It's About Time C

[jordannleigh]

So, I noticed that most of you all are building pendulums. Well, my partner and I were sort of thinking we were going to go with the water clock, and the first time, we used a GIANT device, which used two water cooler bottles, and way too much water. The second and third time we've competed we've used a buret, but we have found that although it tends to be consistant during testing, as soon as we were in the competition room, the times varies too much. Does anyone know why the times would change? We took in water pressure and how the less water left in the buret, the slower the water line moved, but we dont know why the changing of places would change the time.

With the buret method we ended up just using the mL markings and calibrated it to be exactly 5 min. We then glued the dial-thingy in place so it didn't change at all. We ended up getting second at regionals, but we think it was just because our answers to the questions were really accurate, and the other teams messed up by either spilling, not wearing their goggles, etc.

Now, our teacher wants us to use an IV bag instead of a buret, does anyone have any suggestions on how to measue time with it? Do you think that using a scale would be more effective? or should we just try to make even markings again?

Any suggestions would be great! thanks so much!!

[walkingstyx]

PV=nRT.
In a water clock, you measure the amount of volume that moves from the top to the bottom, and the pressure and temperature affect that. Since you have to measure very small changes in volume to get .1 second accuracies, even tiny fluctuations in temperature and pressure can mess up your clock, and there are bound to be small changes depending on the weather and how air conditioned your host is.
Also, I'm pretty sure that IV bags are classified as commercial counters and thus not allowed.

[Paradox21]

PV=nRT is actually a gas law. So you can't apply it to water in the buret. However, barometric pressure does play a role in the inaccuracies. Greater surrounding air pressure would cause the water to be pushed out of the buret faster. I suppose you could make a table of values for all different atmospheric pressures and measure the pressure at the competition site that day. But it would be a lot of work, and there is a possibility that temperature and humidity also cause changes. Or... you could build a pendulum. If a buret is allowed in competition, I don't know why an IV bag wouldn't be allowed.

[texan92]

just curious, how many of you guys have had questions relating to the theory of relativity, time dilation, continuity of time, etc on yall's written test? in the rubric, it states that questions will be asked about the physics of time but i have yet to take a test that asks us about these topics. (the nationals test last year had a question about a spacecraft traveling at 0.8 c but it wasn't really relating to relativity)

[fleet130]

Greater surrounding air pressure would cause the water to be pushed out of the buret faster.

But isn't the pressure on the water in the burette the same as the pressure opposing the water flowing out of it? In my mind, the pressures should cancel each other, eliminating any effect from barometric pressure. If there is a pressure differential, how does your particular design cause it?

Temperature change could affect the viscosity of the water, which could affect the flow rate. I haven't researched this, but I think the viscosity change of water is insignificant for normal temperature ranges.

Difference in head can produce significant differences in flow rate. Head is simply the difference in height between the top surface of the water and the metering orifice. It’s important that the head be the same at the start of each measurement. If not, the beginning flow rate will be different and affect the amount of water dispensed.

A water clock has 4 major components.

1. The water reservoir holds water to be dispensed for measuring the time interval. It should have as large a diameter as practical to minimize water level change as water is dispensed. Head can be increased by using a piece of hose/tubing between the reservoir and metering device. This reduces the effect of level change in the reservoir.

2. Control valve. This valve starts and stops the flow of water. It's important that it can be fully opened/closed quickly as any delay directly effects the time measured

3. Metering device. This device consists of a small orifice to restrict the water flow.

4. Measuring device. Measures the amount of water dispensed. A digital scale could be used to weigh the water, but in my mind this would violate any prohibition against electrical/electronic devices. One option is to capture the water in a small diameter clear tube as it is dispensed. The tube can be marked with graduations corresponding to time.

If I recall correctly, a small drop of liquid detergent (or photoflow solution) to break down surface tension of the water makes the clock more consistent.

Here's an example of a simple water clock.

I saw this clock in the hotel hall way back in the late 80s the night before the national tournament. It was made from a couple of boards, 2 1-gallon clear plastic jugs, a garden hose shut-off, a piece of small brass tubing, a long peace of clear tubing, tape and fasteners as needed. It’s been a long time and I don’t have any photos, so some details are not too clear. As is often the case, it was completely rebuilt and re-calibrated in the hotel hall the night before the tournament.

The clock was capable of making repeat measurements of around ¼ second resolution. I believe it actually measured in 10ths of a second, but that resolution was not completely reliable.

One jug, used as the reservoir, had a hole cut in the bottom for easy filling with water. A line marked on it identified how much water should be added. This made the “head” constant so the water pressure was the same at the beginning of each time interval.

The control valve was a garden hose shut-off that turned 90 degrees from full off to full on. (Its operation might be made easier by disassembling it and coating the parts with a lubricant such as Vaseline.) The control valve was attached to the reservoir with black electrical/friction tape.

The metering device was a piece of brass tubing (inside diameter ~ 1/16 in). Electrical/friction tape attached it to the shut-off valve.

The measuring component was a long clear tube wrapped several times around a second 1-gallon jug (The only purpose for the jug was as a form to hold the tubing). One end of the tube was fastened under the metering tube to catch the water as it was dispensed. The other end of the tub was held above the greatest height water reached when timing the longest time interval. Marks added at intervals to the jug or tubing to indicated the time measured. Both ends of the measuring tube were open to the atmosphere at all times.

After measuring a time interval, water was removed by lowering the bottom end of the tube, capturing the water in a plastic cup, and pouring it back into the reservoir.

Accuracy is very dependent on the calibration process. Start/stop signals can be a large fraction of a second long (possibly as much as a whole second). An effort should be made to turn the water on/off as soon as possible after the beginning of the start/stop signal.

[jordannleigh]

@ everyone who answered, thanks so much! I hadnt thought about temperature at all, oops!

@fleet130, i think my team might just moidify this idea a bit, it has me thinking... i think we'll use something like this or some variation
and the idea about the detergent will be EXTREMELY helpful, i think it might help us with the reading if we decide to do the buret again.


thanks again!

[fleet130]

Don't overdo the detergent. A single drop in a gallon of water is overkill!

[Balsa Man]

4. Measuring device. Measures the amount of water dispensed. A digital scale could be used to weigh the water, but in my mind this would violate any prohibition against electrical/electronic devices. One option is to capture the water in a small diameter clear tube as it is dispensed. The tube can be marked with graduations corresponding to time.

Actually, the rules do clearly allow it:
3c - "...a battery-operated electronic balance or scale used solely to determine mass..."

A very simple water clock, where you weigh the water, does work remarkably well. We use a 2" plastic tube, fill mark toward the top, with an outlet at the bottom, rubber tube atached to the outlet, with an orifice (1 for each of the time ranges) at the end of the tube. A pincher on the tube between the outlet and orifice is the on-off control. By doing a series of calibration runs - measuring time, and weight of water released, and entering that data in Excel, and producing a graph, and then running a regression, getting the formula of the graphed line, you can use that formula to generate a table of time & weights....

[Bogoradwee]

Greater surrounding air pressure would cause the water to be pushed out of the buret faster.

But isn't the pressure on the water in the burette the same as the pressure opposing the water flowing out of it? In my mind, the pressures should cancel each other, eliminating any effect from barometric pressure. If there is a pressure differential, how does your particular design cause it?

Temperature change could affect the viscosity of the water, which could affect the flow rate. I haven't researched this, but I think the viscosity change of water is insignificant for normal temperature ranges.

Difference in head can produce significant differences in flow rate. Head is simply the difference in height between the top surface of the water and the metering orifice. It’s important that the head be the same at the start of each measurement. If not, the beginning flow rate will be different and affect the amount of water dispensed.

A water clock has 4 major components.

1. The water reservoir holds water to be dispensed for measuring the time interval. It should have as large a diameter as practical to minimize water level change as water is dispensed. Head can be increased by using a piece of hose/tubing between the reservoir and metering device. This reduces the effect of level change in the reservoir.

2. Control valve. This valve starts and stops the flow of water. It's important that it can be fully opened/closed quickly as any delay directly effects the time measured

3. Metering device. This device consists of a small orifice to restrict the water flow.

4. Measuring device. Measures the amount of water dispensed. A digital scale could be used to weigh the water, but in my mind this would violate any prohibition against electrical/electronic devices. One option is to capture the water in a small diameter clear tube as it is dispensed. The tube can be marked with graduations corresponding to time.

If I recall correctly, a small drop of liquid detergent (or photoflow solution) to break down surface tension of the water makes the clock more consistent.

Here's an example of a simple water clock.

I saw this clock in the hotel hall way back in the late 80s the night before the national tournament. It was made from a couple of boards, 2 1-gallon clear plastic jugs, a garden hose shut-off, a piece of small brass tubing, a long peace of clear tubing, tape and fasteners as needed. It’s been a long time and I don’t have any photos, so some details are not too clear. As is often the case, it was completely rebuilt and re-calibrated in the hotel hall the night before the tournament.

The clock was capable of making repeat measurements of around ¼ second resolution. I believe it actually measured in 10ths of a second, but that resolution was not completely reliable.

One jug, used as the reservoir, had a hole cut in the bottom for easy filling with water. A line marked on it identified how much water should be added. This made the “head” constant so the water pressure was the same at the beginning of each time interval.

The control valve was a garden hose shut-off that turned 90 degrees from full off to full on. (Its operation might be made easier by disassembling it and coating the parts with a lubricant such as Vaseline.) The control valve was attached to the reservoir with black electrical/friction tape.

The metering device was a piece of brass tubing (inside diameter ~ 1/16 in). Electrical/friction tape attached it to the shut-off valve.

The measuring component was a long clear tube wrapped several times around a second 1-gallon jug (The only purpose for the jug was as a form to hold the tubing). One end of the tube was fastened under the metering tube to catch the water as it was dispensed. The other end of the tub was held above the greatest height water reached when timing the longest time interval. Marks added at intervals to the jug or tubing to indicated the time measured. Both ends of the measuring tube were open to the atmosphere at all times.

After measuring a time interval, water was removed by lowering the bottom end of the tube, capturing the water in a plastic cup, and pouring it back into the reservoir.

Accuracy is very dependent on the calibration process. Start/stop signals can be a large fraction of a second long (possibly as much as a whole second). An effort should be made to turn the water on/off as soon as possible after the beginning of the start/stop signal.

Wow... That's the longest single post to asnwer a question I've ever seen.

[Galacticdude]

This event can be a real pain. The clock building part is easy, I have done that, but I need more info on the written test. The rules don't help with only three example problems, and I have already gotten all I can get out of the links on the National Website. Could somebody help me with this problem?