The setpoint will have an error rate in the heating direction and the cooling direction. For example, a value in the range of +1.8 to -1.8 will be read for a two percent margin of error at 90 degrees. This value range is called the switching range. If the switching range is added to the temperature value, the upper limit is found. If it is removed, the lower limit is found. If the temperature in the tank is lower than the lower limit at 150 C, draw the ladder diagram that operates the heater and stops the heater when it reaches or exceeds the upper limit.