Add notes about PID temp from cold and minimum layer time for flow calibration.

calibration.html

@@ -110,7 +110,7 @@
         <p>PID autotuning is quick and easy, and relates to the most potentially dangerous components of your 3D printer: the heaters. It makes sense to do it as a first step. This procedure is covered in this video: <a href="https://youtu.be/qCtL0Yd_w0I" target="_blank">Two easy fixes for 3D printer temperature swings</a></p>
         <iframe width="480" height="360" src="https://www.youtube.com/embed/qCtL0Yd_w0I" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
         <p>In Marlin, this is a very straightforward process using <a href="https://marlinfw.org/docs/gcode/M303.html" target="_blank">M303</a>.</p>
-        <p>In a terminal, enter the following to tune the hot end:</p>
+        <p>It is not essential, but you may prefer to start this process with the hot end at room temperature. In a terminal, enter the following to tune the hot end:</p>
         <pre>M303 E0 S200 U1</pre>
         <p>This will tune the hot end at 200 degrees. The <b>S</b> value can be altered to suit your most common printing temperature. The <b>U1</b> means the result is stored to RAM and we can save it immediately to EEPROM by sending:</p>
         <pre>M500</pre>
@@ -308,6 +308,12 @@
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+        <div class="exp">
+            <h5>Special note:</h5>
+            <p>Some other factors may affect the accuracy of the result.</p>
+            <p>Some slicers have a minimum layer time, which on a fast print like this, may slow down the feedrate significantly and alter the wall thickness. You may disable this in the slicer, but if your part cooling system is insufficient, the walls may become very hot and deform.</p>
+            <p>To overcome this, you may scale up the X and Y dimensions of the cube. As long as the file is sliced as described above, the wall thickness will not alter from this change in scale and the test will be valid.</p>
+        </div>
         <p>Now slice and print!</p>
         <h2>Interpreting Results:</h2>
         <p>Use digital/vernier callipers to measure the outer wall thickness of the hollow cube. Take measurements in multiple places/sides and average them.</p>