Five Things To Consider Before Purchasing A Greenhouse Alarm

The temperature inside of a greenhouse can rise very quickly on a sunny day if not vented. Greenhouse temperature can also drop rapidly during the heating season if the heating system is not working properly. H.C. Wien of Cornell University reported hourly temperature changes of a high tunnel in early spring. The high tunnel experienced as much as a 20⁰F rise and a 15⁰F drop, respectively, in May. If an equipment malfunction or power outage occurs in the greenhouse when nobody is there to notice, the environment can quickly become deadly to plants. For example, failure of a heater on a cold winter night could allow the crop to freeze. Similarly, a power outage on a hot summer day would shut down cooling systems and cause plants to overheat.

Destruction of a crop is a significant financial loss. To help prevent these types of catastrophes, a grower may choose to install a monitoring system, programmed to send an alert if critical environmental parameters reach dangerous levels. Monitoring systems can track a wide variety of parameters including temperature, ventilation, CO2 level, and humidity. Some systems are also capable of sending alerts in the event of ruptured water lines, power outages, and other equipment failures. A monitoring system may never be needed, but if it is, the system could easily pay for itself after the first use.


Alarms come in two forms: stand-alone or as part of a computerized environmental control system. For example, La Crosse Technology has a stand-alone device that plugs into an existing router to send alarm messages to a computer or smartphone. Other similar systems are available from Sensaphone, Monnit, and others. In contrast, Wadsworth carries an alarm manager that integrates with Wadsworth’s computerized controllers and thermostats. Similar systems are available from Priva, Argus, Link4, Micro Grow, and others. If you already have a computerized control system and are thinking about an alarm, it is worthwhile to investigate if there is an alarm feature that can be activated. Here are four considerations when planning the purchase and installation of an alarm system.

Tailor Alarm Functionality To Your Business Size

The number of parameters that can be monitored is an important factor when choosing an alarm system. Each parameter monitored — temperature, humidity, CO2, etc. — requires its own sensor and input. Some systems are expandable and some come with a fixed number of inputs. Smaller operations may get by with a smaller number of inputs. The number of inputs should be carefully thought out before making a purchase. A business poised for growth should consider an expandable system. In some cases, it is not only useful to monitor conditions, but also to log data. Growers can use the data to identify patterns or trends that could improve operations.

Alarm systems communicate through a phone call, text message, or email. Find out from employees which method of communication will be most effective, to ensure the system purchased meets the communication preferences of the individuals receiving potential alerts.

The most basic alarm system communicates only in the event of a problem. More advanced alarm systems will also log data. Alarm systems with the most functionality offer two-way communication such as real-time monitoring and real-time control. Real-time monitoring allows a grower to remotely check current environmental conditions in the greenhouse. Real-time control allows a grower to remotely make changes to environmental parameters by adjusting set-points. Two-way communication adds an additional layer of protection since a failed attempt to check the system indicates a problem.

Protect Your System From Cyber Attacks

Any system that communicates with the outside world opens up an entry point for cyber-attacks. If you can reach the system from your home, anybody can reach the same system from anywhere. More simple alarm systems that call out only and are behind a firewall carry the lowest risk. When two-way communication takes place, for example to monitor greenhouse conditions remotely, the risk of attack increases. Although phone lines and cellular connections are less likely to be attacked than an Internet connection, there is no such thing as a completely safe connection. To prevent hackers from entering your system, use multiple access control systems such as long passwords, firewalls, and an independent network for the alarm through dual network interface cards in the computer.

Although wireless installation is easier for a pre-existing greenhouse, it adds another attack vector. Anybody with the right equipment can pick up a wireless signal. Hard wired communication is not only more reliable, it is also safer from a security standpoint.

A lot of hacks happen because people are curious. People with the know-how will hack a system without knowing whether or not there is valuable information inside. They attack because they can. This means while a small operation is less likely to be attacked compared to a large operation, the risk is always there. A devious individual may hack a system and simply adjust the temperature set point from 65°F to 120°F.

Determine If Your Alarm Signal Is Reliable

Environmental monitoring and alarm systems can be wired or wireless. A wired system will be more reliable. There are more components in a wireless system that may fail in contrast to hard-wired sensors and land-line communications. However, installation of cables may be expensive to install after a greenhouse is constructed. Hardwiring may involve trenching between the sensors collecting environmental data and the base unit monitoring the sensor data. In contrast, wireless systems are easy to set up in an established structure. For wireless, it is important to consider the wireless range and directionality of the signal so that all sensors can communicate with the base unit. For all types of systems, it is imperative to recognize that the greenhouse environment is harsh with high temperatures, high humidity, strong ultra-violet rays, and the presence of agri-chemicals. Components of a greenhouse alarm system must be built to withstand conditions that are not typically present in other indoor applications.

All alarm systems will have a base unit that communicates with the outside world. The base unit can work with a phone landline, through cellular communications or an Internet connection. A landline is the most reliable connection because it will continue to function even when the power goes out. Unfortunately, many greenhouses do not have a landline.

The next best thing is sending messages through cellular communications. This may seem contrary to popular belief as many of us are familiar with how often cell phones drop calls or operate with poor voice quality. The difference is that cell phones are mobile and the alarm system is stationary. If a strong signal exists when the unit is installed, the signal should always be there. Do not employ an alarm system that works through cellular communications if the signal strength is poor. In the event of a power outage, the system will require a battery backup. A wireless alarm system that uses the Internet will be the most vulnerable to power outages. This type of system will require an additional battery backup for the modem.

Limit False Alarms By Locating The Temperature Sensor Properly

Limit false or delayed alarm messages by ensuring reliable sensor readings. Locate sensors where they will be representative of the environment. For example, direct sunlight on a temperature sensor may generate a false alarm for a high temperature. Multiple false alarms will cause individuals to begin ignoring the alarm messages.

Do not allow direct sunlight, heating vents, or cooling pads to interfere with temperature readings. Place the sensor near the plant canopy in an aspirated radiation shield to ensure a representative measurement. The base unit, which the sensors communicate with should be protected from the elements as well. If there is a battery backup for the system, ensure it is also in a protected location. If the system does not have a battery backup, consider purchasing one. Be careful not to use the battery backup in such a way that it fools the system into thinking the power is on during an outage.

Factor In System Costs

Costs associated with an alarm system include infrastructure, equipment, and operating costs (maintenance). For systems that are hard-wired, infrastructure includes cables such as Ethernet or phone lines. Wires must be installed to connect sensors in varied locations or structures to the base unit. In some cases, especially where the greenhouses are already constructed, it is more cost effective to purchase a complete wireless system than paying to bury wires. In cases where new facilities are being planned or built, it may be advantageous to install a hard-wired system, such as a landline, because of the increased reliability.

All systems will need a base unit to send out the alarm signal and sensors to gather the data. Wired base units cost more than wireless, but they usually have more functionality, such as built in data logging. Hard-wired base units run anywhere from $800 to $2,500, and the sensors average around $50, ranging from $10 to $120.

There are many hard-wired and wireless alarm systems available on the market. Specific alarms are listed in this article as examples of system capability and respective consumer cost in 2016. We are not endorsing systems mentioned here over others. Each system exampled here was designed for 18 separate greenhouses, each monitored for temperature.

The wired Express II system from Sensaphone communicates through a phone line and can have between eight and 40 channels and does not log data. Sensaphone’s SCADA 3000 offers between 16 and 136 channels with data logging. This system can also communicate through cellular 3G and Data Radio. The onset HOBO RX3000 offers between 10 to 18 channels and can communicate through Ethernet, WiFi, or 3G. These systems can run anywhere from $3,800 to $4,500.

Wireless base units cost around $200. Wireless system sensors are more expensive than hard-wired sensors because they include a wireless transmitter. The Omega ZW-ED provides 128 channels for about $4,000. The wireless range is 3,000 feet and a computer is required for Internet communication. The MadgeTech Therm-a-lert, for approximately S4,400, offers fewer channels but has an extendable range. Omega’s UW Series, for approximately $4,350, has an extendable range of 5 miles, offers 32 to 48 channels, and does not require a computer for its Internet connection. Sensaphone offers a wireless system, the WSG30,  that is also capable of Internet communication without a computer. The Omega, MadgeTEch Therm-a-lert and Sensaphone systems can run from approximately $4,000 to $5,000.

In a wireless system, repeaters will be needed if the signal cannot reach the furthest greenhouse. A repeater boosts the wireless signal so that a sensor may communicate with the base unit if the two devices are beyond the signal range. Repeaters cost around $150. Only one base unit and a couple repeaters will typically be needed. Systems that log data usually require a computer, adding to the cost. Most companies offer free software but some do not.

Types Of Alarm Sensors

Most alarm systems can be configured to monitor a variety of conditions, such as temperature, humidity, water flow, and pressure. The price of the sensors vary with each type. For example, air velocity sensors run around $300, while tilt sensors run less than $100.

4-20mA Signals DC Current Open/Close State
AC Current Differential Pressure Temperature*
AC/DC Voltage Dissolved Oxygen Tilt/Acceleration
Absolute Pressure Gauge Pressure Water Flow
Air Velocity Leaf Wetness Water Level
Amp Hour Pulse Water Temperature*
Amps Rainfall* Watt Hours
Barometric Pressure Relative Humidity* Watts
CO2 Run-time Weather
Compressed Air Flow Soil Moisture* Wind Speed
Conductivity Solar Radiation* Wind Direction
*Most commonly used

Operational costs include electricity, telecommunications service, and annual fees if you are data logging in the cloud. To properly maintain an alarm system, it is most important to replace the batteries (average every two years) in the sensors. Batteries that are not replaced in the recommended timeframe can leak and damage the sensor.

What to do in the event of a power outage should also be considered. Many systems have a specialty sensor to detect the loss of power. Including these sensors could be vital to maintaining a crop. While being notified of a power outage is the first step, there needs to be a way to deal with a power outage. Most systems have a backup battery available for purchase. The cost of these vary with the size of the system that is being used