They are placed inside the breathing system and rely on a negative pressure downstream from the vaporiser to create the flow required to entrain the agent. This negative pressure is generated either by the patient’s own inspiration or by a self-inflating bag

So they must have a very low resistance to flow to avoid additional resistance to the patient’s breathing.

Goldman vaporiser, the Oxford miniature vaporiser (OMV) and the Epstein MacIntosh vaporiser (EMV) etc are draw-over vaporisers. The triservice apparatus, used by the military, incorporates two OMVs

They are simpler, lightweight, smaller and less expensive.

As it is not possible to calibrate for the large range of tidal volumes created by the patient/ self-inflating bag, they are inaccurate

So they are not generally used in hospitals, and are reserved for ‘in-the-field’ use, where portability is required.

Reference: Al-Shaikh B, Stacey S. Essentials of Anaesthetic Equipment, 2nd edn. Edinburgh: Churchill Livingstone, 2002 . Davis PD, Kenny GNC. Basic Physics and Measurement in Anaesthesia, 5th edn. Oxford: Butterworth–Heinemann, 2003 .