The Public sector units(like ONGC, GAIL(INDIA)LTD, IOCL, HPCL,NTPC...) provides a great opportunity for Instrumentation engineers. 
The reasons are as below..
1. You are working in your core area of engineering.
2.The number of vacancies on a whole are high.
3. Directly/Indirectly, you are serving the nation.
4. the packages are quite good( Considering all the facilities, they provide, its actually very good)
5.The work environment is  challenging
5.Balanced life, if you are able enough to manage..