The MASTER-53M are the successor model of the conventional model, N-50E. Measurement range becomes wider from the N-50E. MASTER-53M are a good alternative choice when the water resistant feature is not a priority and the Temperature Compensation will be carried out manually. Not equipped with the Automatic Temperature Compensation feature.
The MASTER-53M is made of metal, and the MASTER-53PM is plastic. The specifications are exactly the same. For salty and acidic samples, choose the plastic body, MASTER-53PM. 

*Verification with the 10% Sucrose Solution (RE-110010), 20% Sucrose Solution (RE-110020), 30% Sucrose Solution (RE-110030) , 40% Sucrose Solution (RE-110040) or 50% Sucrose Solution (RE-110050).