[Mtntrek]

I'd confirm there only being 1 ea. grey.
Although the SeaLevel gauges are an major improvement over the older style(s) w/probes. They are still not 100%. Garnets site does contain a manual with a troubleshooting guide. The external stick-on sensors "can" become loose & ineffective etc. I was not able to get the page to load currently. Attd. copy, you may gain some additional knowledge.

We've tended to notice that when we stay extended times in areas with very hard water the sensors give up some accuracy? Currently our Blk gauge has a sluggish delay, but we're connected to some very very hard water.

Best of luck.