The biofouling resistant coatings of hydrogel containing the cationic surfactants benzalkonium chloride and Arquad 2C-75 both extended the fouling free period in marine temperate waters. In the case of BAC the coating stayed clean for 10-12 weeks and the Arquad 2C-75 for 12-14 weeks. Due to the longer life of the hydrogel containing the Arquad 2C-75 instrument trials were carried out using this material. An effective method of attaching the coatings to the optical and membrane ports of sensors was developed and allows the coating to be either held in either a screw down or bolted polymer ring. The diffusion coefficient of cationic surfactants in seawater is reduced compared to diffusion coefficients in freshwater. In seawater the diffusion coefficient of benzalkonium chloride was found to be 2.44 x 10-6 cm2 s-1 compared to 7.78 x 10-6 cm2 s-1 in distilled water at 25°C. Careful choice of gas permeable membrane can result in a slightly longer biofouling lifetime, but only by 1-2 weeks. At 6 weeks all gas permeable membranes had significant fouling which affected their gas permeability. The diffusion rates of ammonia gas, a gas commonly measured in the sea, through PTFE gas sensor membranes varied between PTFE manufacturers with flux measurements ranging from 0.05-1131 µg cm-2 h-1. In addition to the hydrogel testing on instruments within this project a variety of external research groups and environmental agencies are currently testing the hydrogels on their instrumental ports.