Temperature Control

Temperature control is achieved using dual Peltier devices on the sides of the IRGA / sensor head. These devices heat or cool the air that is circulated through the leaf chamber. When f4 (level 2) is pressed in New Measurements mode, the following control options are presented:

Table 7‑3. Temperature control options.

Option	Description
Off	Turns off the chamber coolers.
Block Temperature	Maintains a constant block temperature. Good default option
Leaf Temperature	Maintains a constant leaf temperature.

Constant Block Temperature

Block temperature is measured in the wall of the IRGA, and is primarily used for the feedback control on the coolers. This control loop is fairly straightforward, and should be able to hold temperatures at targets that are within 7 degrees of ambient (larger if warming, since warming is more efficient than cooling).

Constant Leaf Temperature

The constant leaf temperature option is not a tight control loop, for two reasons: 1) the control of leaf temperature is indirect, via air temperature, and 2) there are factors beyond the reach of the controller that affect leaf temperature, including leaf transpiration rate and incident radiation. For these reasons, probably the best use of the leaf temperature control option is to maintain leaf temperatures at or near ambient levels in the face of changing light levels or transpiration rates.

Note: The feedback signal for this option comes from the leaf temperature thermocouple, regardless of whether that signal is actually being used for leaf temperature or not1 For example, using the leaf temperature thermocouple to measure air temperature and calculate leaf temperature from an energy balance..

Condensation

It is possible (but certainly not advisable) to specify a target temperature that will cause the coolers to bring the IRGA below the dewpoint temperature.

While in New Measurements mode, OPEN keeps track of the humidity in the IRGA and leaf chamber, and if it gets above 95%, will display a blinking warning

For a complete discussion of this message, see “High Humidity Alert”.