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Liquid Flow

Measuring the volumetric flow rate of a liquid, usually water, is a common practice in field tests. Applications for measuring liquid flow include measuring whole-house hot water use, hot and cold water use at individual fixtures throughout the house, and measuring the condensate generated by an air conditioner or heat pump. Due to the wide range of flow rates that these measurements may include, there are different types of flow meters. Due to cost requirements, volumetric flow meters are typical even though there are flow meters that can measure mass flow rate directly.

The image shows the bottom of a sink with pipes underneath and a flow meter that has been connected.

A turbine flow meter, installed under a sink, is used to measure the hot water flow rate and total usage. Credit: Dane Christensen, NREL

The most common type of flow meter for residential field test applications is a turbine flow meter. Turbine flow meters come in a variety of size and resolution options and are typically used in conjunction with hot and cold water flows. Measuring condensate flow rate, however, is not a measurement that turbine flow meters do well. Condensate is generated at a very slow flow rate and is not necessarily a continuous flow. There are ways to very accurately measure condensate flow rate in a laboratory setting, but they are cost-prohibitive for a field test. A pumped reservoir is a type of flow meter that is appropriate for field tests when measuring very low liquid flow rates. A tipping bucket rain gauge is another device that can be used to measure condensate generation rate.

There are a number of other ways to measure liquid flow rate, some of which are not expected to be used often in field tests. The following list of flow meters includes those that are commonly used in field tests and some that are not.

Costs apply to equipment designed for flow rates of up to about 15 gpm, except in the case of the pumped reservoir, which is most likely to be used for condensate measurement at much lower overall flow rates.

Turbine Flow Meter

A turbine flow meter has an internal rotor that is spun by the kinetic energy of a fluid flowing through it. The rotational speed of the turbine varies linearly with the mean flow velocity of the liquid. The rotational speed of the shaft is often measured by monitoring a change in magnetic field. As the turbine rotates, voltage is induced in a magnetic pickup coil and as each blade passes the coil, a voltage pulse is generated. Each pulse corresponds to a known volume, which allows the flow rate to be measured. Another similar design for turbine flow meters uses a Hall Effects sensor to measure the rotational speed of the rotor.

Nutating Disk Water Meter

Nutating disk water meters have an internal disk that "wobbles" as flow passes around it. Disk meters are a type of "positive displacement" meter, and will respond to very low flow rates. Water utilities have traditionally used this type of meter, and they are sometimes called "utility meters."

Pumped Reservoir Flow Measurement

Measurement of air conditioner condensate flow is of interest in many field tests. The rate of condensate flow is often higher than can be accurately metered using low-flow meters, such as the tipping bucket, and lower than can be accurately measured using standard flow meters, such as turbine flow meters. Thus, for HVAC condensate flow, a pumped reservoir is recommended.

Magnetic Flow Meter

Magnetic flow meters are a type of volumetric flow meter that can only be used with conductive or water-based liquids. This excludes liquid hydrocarbons and distilled water, among other things. Magnetic flow meters have no moving parts and so do not induce a pressure drop and have little need for maintenance. The principle used in magnetic flow meters is based on Faraday’s Law.

Ultrasonic Flow Meter

An ultrasonic flow meter measures the flow rate of fluids containing either bubbles or particulates. Ultrasonic flow meters do not use any moving parts and thus do not induce a pressure drop. Also known as non-intrusive Doppler flow meters, ultrasonic flow meters use the Doppler Effect to measure the flow rate of particles within the fluid.

Rotameters

Rotameters are a type of differential pressure flow meter used to measure (and sometimes control) the volumetric flow rate of liquids or gases. A rotameter consists of a tapered tube made of either glass or plastic and a float. When fluid is flowing through the tapered tube, the float rises until gravity is equal to the drag exerted by the fluid.

Tipping Bucket Rain Gauge

A common sensor for measuring rain or condensate is a tipping bucket rain gauge. Rain is collected in a funnel on the top of the assembly and is directed into the "tipping bucket." The tipping bucket is shaped like a roof truss, with a fulcrum in the middle like a seesaw. The truss will tip when a preset amount of water is collected on one side and the other side will begin filling. This action will trip a reed switch sending a pulse to the data logger. Since the volume held in the bucket is known, the rate of pulses is used to determine the rate of rain falling or the rate of condensate production. This method works well for low flow rates and becomes less accurate for higher flow rates. As one example, the tipping bucket rain gauge made by Texas Electronics does not accurately capture flow rates higher than 1.5 liters/hour.