Freezer Defrosters and Monnit Sensors

If a Monnit sensor probe or the sensor body itself is operating near a freezer or cooler defrosting unit, there can be an unintended influence on your sensor operation. Since defrosting units often consist of electrical coils, the defrosting unit can affect the sensor radio communication in the form of EMI interference. The defrosting unit can also cause unexpected peaks and valleys in your sensor’s reading history. If you have Rules set to trigger as a result of temperature excursions but do not account for the temperature fluctuations as a result of the defrosting unit, you may receive alerts unexpectedly. This article will provide details on such an issue.

Patterns

Freezers often have defrost cycles that occur every 3, 6, 12, or 24 hours (though this can vary extensively). If you find your sensor is consistently losing connections, look for patterns in the signal history and temperature oscillations. You will generally see a significant signal loss, then a signal recovery after the defrost cycle has completed. You may also see temperature oscillation during the time the defrosting unit is running. If you are able to review the defrost cycle schedule or check with your freezer manufacturer, it is worth comparing the defrost cycle schedule with your sensor’s signal history.

Missed Communications

When a sensor is installed and operating near a defrosting unit, the EMI interference will cause the sensor to miss communications with the gateway. This may also cause the sensor not be able to reconnect to the gateway. In the below example, a customer’s freezer would enter a defrost cycle every 3 hours. As a result, the sensor would often have signal degradation during these times. Reviewing the signal history and considering patterns can help determine this.

Temperature Spikes

In addition to the signal fluctuations, you may also see temperature fluctuations occur if your sensor’s temperature probe is in range of the defrosting unit. This temperature fluctuation can often cause a temperature variance of tens of degrees. If you see unexpected temperature oscillation, this is often the cause. Moving the sensor probe away from the defrosting unit might be necessary to avoid sensing the defrosting cycles.

Unintended notifications

You may find notifications triggered as a result of Inactivity or Temperature notifications being triggered from the defrost cycle. You may consider configuring Delayed Actions or using the Notify After Aware Period notifications to address this.

If you need additional guidance, please contact support@monnit.com.