Which of the following is NOT a method for obtaining pressure for injection of arterial fluid?

The correct answer is hydro aspirator, as it is not typically used for obtaining pressure for injection of arterial fluid. A hydro aspirator functions by utilizing water pressure to create a vacuum, which aids in the aspiration process, often in the context of removing fluids or other materials, rather than injecting the embalming fluid under pressure into the vascular system.

In contrast, hand pumps, gravity flow, and mechanical pumps are all established methods utilized for the injection of arterial fluids. Hand pumps allow the operator to manually create pressure to facilitate fluid injection. Gravity flow relies on the force of gravity to move embalming fluid from a higher container into the body, ensuring a steady flow. Mechanical pumps are designed specifically to inject fluids under controlled pressure, making them effective and efficient options for arterial injection. Thus, while all these methods pertain directly to the process of injecting arterial fluid, the hydro aspirator's primary function is not suited for this purpose.