Corning6795-620KIT PC-620D Digital Stirring Hot Plate Kit with 10" x 10" Pyroceram Top, 5 to 550 Degrees C, 120V/60Hz

I recently purchased the Corning® 6795-620KIT PC-620D Hot Plate/Stirrer for engineering use in our lab. I evaluated several brands online and decided to give this one a try. Having used it now for several days, I have some first-hand experience I'd like to share with the Amazon community. Hopefully Corning engineers will review this as well and chime in with their feedback.First impressions: Overall, the unit is well built and packed nicely in the box. It features a large ceramic top, neutral-gray cast base, and easy-to-read red LED digital displays for stirring speed and temperature. The design is sleek and modern. It looks good in the lab. The AC cord is a heavy-duty Euro-style detachable three-prong cord, which I like, as the cord is easy to replace if needed. It is also consistent with the types of cords found on test and measurement equipment. This unit operates on 120VAC and can draw over 1.1KW at full temperature.The Kit version of the PC-620D also includes a magnetic stirrer, 18" vertical support rod (it screws into the rear of the base), as well as an external probe-type temperature sensor. After comparing other models on the market, it was the external temperature sensor that sold me on this Corning model. There is a comparable Corning model in a smaller footprint that also supports this sensor, but I opted for the additional power and heating area.For those not familiar with hot plates, the external sensor is very important to maintaining reasonable temperature control of the medium. Let me explain. On most hot plates, the temperature of the LED display is the spot temperature on the underside of the ceramic (or sometimes aluminum) substrate. In the case of ceramic, there is a substantial temperature gradient from the center to the edges. I measured and confirmed this using a Fluke IR Thermometer and found the temperature to vary substantially depending on the location. Aluminum substrates are more uniform, but there will always be some differential between the edge and the center. Therefore, without an external sensor, this temperature setting is almost meaningless. What matters is the temperature of the medium that you're measuring or monitoring. Many things get in the way of thermal transfer from the hot plate to the medium, including the type of substrate and thermal mass, the contact between the substrate and the carrier (such as a Pyrex® beaker), air gaps, and absolute position of the glass beaker on the hot plate.As a result, suppose you wanted to heat your medium to 50 deg C. The hot plate thermal sensor has no idea what the medium temperature is. It may set the hot plate to 50 deg C, but the medium could be much cooler or take longer to come up to temperature. Conversely, by using an external sensor that you immerse in the medium directly, the feedback loop to the temperature controller is now a closed-loop -- with a direct reading of the medium. In theory, this would provide better temperature control of the target medium. And it does. Without this, a human would have to continually monitor and fiddle with the temperature knob to "dial in" the right temperature. This is tricky due to the long thermal lag between the time you change the dial and the change in medium temperature.To test the performance of the Corning hot plate, our test setup consisted of the hot plate, a 600 ml Pyrex® beaker (Corning Model 1000), 500 ml of filtered water, VWR Talon clamps to hold both the Corning probe sensor, a Buildera ThermaDur Class-A 4-wire RTD-type PT-100 temperature probe (316 type Stainless Steel), and an Agilent 34461A logging multimeter with trending and histogram displays. The 4-wire Kelvin connection from the Buildera ThermaDur temperature probe is important to ensure that the sensor cable doesn't introduce resistance into the measurement, which would otherwise add about 1.5 degrees of error at 100 deg C (when compared to a 2-wire measurement).In the first test, we set the target temperature to 50 deg C (starting from room temp) and set the stirring rate to 200 RPM to keep a more consistent medium temperature. We noticed that the magnetic linkage was rather weak -- and although the apparatus was well centered, it sometimes struggled to maintain control over the spinner, especially at low velocity, such as 60-100 RPM. This surprised us considering this was only water. What would happen in a more viscous fluid? How well will it stir? But that wasn't the purpose of our test, which was more focused on temperature control. That said, we found the stirrer to be less than stellar, which has been mentioned in other reviews as well. I have to agree. Possibly a stronger magnet would improve results and this is a future experiment we can try. Also, the manual does state the bottom of the carrier must be very flat. The Corning beaker is quite flat and not too thick, so it should be a reasonable test vehicle.The test results were quite interesting. The temperature of the medium gradually ramped from 18 to 50 deg C in about 30 minutes (based on the 500 ml of water in the beaker). This was with the controller set also to 50 deg C. However, the temperature overshot the target by >+5 degrees (to just over 55 deg C, then oscillated between 50-53 degrees in a decaying pattern. The period of the oscillation varied slightly, but was typically in the 15-minute range. Any thermal system will have some hystereses to turn on and off the heating element in order to maintain an average temperature. In this case, the average temperature appears to be around 51.2 degrees or so -- certainly within the specified limits of accuracy of this hot plate. Interestingly, after about 2 hours and somewhat stable temperature around 51 deg C, there was drop down to 50 deg C. It's unclear why the controller must have shut off for a longer period, but apparently it did. That said, the hot plate really cannot be used for an ultra high precision measurement as the sophistication of the closed-loop temperature controller is limited in this application.After the water cooled back to room temperature (approximately 23 deg C at this point), we then cranked the knob up to 100 deg C and turned off the stirrer altogether. In this case, we expected a different result because the maximum water temperature is 100 deg C, regardless of the hot plate temperature, because this will be the boiling point of water (or thereabouts). Indeed, we saw a fairly rapid rise in water temperature as the unit approached 100 degrees. However, it took nearly 45 minutes to fully reach 100 degrees (it was hovering just below that for a while), at which point the LED on the front panel finally stopped flashing (indicating that it reached thermal target of 100 deg C). The actual measured temperature of the water at equilibrium on the Agilent DMM was just over 100 deg C (about 100.3 degrees) which is within the accuracy of a Class A sensor, measuring device, as well as accounting for some non-purities in the filtered water (e.g. dissolved salts). We could have done this experiment with distilled water, but we chose to use high quality filtered tap water for convenience.In summary, the temperature control of the Corning hot plate using the external sensor provides an acceptable average temperature level within a few degrees of accuracy of the dial setting. This method is preferred to open-loop measurements without feedback. The magnetic stirrer seems weaker than expected and further experimentation will be required to determine whether this can be improved with a different (stronger) magnetic stirring element. The unit is overall very quiet -- just some buzz from the motor and whirling of the stirrer, but nothing unexpected or objectionable.In all, I rate the Corning PC-620D four stars and recommend it particularly when used with the external sensor. The deduction is due to the stirrer, which could be improved. The temperature regulation was good, although not spectacular, but within stated specifications. There may be some possibility for Corning engineers to improve the temperature feedback algorithm in a future version of the product. I don't know whether this is firmware upgradeable -- probably not by the consumer -- but it would be a nice bonus were that the case.

expensive & worth it. been running almost 24/7 for 8 months. ordering another one