The HI4001-51 replacement membrane kit contains 20 loose PTFE membranes for the ammonia gas sensing ISE. When the ammonia ISE is first being assembled, or after a membrane is worn, the HI4001-51 membrane kit allows for easy assemblage or replacement. With the help of HI740159 plastic tweezers and a pair of laboratory gloves, the PTFE membrane is ensured to remain dry and undamaged for more reliable, accurate measurements. The loose PTFE membrane is placed on the outer probe body and pulled taut once the membrane cap is screwed on.