His stress levels are indicated by his emotional meter. When his anger has built up, the number on his emotional meter goes up.
この場合、次のような言い方ができると思います。
ーHis stress levels are indicated by his emotional meter. When his anger has built up, the number on his emotional meter goes up.
「彼のストレス度は感情メーターで表される。彼の怒りが蓄積すると、感情メーターの数字が増える」
one's stress levels「人のストレス度」
to build up「たまる・積もる」
to go up「上がる」
ご参考まで!