There are two ways this can happen. (1) You are accidentally telling the DataMate that you have a metric (or English) system, or (2) the installation information in the DataMate is incorrect.
Case 1: When you start a survey using the DataMate, you choose an installation and then you must step through the installation parameters by pressing the Enter key. If you press the down arrow by mistake, you’ll change the value of that parameter. It seems logical that you could press the down arrow to scroll through the readings, but you can’t. Always press the Enter key to step through the parameters.
Case 2: You should check that installation information in the DataMate is correct. Switch on the DataMate. Choose Read to display the Read menu. Then choose Installations. Scroll through the list of installations and choose the one that is causing problems. Then step through the installation parameters until you see Units. Set this to English or metric, depending on the type of probe that you have. Press Enter to see the next parameter, Ins Constant. Set this to 20000 for English probes or 25000 to metric probes. Also, check your DMM database to see that probe type and instrument constant are set properly for that installation.