# Time Constant of Water Level

1. Oct 23, 2009

### getthoboy

I'm curious:

"A group of students needs to determine the time constant of a water tank. FIrst, the student establish an equilibrium level, with a known flow rate into the tank and both valves (solenoid and hand-operated) fully opened. One of the students reduces the input flow and the group records the change in water level as the level drops. after the level reaches a new equilibrium level, the student closes the hand valve. The group then recods the data as the level rises. analysis of data yields that the time constant for the tank when the level is falling is 9 minutes, while the time constant for the rising level is 11.5 minutes."

Why is that so? why is the time constant value different?

2. Oct 23, 2009

### Staff: Mentor

Welcome to the PF. The inflow has one rate, and the outflow has another rate. They were combining rates in some cases and not in others...

3. Oct 23, 2009

### getthoboy

yep, they do have different rates. but why is one higher than the other?

4. Oct 24, 2009

### Staff: Mentor

Um, because if they're different, one has to be higher than the other?

I guess I'm not understanding the crux of your question. I guess all I can say is that the inflow rate is different from the outflow rate? Sorry if I'm not getting your question.

5. Oct 24, 2009

### getthoboy

well. the point is, the time constant of the rising water level is different than the time constant of the falling water level.
but why is that so? what makes the time constant for the falling process smaller than the other?

6. Oct 25, 2009

### getthoboy

7. Oct 25, 2009

### Redbelly98

Staff Emeritus
Are the two valves (solenoid and hand-operated) on the outlet or inlet for the tank?

Also, do you know what determines the time constant for this process?

8. Oct 25, 2009

### getthoboy

the valves do not have any effect on the value of the time constant. But the time constant could be determine by plotting the record on the excell.
But again, why does the values of the time constant differ? why is one higher than the other?

9. Oct 26, 2009

### Redbelly98

Staff Emeritus
Without seeing a figure describing your setup, it's hard for us to tell just what is going on. Are the two valves (solenoid and hand-operated) for the outlet or inlet of the tank, or is one for the outlet and the other is for the inlet? The outlet valve(s) do affect the time constant.