vb.net SerialPort DataReceived data encoding jumbled garbage from RS232

I have created a custom windows software in vb.net, VS 2005.  I am reading data from a Pennsylvania scale (model 7300) via my development computer's RS232 port.  I am using the SerialPort object and Handling the DataReceived event.  I first send the scale a command and then immediately use SerialPort.ReadExisting to get the weight from the scale.  Each time I click the GetWeight button I get different results AND it is garbage, like  ????, then ?/-=, then ?-=?, etc  - usually different every time.  Please advise me on how to get the weight out of the characters returned.  Here is the code.

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        ' open the serial port   
        ' I have dragged and dropped the SerialPort obj to the form
        If mySerialPort.IsOpen Then
            mySerialPort.Close()
        End If
        Try
            With mySerialPort
                .PortName = myPortName
                .BaudRate = 2400
                .Parity = 0
                .DataBits = 8
                .StopBits = IO.Ports.StopBits.One
                .Handshake = IO.Ports.Handshake.None
                .DtrEnable = True
                .RtsEnable = True
            End With
            mySerialPort.Open()
        Catch ex As Exception
            MsgBox(ex.ToString)
        End Try
    End Sub

    Private Sub cmdGetWeight_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdGetWeight.Click
        ' this command makes the scale immediately return the weight  
        Control.CheckForIllegalCrossThreadCalls = False
        mySerialPort.Write("SGW" & vbCrLf)
    End Sub

    Private Sub myDataReceived(ByVal sender As Object, ByVal e As IO.Ports.SerialDataReceivedEventArgs) Handles mySerialPort.DataReceived
        ' this fires right after the Write(sgw) thing. the app gets here fine
        Dim myScaleOutput As String = ""
        myScaleOutput = mySerialPort.ReadExisting
        ' characters are written to the textbox
        boxMultiLine.Text = myScaleOutput
    End Sub