Temperature alarms

This page contains information about equipment I have build for various computers which are related to temperature.

SMS Fridge alarm

The Gentronics SMS Fridge Alarm checks both for a mains failure as well as when the fridge temperature goes above a user settable threshold. Optionally the alarm also has an external input for the alarm output of some freezers and incubators. This alarm has a measurement range of -100 degrees Celsius to 80 degrees Celsius. Although this alarm has been designed to notify a user if a fridge or freezer's temperature is above a certain threshold, it can be used in any instance where a temperature alarm is needed. Examples includes incubators, server rooms, room temperature.


The advantage of this unit is that it is completely self contained, it does not rely on external alarms or networks. It also contains its own lead-acid battery backup. It automatically test the battery using a load test every 24 hours to make sure that the battery is still good. If not, it will send an SMS indicating that the battery should be replaced.

Installation of the unit is straight forward. The unit's power cable is connected to the same supply as the fridge (preferably using one plug for both the fridge and SMS alarm.) The temperature probe (which is connected to the unit via a flat 3-core ribbon cable) is placed inside the fridge.

The temperature threshold is adjusted on the unit itself, while the numbers to which the SMS's should be sent is programmed into the unit via specially formatted SMS messages send to the unit.

When an alarm condition occurs, there is a 90 minute time delay before the unit sends any SMS's. This is to limit false alarms in case their is a short power failure or if someone has opened the fridge for a long time.

Temperature alarm

This temperature alarm is used to monitor the temperature of freezers and refrigerators used in laboratories for storing chemicals and samples. If the temperature rises above a user settable value, an alarm output is triggered. I have build two versions of these temperature alarms. The standard range unit  has a temperature range from -40 degrees centigrade to +50 degrees centigrade. The ultra low range has a temperature range of -100 degrees centigrade to 0 degrees centigrade. The alarm output is in the form of a normally closed relay, which may be connected to an alarm circuit. The alarm has a time delay function. This time delay function is needed since if a freezer is opened to insert or remove samples, the temperature will rise. Only if the temperature is above the set point for about 60 minutes on the current units, the relay will open to indicate the alarm condition. During the time delay the display flashes between the current temperature and the time which has so far elapsed since the temperature limit has been exceeded.






Server room temperature alarm

This temperature alarm has been designed to monitor the temperature of a computer server room. If the temperature goes above a certain threshold, the alarm is triggered. The alarm has a remote siren and strobe which is located outside the server room. There is also a remote mute button to silence the siren. Below is a picture of the main controller unit. This unit contains the main electronics and power supply. Since this temperature alarm is used in conjunction with a network server, it does not have any battery backup supply since it gets power from the UPS which is used to power the server.



The temperature is displayed at the left with the set point adjust buttons just to the right of the display. The silver button at the bottom is used to mute the siren. The ALARM indicator will flash as long as there is an over temperature condition. The main controller unit is mounted inside the server room. Below is a photo of the remote strobe/siren unit. This unit is mounted outside the server room, the distance is not that critical.



Below is a photo of the remote MUTE button. Pressing this button only mutes the siren, but the strobe will keep flashing as long as there is an over temperature condition.



The main controller also has a relay output which may be connected to an external alarm system or even an auto dialer.

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