C#清除缓冲区数据（急）

switch (ls_CmdType)//如果是开阀就按此格式添加
                    {
                        case "K"://开/关阀
                            ls_str = "I" + ls_teradds.Substring(ls_teradds.Length - 4, 4)
                            + ls_TerRe
                            + isml.Substring(isml.Length - 2, 2)
                            + ls_CmdType
                            + ls_CmdTypeStr;
                            break;                                        
                        case "V"://检查温度
                            ls_str = "L" + ls_teradds.Substring(ls_teradds.Length - 4, 4)
                            + ls_TerRe
                            + isml.Substring(isml.Length - 2, 2)
                            + ls_CmdType;                         
                            break;
                        case "T"://检查时间
                            ls_str = "L" + ls_teradds.Substring(ls_teradds.Length - 4, 4)
                            + ls_TerRe
                            + isml.Substring(isml.Length - 2, 2)
                            + "&8013";                          
                            //MessageBox.Show(ls_str);
                            break;                                            
                        default:
                            ls_str = "0027";
                            break;
                    }
                    string model = protocol.Getinstance(ls_str)._Model;
                    if (!spPort.IsOpen)
                    {
                        OpenComm();
                    }
                    spPort.Write(model);//将指令写入串口缓冲区                  
                    System.Threading.Thread.Sleep(500);
                  
                    dt.Clear();
                    dt.Dispose();
                    spPort.DiscardInBuffer();//清理输入缓冲区
                    spPort.DiscardOutBuffer();//清理输出缓冲区                    
                //}
                
             }
            }
            catch (System.Exception e)
            {
                MessageBox.Show(e.ToString(), "警告");
            }        }
 void spPort_DataReceived(object sender, System.IO.Ports.SerialDataReceivedEventArgs e)
        {           
            int bytes = spPort.BytesToRead;
            // 创建字节数组
            byte[] buffer = new byte[bytes];
            // 读取缓冲区的数据到数组
            spPort.Read(buffer, 0, bytes);
            string data = Encoding.ASCII.GetString(buffer);//ByteArrayToHexString(buffer);
            isstr += data;
            isstr = isstr.Replace(" ", "");           
           
        }
当我触发检测时间按钮时，我发现缓冲区里数据还有时间的返回值，导致时间解析错误，
#B0035290&8013000000000014000137039052000000000000005D!#B0035290V000001002390CD!
#B0035290&8013000000000014000137039052000000000000005D!
这是我在逐步执行中捕捉到的数据，我触发一次命令，返回一个数据，但是导致数据在缓冲区积累，让我无法解析，怎么才能执行一次就清除缓冲区呢？c#缓冲区