tube (to get to the same level as the empty tank) then that tube is always going to be full of water and the gauge is going to read that water pressure, ie never empty. Having said that my gauge reads empty when its quarter full! Perhaps I should get one of my kids to explain the physics to me.

[grahame r]

Is it one of these?

 

http://www.hendersons.co.uk/boattank/HP_Install_inline_graphics_gauge.pdf

 

We've got one and it always reads empty when its actually about quarter full. I've been looking at the installation literature and think there may be 2 possible causes: 1) the sender is mounted too high, which I guess is what you're worried about or 2) the breather pipe (to the overflow skin fitting) is holding water and is causing a decrease in pressure in the tank. I'll be investigating this further in the next day or so and will let you know of any amazing discoveries.

 

Well, it's taken a month but I have made an amazing discovery. It says in the fitting instructions that the sender can be fitted to the end of either a vertical or horizontal pipe and there's a diagram showing which part of the sender should be lined up with the lowest level in the tank. Now to calibrate the full mark you have to turn a little screw that is located below a cover on the face of the sender, opposite the pipe connection. If you mount the sender on a vertical pipe and the tank is on or near the floor then you can't get at the calibration screw so it's easier to mount it horizontally which is how mine was fitted. As I mentioned mine was reading empty when it was in fact quarter full. I've altered the pipework now to a vertical pipe but kept the position of the sender relative to the lowest water level the same and hey presto, my gauge now reads accurately at full and empty. It's a bit of a faff but it appears that if you want it to work properly you need to mount it to a horizontal pipe and calibrate the full mark then put an elbow in the pipe and remount it vertically. I know people say "we know how much water we use and when it needs filling" but we have lots of guests and its really handy knowing exactly when the water will run out.

[RobinJ]

I've altered the pipework now to a vertical pipe but kept the position of the sender relative to the lowest water level the same and hey presto, my gauge now reads accurately at full and empty. It's a bit of a faff but it appears that if you want it to work properly you need to mount it to a horizontal pipe and calibrate the full mark then put an elbow in the pipe and remount it vertically.

Possibly with a horizontal pipe there is more chance of build up of particulates which interfere with it?

[blackrose]

I thought the idea of having the outlet a few inches above the bottom of the tank was so that any crap that accumulates in the tank isn't drawn into the pipe, which means it's unlikely to dip down to the bottom of the tank. So in the case of any gauge you're fitting, isn't the height of the outlet pipe effectively the bottom of the tank?

Edited June 28, 2012 by blackrose

[jonathanA]

two things,

 

theses gauges are actually made by MCS so might be worth a chat with them about installation/calibration.

 

They are supposed to work on pressure (i.e head of water ) not flow so horizontal/vertical shouldn't make that much difference its the pressure exerted by the weight of water that makes it work.

 

I'm not sure how moving from a horizontal to a vertical pipe with an an elbow works - you must have moved the sensor position relative to the highest/lowest water level surely ?

 

I guess if it worked for you thats the proof but i wonder if whats actually going on is that the tank is not actually a flat bottomed regular shape, perhaps coupled with the boat not being level fore-aft was giving you a false reading and now you've compensated for that by moving the sensor position ?

 

I'm currently interested in fitting the waste tank version as thats a more of a problem than the water tank level !

[RobinJ]

They are supposed to work on pressure (i.e head of water ) not flow so horizontal/vertical shouldn't make that much difference its the pressure exerted by the weight of water that makes it work.

These type of sensors (working on force or strain) have a rubber (neoprene) membrane to protect them. If they are connected to a fluid and laid horizontally, then any deposit settling out can get trapped between the membrane and the casing, imparting a false 'zero'.

When mounted vertical then any deposits will settle away from the sensor!

If you are planning to fit one to a waste tank than it will be more important to ensure the sensor is kept free from contamination!

[grahame r]

two things,

 

theses gauges are actually made by MCS so might be worth a chat with them about installation/calibration.

 

They are supposed to work on pressure (i.e head of water ) not flow so horizontal/vertical shouldn't make that much difference its the pressure exerted by the weight of water that makes it work.

 

I'm not sure how moving from a horizontal to a vertical pipe with an an elbow works - you must have moved the sensor position relative to the highest/lowest water level surely ?

 

I guess if it worked for you thats the proof but i wonder if whats actually going on is that the tank is not actually a flat bottomed regular shape, perhaps coupled with the boat not being level fore-aft was giving you a false reading and now you've compensated for that by moving the sensor position ?

 

I'm currently interested in fitting the waste tank version as thats a more of a problem than the water tank level !

 

If you look at the instructions then you'll see that they show the sender unit both vertical and horizontal with a line showing where it should be level with lowest water level. Presumably this is the point where the sensor is. Originally mine had a long (about 2 feet) horizontal pipe that dipped down to required level. I replaced this with an elbow and vertical pipe which in effect turned the sensor through 90 degrees but still level with the bottom of the tank. Admittedly it also moved 2 feet back and the natural "nose up" boat attitude would have some affect over that 2 feet. However the difference really is very marked, its gone from being accurate to a quater of a tank to a couple of gallons. In my opinion this sender unit simply doesn't work very well if attached to a horizontal pipe and is superb if attached to a vertical one.

When you say a waste tank do you mean a toilet tank? If so I've got one of those waiting for me to fit it so it'll probably be a couple of years before I get round to it. These work on a completely different principle, you need to stipulate the depth of tank when ordering and they supply a suitably sized probe so there's no need to calibrate anything. Obviously on a water tank it's the empty reading that you are really interested in whereas with a toilet tank its the full reading.