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xliu63 · Feb 5, 2009, 06:38 PM

Hi,
I bought a Buchi Rotavapor. It is a piece of scientific equipment to get solvent out. It has a heating bath. The temperature control malfunctioned. I wonder any expert here could point me the way to troubleshoot?

The unit is supposed to set to specific temperature. On the heating element is a temperature probe, I guess the working principle should be the same as a household oven. I opened the unit up and could not see anything wrong. I checked the voltage on the two leads to the heating element, when power is switched on, the dial is set to close to off, the voltage reading is 120v, when I turn the dial to higher reading, the voltage dropped to zero. I expect the voltage increase from low voltage when the dial moves away from the home position. Just looking at the dial structure did not give me any clue.

I have to find the reason and fix it so I can use the unit to finish up an important part of a research project.

Any help is appreciated. If you need some photo graph of the unit, please drop me an email.

Thanks.

Stratmando · Feb 5, 2009, 06:50 PM

If have voltage to the element, it should draw current an an amprobe will show current, unless open.
Sounds like your saying there is voltage to element, but when temp is increased, it shuts off, almost sounds like your reading across a line voltage thermostat.
If the element is open, you shouldn't have seen voltage, unless something operates WITH the element, like an indicator?

Flying Blue Eagle · Feb 5, 2009, 07:34 PM

xliu63 - does this unit have a printed circuit board? If so you may have a shorted circ uit in it and causeing the trouble < good answer also <stratmando: have a nice day and god bless ::f.b.e.

MarkwithaK · Feb 5, 2009, 07:43 PM

Originally Posted by xliu63

Hi,
I checked the voltage on the two leads to the heating element, when power is switched on, the dial is set to close to off, the voltage reading is 120v, when I turn the dial to higher reading, the voltage dropped to zero.

Assuming the supply voltage is 120VAC then this sounds correct. When the switch is off you should be seeing that 120VAC and of course when the switch is opened you no longer have that potential because you are reading the same leg. As far as basic operation I wold say the switch is functioning but the accuracy is another issue. I would look more towards the elements themselves.

xliu63 · Feb 5, 2009, 07:45 PM

Originally Posted by Flying Blue Eagle

XLiu63 - DOES THIS UNIT HAVE A PRINTED CIRCUIT BOARD?? IF SO YOU MAY HAVE A SHORTED CIRC UIT IN IT AND CAUSEING THE TROUBLE < GOOD ANSWER ALSO <STRATMANDO: HAVE A NICE DAY AND GOD BLESS ::F.B.E.

no. It does not have printed circuit.

Here is the picture of the unit -

More Pictures

I think you can figure out how it works by looking at this picture.

Thanks.

MarkwithaK · Feb 5, 2009, 07:51 PM

Originally Posted by Stratmando

If have voltage to the element, it should draw current an an amprobe will show current, unless open.
Sounds like your saying their is voltage to element, but when temp is increased, it shuts off, almost sounds like your reading across a line voltage thermostat.
If the element is open, you shouldn't have seen voltage, unless something operates WITH the element, like an indicator?

I think one of us misread the provided info, not sure which lol. From what I read he is getting 120 across the terminals of the switch when it is off and nothing when it is on. Pretty much like a basic light switch. Maybe I misunderstood?

xliu63 · Feb 5, 2009, 08:09 PM

Originally Posted by MarkwithaK

I think one of us misread the provided info, not sure which lol. From what I read he is getting 120 across the terminals of the switch when it is off and nothing when it is on. Pretty much like a basic light switch. Maybe I misunderstood?

I think I did not spell out an important fact. The heater works, it heated water to boiling at very low setting. The dial could not set temperature.

I opened up the dial part, it is a disc, on top of it are two copper contacts

MarkwithaK · Feb 5, 2009, 08:18 PM

Is the a small tube leading out from the stat? Maybe 1/8" in diameter?

xliu63 · Feb 5, 2009, 08:54 PM

Originally Posted by MarkwithaK

Is the a small tube leading out from the stat? Maybe 1/8" in diameter?

Yes. Maybe a 1/16" thermostat wire

MarkwithaK · Feb 5, 2009, 09:01 PM

Originally Posted by xliu63

Yes. maybe a 1/16" thermostat wire

I was actually referring more to a capillary tube. Some styles of dial thermostats like this have that kind of tube as a means of sensing temperature. I guess what I am trying to ask is what is sensing the temperature on your piece of equipment?

hvac1000 · Feb 5, 2009, 09:04 PM

Replace defective thermostat. The thermostat has lost its ability to control the heating element/the contacts may be welded shut from use or they need to be burnished with a special file. I am sure the process requiring the use of this device depends upon accurate temperature control and that is why I recommended you replace the thermostat.

MarkwithaK · Feb 5, 2009, 09:07 PM

Oh sure... sum up my point in a direct way. :p

hvac1000 · Feb 5, 2009, 09:11 PM

Originally Posted by MarkwithaK

Oh sure....sum up my point in a direct way. :p

Like taking candy from a baby:D

MarkwithaK · Feb 5, 2009, 09:15 PM

Originally Posted by hvac1000

Like taking candy from a baby:D

Don't you have royalty checks to cash? ;)

hvac1000 · Feb 5, 2009, 09:47 PM

I have direct deposit since they come in now every day.

xliu63 · Feb 6, 2009, 05:58 AM

Originally Posted by MarkwithaK

I was actually referring more to a capillary tube. Some styles of dial thermostats like this have that kind of tube as a means of sensing temperature. I guess what I am trying to ask is what is sensing the temperature on your piece of equipment?

There is a temperature probe tied to the heating element. The probe has diameter close to the heating element, at end of the probe is a thin wire 1/16 inch linked to the dial.