A Brief Study of Boiler Temperatures


PID HOME CONTACT US KITS AND PRICING ORDERING MY PID STORY eBAY LISTINGS


The graph above very clearly shows the improvement in temperature stability provided by a PID controller.

Recently, a customer ordered a brand new Silvia with the SD3 kit pre-installed. During our tuning and testing we instrumented the boiler with a temporary thermocouple right below the brew thermostat. This plot is the result of that testing.

The red line shows how the boiler temperature varies when under the control of the original brew thermostat. That thermostat allows the boiler temperature to swing from a low of 196F to a high of 239F - a 43F variation! Later, after the new SD3 kit was installed on this same brand new machine, the blue line shows the remarkably stable boiler temperature this controller is able to routinely achieve.

In addition to the obvious improvement in temperature stability, this plot shows two other interesting facts. First, on the right side of the graph, there is a sudden drop in temperature. This is the result of the standard "shot recovery" test we perform on each of our kits before it is shipped. (Normally, this test is performed on our bench Silvia).

Second, if you examine the lower left of the graph, you will see that the red line is very slightly steeper than the blue line (the steeper the line, the faster the boiler heats).

The primary reason for this difference (at least until the temperature begins to approach set point) is the very slight voltage drop in the solid state relay. We have measured this drop to be consistently 0.6V on the SSR's we use, so very little energy is wasted. In addition, our kits all use oversize wire leads to and from the SSR, to further minimize loss of energy to Silvia's heating element. So if you were worried about whether the addition of a PID will slow down Silvia's impressive heating capacity, the plot above should put your mind at ease.

In addition to holding steady temperature, which any decently tuned PID should be able to do quite easily, we tune our controllers to:

  • recover to stable control at the set point within 2 minutes
  • exhibit little (<2F) or no overshoot on recovery
This plot shows that the SD3 easily passes both of those tests.

If you are considering a different brand PID controller, be sure and satisfy yourself that it can (and will) be tuned to both hold set point and recover quickly after a shot. We believe that the Watlow controllers, and in particular the SD3, the 96, and the 93, excel in both of these important aspects of boiler temperature control. Our experience with some other brands of controllers, notably Fuji and Cal Controls, has not been quite as good.


PID HOME CONTACT US KITS AND PRICING ORDERING MY PID STORY eBAY LISTINGS

Copyright (c) 2006 MLG Properties, LLC