c#中涉及秒表的变量作用域

我正在visual studio中创建一个web服务。即使我在studentRead之后调用studentSave 1000毫秒，秒表返回0毫秒。我猜这和瞄准镜有关，但我看不出是什么!我做错了什么?

 public class Service1 : IService1
{
    Database db;
    Stopwatch sw;
    public Service1()
    {
        sw = new Stopwatch();
        db = new Database();
    }

    public string StudentRead(int id)
    {
        sw.Start();
        return db.getSentenceAtId(id);
    }
    public bool StudentSave(int id, int sentenceId, int acc, int speed)
    {
        sw.Stop();
        System.Diagnostics.Debug.WriteLine("ElapsedMilliseconds: " + sw.ElapsedMilliseconds); 
        return db.saveStudentResult(id, sentenceId, acc, speed);
    }
}