Why not lessen the risk of freezing damage to your filter with this Handy Thermal sleeve. One size fits the filter in any of the SP124, SP128, SP129, SP131 or SP181 models Note: This sleeve will help to prevent your filter from freezing in sub-zero temperatures. This sleeve is an aid to prevent freezing, and does not in any way guarantee that the filter will not freeze. Please follow these guidelines to prevent freezing: Take it into your sleeping bag at night time. Carry it in an inside pocket next to your body when walking. If you are using it in line on a hydration pack, take frequent sips to keep the water moving within the filter and line. If you suspect it has frozen replace it. This sleeve will also give enhanced impact protection!