Monitoring Main Sensors for Temperature and Fans

Main Sensors: Introduction

Every up-to-date Macintosh computer contains a certain number of sensors that acquire physical data of the hardware, e.g. the voltage supply of specific parts, or the temperature at selected probe locations. The equipment of sensors can be very different depending on model type. Some Macs have less than 20, others more than 100 sensors. Interpreting each reading can require special technical knowledge, which is partially a trade secret of Apple. System Monitor analyzes the sensor equipment of your system and automatically determines the most important main sensors which are characteristic for the cooling behavior of the computer. The application can display readings for the following sensors:

If you like to monitor these main sensors of your computer, perform the following steps:

  1. Ensure the control window of System Monitor is open.
  2. Select the item Information Sources > Main Sensors in the sidebar.
  3. Make sure the check mark Monitor this item is set at the right side of the window.

The data shown is acquired live and is updated approximately every 5 seconds.

For all computers designed after mid–2007, the communication line between the main processor and the auxiliary processor responsible for sensor data acquirement was not designed by Apple for continuous data transfer. Transferring sensor readings is a costly operation, and consequently, it can put considerable load on the operating system. In order to avoid that recording sensor data may significantly reduce your computer’s performance, System Monitor is not designed to refresh sensor readings in time intervals shorter than 5 seconds.

Main Sensors: Menu Bar

The box Menu Bar controls in which form sensor values should be displayed. System Monitor offers a variety of icons and functions that you can combine in any order. The following elements can be used:

The preferred unit to display temperature readings can be selected with the item General Settings.

If your computer does not contain a sensor which is capable of retrieving typical CPU temperature readings, or this sensor is defective, the display — will be shown instead of a value.

Within the box, the upper bar simulates as a sample how this monitor section should be presented in the menu bar. You can grab the individual items by mouse and move them into a different order. You can remove an element by pulling it out of the bar. The lower bar contains the stock of items available. If you like to add a certain item to the menu bar, use the mouse to drag it from the lower to the upper bar. Pressing the button Default causes your current settings to be deleted, replacing it by a default suggestion for the menu bar.

Spacers are shown with dotted rectangles to make it easier for you to recognize and to move them. In the actual display, the rectangle will become invisible, so the spacer does its job providing a blank area within the menu bar.

If you mistakenly have removed all items of this monitoring section from the menu bar, a three-star-icon (⁂, asterism) will appear as a placeholder. This ensures that you still can open the associated menu.

You cannot drag items directly into the real menu bar.

Main Sensors: Menu Items

The following items will be shown for each air blower (fan) which is monitored and controlled by the hardware:

To display the history graphs for each of the fans, set a check mark at Show history graph for fan speeds within the information item Main Sensors. In default setting, the graphs are zoomed automatically, so the value range for readings acquired in the last 150 seconds will be shown in maximum detail. Alternatively, you can enable the option Use fixed scale according to fan specifications to avoid the zoom. In this case, each graph will reflect the maximum number of revolutions each individual fan has been specified for. For example, if the maximum speed of a particular fan is specified as 6,000 revolutions per minute, the bottom line of the corresponding graph will indicate 0 RPM, while the top line will indicate 6,000 RPM, and this scale is kept.

When using fixed scale mode, the graph will be a flat line most of the time because the changes in fan speed are small under normal operating conditions.