I find these thermocouples very interesting
This looks similar; http://contentinfo.autozone.com/znetcs/product-info/en/US/atm/5248/image/3/
Dunno if the link will break, Auto Meter Pyrometer probe 5248
Ran across these, thought they maybe good for you all that monitor grate temps. https://www.ebay.com/itm/8-5-High-Temperature-K-Thermocouple-Sensor-Ceramic-Kiln-Furnace-2372F-1300C/232595700329?rt=nc&_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D2%26asc%3D49918%26meid%3Da29e7cae8943494b96f6b1941a336ab0%26pid%3D100005%26rk%3D5%26rkt%3D6%26sd%3D311991180006%26itm%3D232595700329&_trksid=p2047675.c100005.m1851 https://www.ebay.com/itm/High-Temperature-K-Type-Thermocouple-Sensor-for-Ceramic-Kiln-Furnace-2372-1300-/352168951729?_trksid=p2349526.m4383.l4275.c10
The main problem I see here is they are not airtight, and the ceramic might not hold up in the grate area.
Good price, might last good enough too get an idea of what your temps are on bottom end, and compare too top end readings. And put inside a pipe too keep from being damaged from grate stiring rods ECT.NEVER knew they made them ceramic.! Unless there not air tite or smoke tight. As Carl mentioned.