178-397 Mitutoyo SJ-400 Series Standard Detector 4mNMitutoyo Surface Roughness Standard Detector Replacement The Mitutoyo Surface Roughness Tester model SJ 401 is available in 2 different models, the Standard Force model and the Low Force model. The Standard Force model uses 4mN measuring force during the measurement. The Low Force model uses 0. 75mN measuring force during the measurement. This Standard Detector for the Mitutoyo Surface Roughness Tester model SJ 401 is for the Standard Force model SJ