Different Types of Anemometers (Reed Switch vs. Sine Wave)
In an earlier blog, we discussed the general different types of anemometers– Discussion of the cup anemometer noted that the design was invented by Dr. John Thomas Romney Robinson in 1845 and consisted of four hemispherical cups mounted on one end of four individual horizontal arms. These arms were then mounted to one another on a vertical shaft at equal angles. As wind flows past the cups, regardless of the horizontal direction, the shaft turns proportional to the speed of wind. By counting each turn of the shaft over a specified period of time, average wind speed is determined. Today, professional anemometers such as the Comptus A75-101 and A75-104, are regularly used as part of assessing wind resources for utility scale wind farms.
There are several different signal generating three-cup anemometers available. Two common types are the sine wave and reed switch type anemometers. How are they different? Which is better for what type of applications?
Both types of anemometers use a rotating magnet that interacts with other components to generate a signal. The reed switch anemometer utilizes a very sensitive switch which closes and opens with each pass of the rotating magnetic poles. The contact transfer rate varies linearly with wind speed. The sine wave anemometer uses a copper coil alternator that generates a sine wave output signal with frequency changing linearly with wind speed.
Provided the profile of the housing and rotor assembly are the same, overall accuracy, starting threshold and cup distance constant should be very similar. The sine wave unit generates its’ own power and is well suited to remote installations that require very low power consumption. The reed switch output is simply a switch readily measured by most data logging devices in use today. The A75-104 generates 2 volts at 60 cycles. The A75-101 switches up to 10 VDC at 10mA. Sine wave anemometers are subject to minor accuracy variations at low and high speeds. Reed switch anemometers have a consistent linear response across all speeds.
Quality commercial anemometers are accurate and designed to withstand harsh conditions for years. For example, reed switches like the ones used in the Comptus A75-101 have an expected lifespan of 109, or 1 billion, cycles. The sine wave generator within a Comptus A75-104 can theoretically run indefinitely. That said it is wise to change out sensors periodically to ensure accurate data collection.
Deciding between a reed switch or sine wave anemometer comes down to data logging capability and the number of anemometers you will be connecting. Top quality data loggers are able to receive signals from either sensor. However, some data loggers provide more inputs for one signal type over another. If you will be connecting multiple anemometers to your data logger, confirm the number of inputs you will need. Then make your decision on whether you will use reed switch or sine wave output anemometers.