Print material level sensing

A print material container comprising:

• a chamber to hold a volume of print material;
• a series of print material level sensing devices disposed at intervals to detect a presence of the print material at successive depth zones in the chamber, wherein each print material level sensing device includes a heater to emit heat at its depth zone and a sensor to sense heat at the depth zone and to output a signal based on the heat sensed;
• and
• control circuitry to, for each print material level sensing device to be calibrated, turn on the heater for an initial time duration set by an initial heat count and iteratively adjust the time duration for which the heater is turned on in accordance with an adjusted heat count, until the signal output from the sensor indicates that a target value has been reached in that depth zone.

A method, comprising calibrating print material level sensing devices disposed at successive depth zones in a container holding a volume of print material, wherein the calibration includes, for each print material level sensing device to be calibrated:

• turning on, for an initial time duration, a heater of the print material level sensing device to emit heat at the depth zone of that print material level sensing device;
• sensing heat received by a thermal sensor at the depth zone of that print material level sensing device;
• and
• iteratively adjusting the time duration for which the heater is turned on until a signal output by the thermal sensor indicates that a target value has been reached in that depth zone.