Exactly how much water do I have left in my pressure tank?

[WyomingMan]

Ever wonder exactly how much water you have left in your well pressure tank? Have enough for a shower? Flushing a toilet too?

I have a deep well and a pressure tank. I have a 40/60 PSI turn on/off switch. Typical. I have an emergency generator to operate the well pump but have to roll it out from the garage, fuel it, and connect the big cable. When the power goes out I'd just as soon ration my remaining water in the pressure tank rather than firing up the generator.

Assume you have a 44 gal pressure tank, fairly typical. Assume a 40/60 switch. Assume you live at about 1,500 ft above sea level where the atmospheric pressure is 14 PSI.

Boyle's Law says that the pressure times the volume of a gas is a constant. i.e. if you halve the volume the pressure doubles. But you have to use absolute pressure, the pressure that the molecules "feel", not what a pressure gauge shows (a tire with 30 PSIG (PSI gauge) has a true pressure of 44 PSIA (PSI absolute) which is 30 + 14 = 44.
PSIA = PSIG + 14 in this example. We have to use PSIA because obviously increasing a tire (or tank) from 1 PSIG to 2 PSIG doesn't cram the molecules together twice as much.

In a normal pressure tank set up you precharge the air bladder (when empty) to 2 PSI less than the switch turn on point. So with a 40/60 switch this is 38 PSIG. You now have 44 gallons of air at 38 + 14 = 52 PSIA. If you have 44 gallons of air you must have 0 gallons of water!

Now, assume the gauge reads 40 PSI and the pump is just about ready to come on.
If you have 40 PSIG you have 54 PSIA. The pressure has increased from 52 to 54 PSIA which is 1.04 times so the volume of air must decrease by 0.96, the reciprocal. 44 gallons X 0.96 = 42.4 gallons of air allowing 1.6 gallons of water. Yup, you have a whopping 1.6 gallons of water when your gauge reads 40 PSI. Not much.

Assume your gauge reads 50 PSI, half way between turn on and turn off. How much water? If you have 50 PSIG you have 64 PSIA. That's 1.23 times the empty 52 PSIA. So 1/1.23 = 0.81 and that times your 44 gallons is 35.8 gallons of air and 8.3 gallons of water (figures are rounded).

Lastly, assume you're lucky and your pump just turned off after reaching 60 PSIG before the power failed. What now? 60 PSIG = 74 PSIA and that's 1.42 times your empty tank air pressure. 1/1.42 = 0.70 and that times 44 gallons is 30.9 gallons of air leaving 13.1 gallons of water. That is the MOST that will EVER be in your 44 gallon tank! This surprises most folks.

Notice that your available water is not linear with pressure, i.e. the water you have at 50 PSIG is not the half way point between what you have at 40 or 60.

I actually installed an 86 gallon tank for this very reason. More available when the power goes out and reduces the on/off cycling of the pump. Even at that there's not a tremendous amount available. You can look up your own atmospheric pressure if you want to refine your calculations (if you're a nerd like me). At 5,000 ft. I have only 12.23 PSI of air above me.

Lastly, be aware that when you run out of water (at 38 PSIG in this example), it just STOPS, you get no notice. Sorry for the long winded dissertation. Just thought it might be useful info.

 

[Proud Prepper]

He's back, thanks for the info.

 

[Arcticdude]

Too much math for me. I have 2 ea 119 gallon pressure tanks in my pump house. I guess I get about 75 gallons of water out of each tank. My well is 650 feet deep and have a 5 hp pump. The pump is run off an 8700 watt generator and is pumped over a quarter mile to the house site with over a half mile total of buried water lines.

 

[KateMTx]

Ever wonder exactly how much water you have left in your well pressure tank? Have enough for a shower? Flushing a toilet too?

I have a deep well and a pressure tank. I have a 40/60 PSI turn on/off switch. Typical. I have an emergency generator to operate the well pump but have to roll it out from the garage, fuel it, and connect the big cable. When the power goes out I'd just as soon ration my remaining water in the pressure tank rather than firing up the generator.

Assume you have a 44 gal pressure tank, fairly typical. Assume a 40/60 switch. Assume you live at about 1,500 ft above sea level where the atmospheric pressure is 14 PSI.

Boyle's Law says that the pressure times the volume of a gas is a constant. i.e. if you halve the volume the pressure doubles. But you have to use absolute pressure, the pressure that the molecules "feel", not what a pressure gauge shows (a tire with 30 PSIG (PSI gauge) has a true pressure of 44 PSIA (PSI absolute) which is 30 + 14 = 44.
PSIA = PSIG + 14 in this example. We have to use PSIA because obviously increasing a tire (or tank) from 1 PSIG to 2 PSIG doesn't cram the molecules together twice as much.

In a normal pressure tank set up you precharge the air bladder (when empty) to 2 PSI less than the switch turn on point. So with a 40/60 switch this is 38 PSIG. You now have 44 gallons of air at 38 + 14 = 52 PSIA. If you have 44 gallons of air you must have 0 gallons of water!

Now, assume the gauge reads 40 PSI and the pump is just about ready to come on.
If you have 40 PSIG you have 54 PSIA. The pressure has increased from 52 to 54 PSIA which is 1.04 times so the volume of air must decrease by 0.96, the reciprocal. 44 gallons X 0.96 = 42.4 gallons of air allowing 1.6 gallons of water. Yup, you have a whopping 1.6 gallons of water when your gauge reads 40 PSI. Not much.

Assume your gauge reads 50 PSI, half way between turn on and turn off. How much water? If you have 50 PSIG you have 64 PSIA. That's 1.23 times the empty 52 PSIA. So 1/1.23 = 0.81 and that times your 44 gallons is 35.8 gallons of air and 8.3 gallons of water (figures are rounded).

Lastly, assume you're lucky and your pump just turned off after reaching 60 PSIG before the power failed. What now? 60 PSIG = 74 PSIA and that's 1.42 times your empty tank air pressure. 1/1.42 = 0.70 and that times 44 gallons is 30.9 gallons of air leaving 13.1 gallons of water. That is the MOST that will EVER be in your 44 gallon tank! This surprises most folks.

Notice that your available water is not linear with pressure, i.e. the water you have at 50 PSIG is not the half way point between what you have at 40 or 60.

I actually installed an 86 gallon tank for this very reason. More available when the power goes out and reduces the on/off cycling of the pump. Even at that there's not a tremendous amount available. You can look up your own atmospheric pressure if you want to refine your calculations (if you're a nerd like me). At 5,000 ft. I have only 12.23 PSI of air above me.

Lastly, be aware that when you run out of water (at 38 PSIG in this example), it just STOPS, you get no notice. Sorry for the long winded dissertation. Just thought it might be useful info.

Growing up we had a well with a tank like this. The first one we had was only 50 gallons and it would run out of water very quickly whenever the power went out. Eventually we had a deeper well drilled and a much bigger tank. I don't know how many gallons but it's quite a bit taller than I am and it takes a while to empty out.

 

[WyomingMan]

Too much math for me. I have 2 ea 119 gallon pressure tanks in my pump house. I guess I get about 75 gallons of water out of each tank. My well is 650 feet deep and have a 5 hp pump. The pump is run off an 8700 watt generator and is pumped over a quarter mile to the house site with over a half mile total of buried water lines.

The point of my long post is to KNOW, not guess, how much water one has. If you look at my calculation for 50PSIG, half way between the 40 and 60 set points, you'll see the tank is about ONE-FIFTH full of water, so in your case you have about 25, not 75, gallons of water in each tank. Most folks guess they have much more than reality.

 

[Arcticdude]

The point of my long post is to KNOW, not guess, how much water one has. If you look at my calculation for 50PSIG, half way between the 40 and 60 set points, you'll see the tank is about ONE-FIFTH full of water, so in your case you have about 25, not 75, gallons of water in each tank. Most folks guess they have much more than reality.

My guestimate is based on the fact that I have a 65 gallon water trough at the barn and I can fill it from the pressure tank without starting the pump. There's a quarter mile of 1-1/2" water line between the well and the barn and a half mile total water lines on the property. This holds a certain amount of water in reserve too. I'm in the process of adding a second 119 gallon pressure tank at the well and eventually an auto start generator so that we'll have water without manually starting the generator.

 

[KateMTx]

There are other places you can store water than just the pressure tank though. I think it is rather foolish to rely on it as your plan for keeping water, especially since I've seen a few of those tanks spring a leak and all the water drained out. Also, I don't think you will be able to access ALL the water in the pressure tank anyway. Due to science reasons like you mention above, as some point the air pressure in the tank will be too low to force more water down the pipes and out to your house. So you need to take that into account and not just the actual # of gallons too. Or just don't plan to rely on the water left in the tank when the power fails. Have alternative methods of storing water and plans for how to make use of the water.

 

[GaRp58]

Good evening all, I have only a 60 ft. deep well, but it provides all I need. It can run day and night and won't go dry. But, how many of you have gotton or are using a "widder" or "ram" pump to move your water? Look in you tube for ram pump or widder pump. Using a simple to build principle and a few one way valves with a small 3 foot water tank for pressure, it can run water even uphill with no electricity, wind, pumps or such...take a look.
In winter all freezes, but in warmer times, you can save a lot of fuel...GP

 

[GaRp58]

#TOMTV #TOMTVManipuriNews #TOMTVEnglishnew Hello again, another project for the fun, building a distillery for moonshine. BUT, if you put plastic bottles and bags into the main tank, you will get a bit of methane gas first and then about 5 liters of petrol from about 7-8 kilos of plastic trash. You can cook on the methane if you collect it into an old rubber innertube and run your generator on the petrol...several videos from India and the Phillipines in You Tube. GP

 

[Brent S]

I am much more concerned with having multiple ways to get more water from the well than I am with what’s left in the tank. Right now I have two generators. Both are dual fuel, propane and or gasoline. Solar panels are in the works before the end of the year to make the 30% tax incentive. If worse comes to worse I also have two creeks on the property. Carrying 5 gallon buckets may suck but it will fill a toilet tank and can go into a Berkey water filter without needing power. Everyone that asks me about prepping is always surprised by my unglamorous advice about water being the number one thing to have a plan for. I like guns, freeze dried foods, and all the other fun survival stuff, but without water everything else is worthless.

 

[tmttactical]

I am much more concerned with having multiple ways to get more water from the well than I am with what’s left in the tank. Right now I have two generators. Both are dual fuel, propane and or gasoline. Solar panels are in the works before the end of the year to make the 30% tax incentive. If worse comes to worse I also have two creeks on the property. Carrying 5 gallon buckets may suck but it will fill a toilet tank and can go into a Berkey water filter without needing power. Everyone that asks me about prepping is always surprised by my unglamorous advice about water being the number one thing to have a plan for. I like guns, freeze dried foods, and all the other fun survival stuff, but without water everything else is worthless.

Brent check out GaRp58' s post about the Ram Pumps. Here is a possible concept. Ram pump in creek closest to house. Build a holding pond above house elevation. Pump water (first ram pump) from creek into holding pond. Pump water (second ram pump) from holding pond to house water holding tank. Drain excess water from ram pump to gravity fed irrigation lines. No electricity required. This will require a bit of actual engineering but could be feasible. JM2C

 

[Brent S]

Brent check out GaRp58' s post about the Ram Pumps. Here is a possible concept. Ram pump in creek closest to house. Build a holding pond above house elevation. Pump water (first ram pump) from creek into holding pond. Pump water (second ram pump) from holding pond to house water holding tank. Drain excess water from ram pump to gravity fed irrigation lines. No electricity required. This will require a bit of actual engineering but could be feasible. JM2C

I actually built one a while back. Just didn’t have enough elevation drop to make it work well. Was able to pump water about half way up to the house. It was an interesting science experiment anyways. You need the elevation change to power the pump and my property is Just too level

 

[tmttactical]

Brent is a flat lander!!! LOL

 

[Brent S]

Brent is a flat lander!!! LOL

Yup, 1100’ elevation, but it’s a flat 1100’ where the creeks flow across the property. 1100 on one side and 1150 on the other would have done it. I think my creek drops about six feet from one edge to the other.

 

[tmttactical]

Yup, 1100’ elevation, but it’s a flat 1100’ where the creeks flow across the property. 1100 on one side and 1150 on the other would have done it. I think my creek drops about six feet from one edge to the other.

Heck I got more rise than that, just going to the mailbox. You need to build you an elevated holding pond, a big one.

 

[GaRp58]

You can regulate the ram pump with a higher (taller) pressure tank and put one of the one-way valves on without a spring. Larger pumps with larger intakes and smaller output pipes change the relationship also. Try, Try again. How many trial and errors did we all need to finally start a fire with a magnesium bar? Can't remember how many times I burned my fingertips or just scattered the powder in all directions...Go for it. GP

 

[Brent S]

You can regulate the ram pump with a higher (taller) pressure tank and put one of the one-way valves on without a spring. Larger pumps with larger intakes and smaller output pipes change the relationship also. Try, Try again. How many trial and errors did we all need to finally start a fire with a magnesium bar? Can't remember how many times I burned my fingertips or just scattered the powder in all directions...Go for it. GP

Before I even built it I did the math and the elevation was not where it needed to be. I wouldn’t say it was a failure as I did get it to pump water, just not as high as I would have liked.

 

[Brent S]

Heck I got more rise than that, just going to the mailbox. You need to build you an elevated holding pond, a big one.

I have hills, just not where the creeks flow across. The front creek has three ponds with spillways between them. I think I have plenty of storage in them, but a solar pump is the best option I can think of for getting it up to the house.

 

[Silent Earth]

I have hills, just not where the creeks flow across. The front creek has three ponds with spillways between them. I think I have plenty of storage in them, but a solar pump is the best option I can think of for getting it up to the house.

Wind and Solar pumps better.

 

[GaRp58]

Damn I'm glad for living on flat land. Remember the old hand pumps on water wells we saw in the old John Wayne films? GOT ONE ! No wind, no solar, no electricity...jus' me and my old dirty hands. The ground never freezes (Arctic Dude) no hills (BrentS) no holding pond (Urbanprep) no holding tanks...Guess I got lucky for once in my nitty gritty little life...GP