Does an hps bulb vary in intensity from hour to hour?

I have plants under a 400w hps and the setup is dialed in so that conditions remain steady - temp, airflow etc. I have a thermometer ung at the top of the canopy and under the light. With all else remaining constant, it will go up 5 degrees for a couple hours, then back down. I see nothing that could cause the heat fluctuation other than the bulb varying in how much heat it puts out. Maybe it could be the thermometer, but that seems unlikely.

Any thoughts?

Does it coincide with watering? Maybe there is a bit of humidity build up or something along those lines. Just a thought.

you will find your temp goes up off your body heat if your in the grow room....from what iv read its up around 5 degrees

Illegal Smile said:

I have plants under a 400w hps and the setup is dialed in so that conditions remain steady - temp, airflow etc. I have a thermometer ung at the top of the canopy and under the light. With all else remaining constant, it will go up 5 degrees for a couple hours, then back down. I see nothing that could cause the heat fluctuation other than the bulb varying in how much heat it puts out. Maybe it could be the thermometer, but that seems unlikely.

Any thoughts?

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Well, if you're experiencing temperature swings, then by definition your temperature is not steady, right?

I strongly doubt that heat variations from your bulb have anything to do with temp swings, particularly of the magnitude you describe.

Ordinary thermostats often have a 2 or 3 degree range in which they'll operate.

Differences in external ambient air temperature (including air you use to vent the room) or external surface (including floor/ceiling) temperatures may be affecting your grow room temp.