My main counter (SBM-20 tube) has been outside for over 6 years and I have never noticed any fluctuations due to temperature. The extremes it has endured are not as much as yours, FSM19. Approximatly -5 degrees C to +30. Generally it would be in the range of +5 to +20 degrees C. It is mounted in such a place that direct sunlight never hits it.
