In order to achieve a high resolution signal, special care must be taken in the coupling of the ultrasonic transducers to the battery being measured. In non-scanning measurements, the transducers can be directly clamped to a battery with a coupling gel on the interface. For scanning measurements, the battery and transducers are usually submerged in a coupling fluid such as water or mineral oil. Bubbles and temperature fluctuations in the coupling fluid can cause variations in measurements and should be avoided. Certain battery chemistries, such as lithium metal cells, require a pressure jig for optimal cycling performance. Ultrasonic measurements can be made through such setups, provided that the plates of the pressure jig are made of a hard plastic rather than a metal since the high acoustic reflectivity of metal plates in a fluid tends to overwhelm signal from the battery.