How to Use the Benchmark Digital Hot Plate
The goal is to achieve a stable temperature in some water and hold it for half an hour. By stable I mean that the temperature in the water does not change much if at all.
Conditions: Work was done in the kitchen for more light and rest for me as I monitored the temperatures. Temperature was 72 as the house was closed and it was cool outside. 2000 ml of water was used in Melita’s old Pyrex 9 cup coffee pot. 2 L is to the top of the band. A digital thermometer was used by hand to measure water temperature always in Centigrade. The Benchmark Digital Hot Plate was used, but not the probe. A long handled teaspoon was used to stir the water a couple times before measuring the temperature. A spiral bound notebook and a ball point pen were used to record data as it was gathered and when it was understood. A paper towel was needed to put the spoon and thermometer onto. A grounded outlet is needed. No lid was used on the pot. No probe was used. Adjustments were by hand and by logic.
The hot plate did not come much instructions nor are any available on line. Experimentation and experience were required to deduce how to use the hot plate.
The machine has an off and on switch on the back. It has one dial on the front. There is a digital read out area. There are two rows of numbers. There are little arrows in the read out area. There is a light.
The dial turns and you also push it in once to change what it does. The dial turns fast or slow. If you turn it one click stop at a time then the temperature will adjust by .5 C. If you turn the dial fast it will jump a lot or a little depending on how fast you changed it. The dial has two settings, Set or off and Actual or on.
The digital readout has two lines of numbers. The bottom one is the Set or where you set the temperature you want to use. The top one is the actual temperature achieved and is the on mode.
A little black triangular pointer jumps up and down between the two as you push the dial over and over. It is located on the right side of the readout area.
The dial also has information on the left side. Another little black triangle points to on or off, depending if the heat is on or off. It will work automatically. You can also make it change by using the knob. There is a light that is red that goes on if the heating pad is over 50 C. It stays on until the temperature drops below 50.5 C. Do not touch the hot plate if the light is on.
The machine has a heat sensor in the hot plate. When you push the dial and turn it to on or to Actual, the top row of numbers is the temperature of the ceramic hot plate. The actual temperature will show degrees and tenths of a degree. The bottom row of numbers is what you set the temperature at. It only displays whole numbers, but there are two click for every change of numbers.
When you first turn on the hot plate, the display will be what was last shown. The top row will display the current temperature, something like 18.4. The bottom row could have been set to 0.00 before turning off the device. Otherwise it will display the last number used.
The hot plate holds heat in the ceramic top. Heat goes up very fast. It cools more slowly. When it is on, the top numbers change as the plate heats or cools. Both sets of numbers will become the same at some point and they will stay that way.
After the plate is at the set temperature it will stay there. The on and off indicators in the left side of the display area will cycle up and down. Not much is needed to heat the plate or to keep it at the set temperature so the indicator is usually in the off or uppermost position.
The probe has a plug in the back of the machine. The probe will read temperatures in the pot of water. The probe is very long. It should not touch the bottom of the pot. The probe was not used for this test. Another set of experiments is needed to determine exactly how to use it and what happens.
If you want to heat water with the hot plate turn it on and spin the dial then push it to turn it on. The top number will rise rapidly. The temperature may be adjusted without pushing the dial. However, it seems that one should turn it off first. Best not confuse the sensor.
Let’s say you set the temperature at 200 C. The heat is on and rising and you see 125.6 on the top row in the display area. Your sample is getting close to your desired degree, as determined by a portable separate digital thermometer and you want to approach the desired temperature more slowly. Push the button and the heat will turn off. Push it again and it will resume heating. But even though you turn the heat off, the sample will continue to warm because of the residual heat in the ceramic hot plate. The quantity of heat in it is what you have to deal with.
When your sample reaches the desired temperature, take the sample off the hot plate and turn off the heat. This is where using the coffee pot is nice because it has a handle. Residual heat in the glass will make the temp rise a little more. The hot plate will start to cool and the top number will drop.
The hot plate must now be cooled to a maintenance temperature that will hold the desired degree in the sample. By trial and error one was determined. To hold 49.5 in the water set the dial at 92, the first half of it.
It takes a long time to cool the plate in the air if you used it to heat the water. So, use another Pyrex coffee pot that has ice water in it to cool the plate rapidly. I’d rather use hot and cold tap water first to get the desired temperature then set the maintenance temperature.
What you want to do is to balance the heat lost from the hot water sample into the air and from the open top with the higher degree setting of the hot plate. You will see the heat on off cycle. The desired temperature can be held to a tenth of a degree for half an hour or more when you get it right.
Now that’s 2 L of water in a 9 cup Pyrex coffee pot, no lid, 72 degree room temperature, dry air, 49.5 C preheated water, and a 92 C hot plate.
Those printed instructions didn’t do much for me. I had to mess around with the machine and try to figure it out myself. The things I discovered were:
A really hot setting heats water fast but it also takes a longer time to cool back down so the pad becomes useful again for the next task.
A really hot pad will continue to heat a sample after the heat is turned off.
The water to use may be made the correct temperature using the faucets before using the heating pad.
Pyrex glass holds a lot of heat, too.
The dial may be used to toggle heat on and off manually.
The pad may be cooled using a coffee pot with ice water in it.
Heat toggles automatically on and off to maintain a temperature after both numbers are the same.
Heat input must balance heat loss to keep a temperature constant.
After the pot of water reaches the desired temperature, take it off the pad and let the pad adjust to the new temperature.
Using a lid on the pot changes everything.
Water can be held at a very exact temperature for as long as you want.
The coffee pot can be cooled by spraying cold water on the outside of it. Then you need to dry it of course.
Stir the pot of water and the heat will go on briefly.
A low heat setting will raise temperature more slowly than a hot setting.
The sensor without the probe measures the pad temperature.
Temperature was determined by when the thermometer dropped a digit. It got hot, slowed down, rose a few more degrees, then dropped one. There.
Using the Benchmark Digital Hot Plate With a Probe
What changes when you plug in the probe? How well does it work?
Set up the hot plate according to the first use or tests. Put 2 L of hot water, 49.5 C, into the Pyrex 9 cup coffee pot. Set the hot plate at 92 and let it stabilize with the water.
This time, I moved from the kitchen down into the darkroom before I started. It is a little colder there but not much. Set it all up then turn off the hotplate. Plug in the probe. Turn it back on. Readjust the set temperature according to the probe reading. It will be different than the hot plate reading without the probe.
When the machine was turned on without the probe, the reading was 18.0 and 0.00. I had zeroed the set setting.
When I turned off the machine, I plugged in the probe and turned it back on. The dial now showed a reading of 14 & 0.00. Push the dial on and it became 14.5.
That is a 4.4 difference.
What to do?
Everything was set up as per the tests that were successful without the probe. The probe was used then and the set temperature was immediately readjusted to match the top reading. Heat was turned on. The top number rose then the heat turned off at 46. I then changed the set temperature of the lower number to match the top number. The two numbers became the same. Heat cycled and the water temperature became 49.5 after a few minuets.
The number the probe gives more closely matches the true water temperature. It is not exactly the same, however. But it is way better than being 40 some degrees different without the probe.
Fine adjustment made the final setting to be 45.0. Water temperature stayed at 49.3 to 49,9 for an hour with occasional stirring.