I have been using a DLJ water meter from the Daniel L. Jerman Co. ( http://www.jerman.com/ )to measure flow rates and flow quantities for my Solar Shed house heating project -- I thought I would pass on what I have learned to date.
The water meter is in the pipe that runs between the outdoor thermal storage tank and the house. It measures the total flow of hot water from the storage tank to the house. This flow rate coupled with the temperature of the incoming water and the temperature of the water returning to tank allows the calculation of the total heat delivered to the house by the collector system.
In addition to showing total gallons on the face of the meter, it also has an internal switch that closes briefly for each gallon of flow. These switch closures can be monitored by a suitable data logger, and used to calculate flow rates and total flow over an interval.
The prices for the DLJ meters start around $50. The hot water capability and the pulse output capability cost extra. The 3/4 inch one I bought (the DLJSJ75CH) was $120 with the hot water and pulse output options.
What's to like about the meter:
Low flow resistance
Registers total gallons down to the nearest 1/10th gallon
Has a spinner dial that does one full rotation per tenth of a gallon
Hot water capability
Appears to be well made
Pulse output for logging
Not to like:
There are some mounting position limitations on some of the meters
You cannot reset the meter total to zero
The odometer gives total gallons, with the last dial being tenths of gallons. The red dial spinner wheel rotates one full revolution for each tenth of a gallon -- timing a couple of rotations of this allows easy calculation of the current flow rate. The metal fittings in the picture come with the meter, and adapt it to a standard 3/4 inch pipe thread.
The meter installed in the incoming hot water line. The T and stub to the right of the meter is the temperature sensor (thermistor) for the Onset data logger. A ball valve is installed just to right of this to allow easy removal of the meter. Note that the position of the pipe, and the fact that the meter face must be up makes it necessary to read the meter with a mirror -- not a big deal, but a little inconvenient.
When logging the flow rates by hand, I note the total meter gallons at the start of the session. During the session I calculate flow rates by counting the time it takes the red pointer to make 10 rotations (1 gallon). At the end of the session, I note the total gallons again.
You could also just count the number of full and partial rotations the red dial makes over one minute, and divide this count by 10 to get flow rate in gallons per minute -- e.g. it the redial goes around 12 full rotations, and is half way through the 13th in one minute, the the flow rate is (12 + 0.5)/10 = 1.25 gal/minute.
The meter has in internal switch which closes for a brief time for each gallon of flow. The switch contacts are brought out to two wires. A data logger that can detect switch closures can be used to record the closure for each gallon.
I use the Onset Computer U12 Event logger to do this. The output from the logger looks like this:
In this case, the pulse is coming about every 40 seconds, indicating a flow rate of 1/(40)/(60)) = 1.5 gpm.
The switch stays closed for what appears to be about 15% of the time to the next pulse. Depending your logger, this may require a little program to process the recorded pulses and turn then into flow rate values.
The total heat flow to the house can be calculated for any given minute as:
Heat Flow = (Flow Rate ) (Tsupply - Treturn) (Specific Heat of water)
For example, with a flow rate of 2 gal/min, Tsupply = 110F, and Treturn = 95F, the heat flow is:
Heat Flow = (2 gal/min)(8.3 lb/gal) (110F - 95F) ( 1 BTU/lb-F) = 249 BTU/min or 14950 BTU/hour