Final answer:
For determining the temperature of a preterm infant under a radiant heater, an axillary sensor is the best choice due to its non-invasive nature and low risk of infection, as opposed to tympanic membrane sensors or rectal thermometers.
Step-by-step explanation:
The most appropriate device for a nurse to use when determining the temperature of a preterm infant under a radiant heater is a axillary sensor. This method is non-invasive and poses the least risk of infection since it does not require contact with any mucous membranes. Furthermore, other methods such as rectal temperature measurements, whether using a mercury glass thermometer or an electronic one, carry a risk of injury and infection to the preterm infant due to the invasive nature of the procedure.
Infrared ear thermometers (tympanic membrane sensors) are an alternative that are quick, generally accurate, and reduce cross-infection risks, but the placement under a radiant heater may affect the ear temperature reading. The tympanic thermometer uses infrared radiation technology, similar to a pyrometer, which measures the emission of infrared radiation to determine temperature. This type of thermometer is used frequently in clinical settings and provides a rapid temperature readout, which is more accurate than traditional alcohol thermometers.
However, due to the specific situation of a preterm infant in a radiant warmer, accuracy and safety are of utmost importance, and thus the axillary sensor remains the preferred choice.