A method, including ejecting irrigation fluid from a distal end of a probe so as to irrigate tissue, and 5 receiving, over a period of time, initial signals indicative of respective temperatures of the distal end, from a temperature sensor in the distal end. The method also includes formulating from the initial signals a temperature range between upper and lower temperature 10 thresholds and, whena further signal from the temperature sensor, received subsequent to the period of time, is indicative of a further temperature above the upper temperature threshold, raising an alarm that a bubble is present in the irrigation fluid.