Slaving Over a Hot Laptop,PID control loops and Sous Vide cooking

///////////////////////////////////////////////////////////////////////////////
//
//  PID.cs
//
//  By Philip R. Braica (HoshiKata@aol.com, VeryMadSci@gmail.com)
//
//  Distributed under the The Code Project Open License (CPOL)
//  http://www.codeproject.com/info/cpol10.aspx
///////////////////////////////////////////////////////////////////////////////

// Using.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace SousVide
{
    /// <summary>
    /// PID.
    /// </summary>
    public class PID
    {
        double m_p = 0;
        double m_i = 0;
        double m_d = 0;
        double m_g = 0;
        double integral = 0;
        double last = 0;
        bool first = true;

        /// <summary>
        /// Setup.
        /// </summary>
        /// <param name="p"></param>
        public PID(double setPoint, double proportional, double integral, double derivative)
        {
            m_g = setPoint;
            m_p = proportional;
            m_i = integral;
            m_d = derivative;
            integral = 0;
        }

        /// <summary>
        /// Update
        /// </summary>
        /// <param name="goal"></param>
        /// <param name="sensor"></param>
        /// <returns></returns>
        public double Update(double sensor, double dt)
        {
            double error = m_g - sensor;

            integral = (0.9 * integral) + error;
            double derivative = first ? 0 : (sensor - last) / dt;
            last = sensor;
            first = false;

            return (m_p * error) + (m_i * integral) + (m_d * derivative);
        }
    }
}