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How Does a Transit-time Ultrasonic Flowmeter Work?

Transit-time meters, as the name implies, measure the difference in travel time between pulses transmitted in the direction of, and against, the flow. This type of meter is also called time of flight and time of travel.

In the example shown in Figure 1, the sonic beam is at a 45º angle, with one transducer located upstream of the other. Each transducer alternately transmits and receives bursts of ultrasonic energy; the difference in the transit times in the upstream vs. the downstream directions (TU - TD) measured over the same path can be used to calculate the flow through the pipe:
V=K*D/Sin2?*1/(To-t)²?T

where:
V= Mean Velocity of flowing fluid
K= Constant
D= i.d. of pipe
?= incident angle of ultrasonic burst waves
To = zero flow transit time
?T = T2-T1
T1 = transit time of burst waves from upstream transmitter to downstream receiver
T2 = transit time of burst waves from downstream transmitter to upstream receiver
t = transit time of burst waves through pipewall and lining

This equation shows that the liquid flow velocity is directly proportional to the measured difference between upstream and downstream transit times. Because the cross-sectional area of the pipe is known, the product of that area and the flow velocity will provide a measure of volumetric flow. Such calculations are easily performed by the microprocessor-based converter. With this type of meter, particles or air bubbles in the flow stream are undesirable because their reflecting qualities interfere with the transmission and receipt of the applied ultrasonic pulses. The liquid, however, must be a reasonable conductor of sonic energy.
Figure 2 shows three placements that can be used for the two transducers. All are identified as single measuring path because the sonic beam follows a single path, and in all three the two transducers are connected by cable to a converter that can output a 4-20 mA DC signal. The selection of one configuration over another is dictated by several factors associated with the installation, including pipe size, space available for mounting the transducers, condition of the inside pipe walls, type of lining, and nature of the flowing liquid.
The Z configuration places the transducers on opposite sides of the pipe, one downstream of the other. Generally, the distance downstream is ~D/2, where D = pipe diameter. The converter uses specific data on piping parameters to compute the optimum distance. The Z method is recommended for use only in adverse conditions such as where space is limited, the fluid has high turbidity (e.g., sewage), there is a mortar lining, and when the pipe is old and a thick scale has built up on the inside wall that tends to weaken the received signals. It is not recommended for smaller pipes, where its measuring accuracy tends to degrade.
In most installations, the V method is recommended, with the two transducers on the same side of the pipe about a pipe diameter apart. The rail attachment that can be clamped on the pipe facilitates sliding the transducers horizontally along the pipe and positioning them the calculated distance apart.
The W method should be considered on pipe 1½ in. down to ½ in. dia. Its main limitation is a possible deterioration in accuracy due to buildup of scale or deposits on the pipe wall-note that the sonic signal must bounce off the wall three times. Turbidity of the liquid also could be harmful since the signal has a longer distance to travel.

About the Author

Jhon Smith is an eminent analyst and writer. How Does a Transit-Time Ultrasonic Flowmeter Work

High TDS reading on Reef Tank?

My 24 gal tank is cycling right now with live rocks and live sand. My well water has a reading of 110 PPM and after it goes through the RO/DI unit it has a reading of 70ppm. When i do a tds reading on my tank it has a reading of 700x18 ppm . IS THIS RIGHT? i have a HM DUAL DIGITAL TDS READING METER

One summer i worked at one of those "students needed for part time work" things that are posted at every college campus. It turned out that we went door to door trying to sell people water softeners by shocking them with high TDS.

The problem is that just having a high TDS does not necessarily mean that the solids are bad for your aquarium (of course TDS stands for "total dissolved solids") it just means that there is stuff other than water in your water.

TDS meters are notoriously inaccurate and even if you do get one that is accurate it must be sent in yearly to be recallibrated if you want "perfect" readings. This being said i would go out and buy a bottle of Aquafina water, the only water that i know of that always has a TDS of 0 and make sure that my TDS meter was accurate. Also when you clean your TDS meter after use i would just pour a little Aquafina on it every time after testing to make sure none of the residual solids in the water that you were testing dried on your meter.

I know that people do TDS readings on tanks, i personally dont since i cant afford to keep one properly callibrated and honestly i dont think it does much good (just my opinion though)

As to your reading out the back of your RO/DI i think that this is pretty poor depending on how much stuff is in your well water to begin with. Here in the high plains of texas you usually get between 700 and 800 tds from well, with a good membrane and 5 stage ro you can get your tds under 100 and using real good di's you can get your tds down to around 1 ppm but you have to replace your resin often.

What i would do if i were you is test my water for phosphates and metals and of course alkalinity, if all of these came back negative, i would just not worry about TDS, i have a buddy that uses straight well water for his mushroom/ leather tank and has no problem except the occasional diatom bloom (with TDS of 720 according to his meter that is 3 years old and has never been callibrated,) he does however use LFS bought water for his zoa, frag, and sps tanks.

ASU hopes to solve ULM defense
JONESBORO — Arkansas State is searching for a way to crack Louisiana-Monroe’s defense. The Red Wolves believe the key to earning their first victory of the season is figuring out a ULM defense that has significantly improved over the last couple of years.

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