Thank you all for answers.
I did not explain thoroughly the problem.Pyrometer is optical.
During the operation of the boiler optical path of pyrometer is blocked with slag that is being created on walls of the boiler. Temperature of combustion in melting chamnber is cca. 1.500 deg.C...
We have problems with optical pyrometer lens fouling.
The boiler is wet bottom boiler.During the operation of the boiler slag deposits accumulate.
Are there any experiencies how to this problem is solved?
Thank you!