ErtelAlsop, the result of a merger between Ertel Engineering, and Alsop Engineering, has been producing depth filter media, plate and frame filter presses, and enclosed disk Pakâ„¢ lenticular filters since the early 1920's. ErtelAlsop is the only US based manufacturer of a complete line of liquid depth filtration products, including laboratory filters, plate and frame filter presses, membrane filter presses, cellulose filter pads, filter sheets, filter paper, Pakâ„¢ lenticular filter cartridges and lenticular depth filtration housings.