bathwater

[Roy & Sue Farman]

come on then all you boffins work this one out. A full size bath draining into a sump around 500mm below the bath through a 38mm plastic drain. In gallons per hour what is the discharge rate. (what capacity pump is required to keep up). Some body must have worked this one out.

[Admin (Chris)]

Er?    I dunno, but I'll try and find out.

But I do know this, it depends on how much water is in the bath. When it's full the flow will be much more than when it is nearly empty from the extra pressure due to weight of water.

So there is no fixed flow rate, you need to work out the weight of water in a full bath (number of gallons x 10lbs) then find out the flow rate through 1/2 a metre of 38mm pipe at this weight and get a pump that can cope.

[Admin (Chris)]

After some research, it turns out the flow rate can't be calculated just from the info given. There are too many variables:

Length of pipe, is pipe straight, material of pipe, surface area of source, depth of source, constrictions such as plug hole grill.

I am reliably informed that the best way to find out is to get a 25ltr plastic container, fill the bath so as to get maximum pressure, put your 38mm pipe into the container and pull out the plug, then time how long it takes in seconds to fill the container.  That will give you 25litres in "x" seconds, devide 25 by "x" and you have your flow rate in litres per second, x 60 for per minute, x 60 again for per hour, then convert that to gallons if you like, but most small pumps these days are measured in litres per minute. But then you are getting on a bit, so gallons would be easier for you oldies

[Roy & Sue Farman]

i dunno how but i know a man that does. he is a young aeronautical engineering student who works evenings in the pub (Trout at Lechlade on Thames Plug Plug) he did it the other night and gave me the bit of paper. Trouble is I lost it and cant remember the answer(I was tired right). next time in I will get him to write in big writing for us gallon thinking oldies and post the calcs on the forum. A calculation like that would come in handy if you never used it.