HOBO U30 Sistema de Alarma de Heladas

Alarmas Automatizadas de Heladas

The basic configuration for frost alarms consists of the HOBO U30 station, a temperature sensor, and a user-supplied autostart controller. The autostart controller is needed to control the irrigation pumps during start-up, allowing the pumps time to prime which protects them from damage. A relay in the HOBO U30 is configured to activate the autostart controller when a frost condition occurs, and to deactivate it when the frost danger has passed. Typically the U30 is configured to activate irrigation at one temperature and turn it off at a higher temperature, to avoid short-cycling the irrigation system. For example, the U30 can be configured to turn on irrigation when the temperature drops below 34°F, and turn off irrigation at 39°F.

Notifications can also be sent to one or more cell phones when the irrigation system is turned on or off, so growers can know immediately when a frost event is happening.

There are several autostart controllers available. Some growers have used the Murphy ASM170 controller. The relay output of the U30 can be connected directly to the autostart's remote start/stop input.

Be sure to use wire of sufficient gauge to handle the current to the relay and make sure it is UV rated for outside use. The autostart's documentation should indicate what wire gauge is required. For example, the Murphy controller recommends using 18 AWG stranded wire. Also, double-check that the power through the relay will not exceed the (30V, 1A). It is a good idea to connect a 1A fuse in line with the relay.

It is also important to know that the pumps at each site are actually working, so it is common to add a pressure sensor to the system. This allows the pump pressure to be monitored remotely via HOBOlink on the internet. This requires the HOBO U30 to have the Analog Sensor Port option, which provides two analog sensor inputs. The 0 to 100 psi gauge pressure sensor (Onset part number T-ASH-G2-100) can be connected directly to the analog sensor port. This sensor has a 1/4-inch NPT fitting for connecting into the irrigation system water lines.

Note that for frost alarms, it is best to use an exposed temperature sensor in the crop canopy, and to not put the sensor in a solar radiation shield. Although a radiation shield improves temperature measurement accuracy during the day, it slows the response of the temperature sensor during frost events. If daytime air temperatures are also needed, a second temperature sensor with an RS3 Solar Radiation Shield can be added.