For an economical alternative consider the Water Sentinel WSK-1.
The Whirlpool WHERF is the correct replacement filter pack for the following water filter systems:
The Whirlpool WHERF Replacement Filter Pack contains the pre filters and post filters that fit Whirlpool reverse osmosis systems WHER-12 and WHER-18. Also can be used as replacements for the SEARS Kenmore UltraFilter Reverse Osmosis Series filters 42-38476, 38476. It is recommended that you replace the WHERF every 6 months based on your water quality and usage.
The Whirlpool WHERF specifications:
This is a genuine Whirlpool Under Sink Reverse Osmosis Replacement Filter. Also known as item # 67663.