Dear Biswanath.
In 1963 Bowie & Dick test was created British bacteriologists Bowie and Kelsey like test of steam penetration in porus load. From this time mostly biggest part of steam sterilizer that creating under EN 285 are included Bowie & Dick test program with parameters 134℃ for 3,5 min. If your sterilizer is not passing Bowie & Dick test you cannot sterilize of MD.
In ISO 15882 Sterilization of health care products — Chemical indicators — Guidance for selection, use and interpretation of results (IDT) (annex A) you can find a lot of demands to Bowie & Dick test.
IMHO: Maybe it will be good for you to check quality of water, heating elements of steam sterilizer, productivity of vacuum pump and rinse steam generator with citric acid. I think if heating elements of steam generator are working well and vacuum pump is working well your sterilizer will passing Bowie & Dick test correctly.

Hope it’s help.

Petr Demidov, Moscow, Russia

Dear petr Demidov,
Thanks for your answer. But, why we can not run it at 121℃?

Dear Biswanath.
This type of test are very connective with transmission of heat calories. Actually all CI for steam sterilization processes are based on accepting of heat calories. In longest time (like your 15 min) you can have a lot of false-positive results.
I was never so Bowie & Dick test 1210C -15 min., on the market.
Petr Demidov Moscow, Russia