MASTER-20M is for low concentration such as juices and cutting oil etc. and is 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-20M is made of metal, and the MASTER-20PM is plastic. The specifications are exactly the same. For salty and acidic samples, choose the plastic body, MASTER-20PM. For the model with Automatic Temperature Compensation please choose MASTER-20α.