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WATER PUMPS, TANKS, TESTS, WELLS, REPAIRS
WATER CONTAMINANT LEVELS
WATER HAMMER NOISE DIAGNOSE & CURE
WATER ODORS, CAUSE CURE
WATER PUMP REPAIR GUIDE
WATER PRESSURE LOSS DIAGNOSIS & REPAIR
WATER PUMP SHORT CYCLING
WATER SOFTENERS & CONDITIONERS
WATER TANK REPAIR PROCEDURES
WATER TANK: USES, TROUBLESHOOTING
WATER TESTS, CONTAMINANTS, TREATMENT
WATER TREATMENT EQUIPMENT CHOICES
WELLS CISTERNS & SPRINGS
WELL FLOW RATE
WELL WATER PRESSURE DIAGNOSIS
WELL YIELD IMPROVEMENT
WINTERIZE A BUILDING
This article explains Water Tank Size - how much water is in the water pressure tank? and we address the question of how much water volume we need to avoid short cycling the water pump? This article series answers just about any question you may have about pumps, wells, and drinking water.
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How to Find Out How Much Water is in the Water Tank, Considering the Space Taken up by Air in a Water Pressure Tank
Even before performing water quantity, quality, equipment function tests, there is an enormous amount we can determine about a building's water supply just by looking at the equipment. Articles here provide details on water pumps, tanks, controls, and wells and water supply inspection, diagnosis, and repair.
The page top photo shows our client holding the open top of his well casing along with some unusual well casing exit plumbing at a drilled well with a modern steel casing. Finding the location of your well and inspecting the condition of the well piping and equipment are an important first step to assure a functional and potable drinking water supply - that is, having enough water supply and having water that is safe to drink. The articles listed below provide detailed advice on diagnosing and repairing problems with water pumps, water tanks, wells, and other water supply equipment.
It's not how much water the tank holds that is useful to know. It's how much water we can get out of the water tank before the pump has to turn on. This is the "draw down" volume, which we can measure or calculate, or we can focus instead on how long (in time) we can run the water before the pump has to turn on. (See our discussion of "short cycling" water pumps at Water Tank Repairs: Diagnose "Water Pump Short Cycling" & Restore Air in a Building Water Tank. If your pump is short cycling you want to fix it, as we explain in that article.
How to calculate the volume of water in a water tank
For the 36" diameter x 99" long water tank above, we calculated the water tank volume as follows:
[3.1416 x (18")2 x 99" = 100,782 cu .in.] / 231 = 436 gallons in this water tank.
That's a big tank storing a lot of water, probably indicating a well with a very low flow rate at the home where this water tank was installed. Or someone was planning to survive a long dry period. The small control connection shown at the 11 o'clock position on the end of this water tank was an air volume control.
Domed water tank volume: If we needed to be precise and if the bottom and top of a water tank are domed at the top and convex at the bottom (usually) we can measure these areas and calculate their volume using the formula for the volume of a sphere (or part of one). But we suggest skipping this detail. Probably the spherical volume lost from the convex tank bottom is about equal to the spherical volume of the tank top, so it's a wash and we can just use the tank's overall height and diameter.
A 30 gallon water tank does not give you 30 gallons before the pump turns on
The photograph shows a one-line jet pump, the water pressure tank, and a water softener. We know from the fact that this is a single line jet well pump that the well is a shallow one, probably less than 27' deep. Well depth may have implications for water quantity and quality and vulnerability to surface water contamination.
Remember that a "30 gallon water tank" used to control water pressure and pump cycling in a building does not hold 30 gallons of water, but something less than that (say 20 gallons of water max and 10 gallons of air at the point of pump cut-off). Then as you draw water out (and the in-tank pressure falls down to the pump cut-on point) the pump is going to come on before all of the water leaves the tank.
So the maximum actual water you get out of the tank is less than the tank size, maybe 15 to 20 gallons max. The bladder-type pressure tank manufacturers cite an "equivalent" draw-down water volume as that provided by the older bladderless tanks.
How to measure the draw-down water volume provided by a water tank
What are some typical water tank draw down volumes?
A 10 gallon water pressure tank that starts fully empty and is pumped up to about 50 psi will contain about 3 gallons of air and 7 gallons of water. The water tank in normal operation does not draw down to 0 gauge pressure before the pump comes on.
The water tank provides out flowing water down to 20 psi (on a 20-40 psi system or down to 30 at a 30-50 psi system) when the pump comes on.
A water pressure tank with a total volume of 10 gallons and operating between 20 psi and 50 psi of pressure will have a draw down water volume of just 4.35 gallons of water.
A typical kitchen water faucet runs between 3 gpm and 5 gpm (varying as the water pressure in the system varies as the water pump cycles on and off), so we can expect to run the water at the tap for about a minute before the pump will come on with this theoretical water tank. Because the pressure drops as the water tank empties and then increases as the water pump comes back on, the water pressure at a faucet or other plumbing fixture will vary between the pump cut-in pressure (typically 20 psi or 30 psi) and the pump cut out cycle (typically 40 psi to 50 psi). quoting from Water Tank Pressure, Temperature, and Air Volume Calculations.
The above-cited article, which we admit is a bit unnecessarily complex (I was answering someone else's query) has the math you need to calculate the actual draw-down volume of water you get with a given sized tank, with a given in-tank water volume when the pump has reached its shutoff point.
Frequently Asked Questions (FAQs)
Questions & answers or comments about the size of water tanks in buildings: what size do we need, what are "equivalent" water tank sizes in gallons or liters and what does that mean?.
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.