(Links)
Chemical Sensor Suppliers
Transducer Technology - electrochemical gas sensors
Figaro Engineering - primarily metal oxide sensors and KE oxygen sensors
City Technology, Inc. - wide variety of electrochemical gas sensors
Mine Safety Appliances
Applied Sensor, Inc - sensor modules
Chemical Sensor Information
Compendium of sensor information from Sensors Research Consulting
MSA Gas Detection Handbook (free textbook)
A slideshow illustrating several sensor types and modes of operation in superficial form.