Compare old and new values of input textbox of cellTemplate in ng-grid in AngularJS

Question

I want to compare the old value with new value of input textbox of cellTemplate in ng-grid (AngularJS).

$scope.templateArray =[
{
    field: "NP",
    displayName: "Nameplate",
    width: "*"
},
{
    field: "Qty",
    displayName: "Qty",
    width: "30px",
    cellTemplate: "<input ng-class=\"\'colt\' + col.index\" ng-input=\"COL_FIELD\" ng-model=\"COL_FIELD\"  ng-model-options=\"{updateOn: 'default blur', debounce: {default: 0, blur: 0}, allowInvalid: false}\" ng-change=\"$scope.UpdateItemQty(row)\" />",
    enableCellEdit: true
},
{
    field: "NPPos15",
    displayName: "Location",
    width: "*"
}];

I tried two ways to achieve it, one using ng-change (in this case I always get new updated value) and second using AngularJS $watch (In this case I don't know what will be the $watch name). Suppose I use *$watch name "inputQty" then where I should use this name to link it with cell template of ng-grid (I tried putting it inside input textbox as id, but it did not work).

$scope.$watch('inputQty', function (newQty, oldQty) {
        console.log('being watched oldValue:', oldValue, 'newValue:', newValue);
    }, true);

How can I achieve it in ng-grid cellTemplate?