In order to avoid leakage at the packing or seal particularly when a liquid is dangerous, toxic, or noxious, diaphragm pumps are used for metering.  Diaphragm pumps have a diaphragm through which repeated compression/decompression motion is transmitted.  The liquid does not penetrate through the diaphragm, so the liquid inside the pump is sealed off from the outside.  Such motion changes the volume of a chamber in the pump head so that liquid enters through an inlet check valve during decompression and exits through an outlet check valve during compression, in a manner similar to piston pumps.  Diaphragm pumps can also be made which discharge at fairly high pressure.  Diaphragm metering pumps are commonly hydraulically driven.
Peristaltic pumps use motor-driven rollers to roll along flexible tubing, compressing it to push forward a liquid inside.  Although peristaltic pumps can be used to meter at lower pressures, the flexible tubing is limited in the level of pressure it can withstand.