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Joined: Feb 2013
Posts: 264
Potseal Offline OP
Member
I have a question regarding the possibility of a program for a piece of equipment "glitching" and losing it's accuracy or calibration. The piece of equipment is an automated hospital bedpan washer/disinfection unit that stopped working last week. The error on the display stated "disinfection not started". Whenever a typical wash cycle is attempted it will complete all the stages leading up to disinfection and then it stops and shows the aforementioned error code. We referred to the manual for troubleshooting and contacted the tech for this equipment but so far we have yet to solve the problem.
What we do know for sure is that in order for disinfection to occur the water temperature must reach 85 Celcius and that the water is in fact reaching that temperature (when measured with a digital thermometer). Herein lies the problem - the display only ever shows a maximum temperature of 67 Celcius. We've replaced the temp. sensor that connects directly to the I/O circuit board and nothing changes. So back to my original question - does it sound reasonable that the program is simply reading the wrong temp. and the I/O circuit board needs to be replaced?

Last edited by Potseal; 04/02/17 08:49 PM.

A malfunction at the junction
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Dwayne
Joined: Jul 2004
Posts: 783
L
Member
Couple of questions:

What type of temperature sensor? There are three common types. RTD, Thermocouple, and thermistor.

Resistance Temperature Detectors are basically a length of wire that changes resistance with temperature. Usually come as 2 or 3 wire versions. Very linear over a wide temperature range.

Thermocouples are two different wires that produce a voltage when one end is hot and the other end is cold. They come in several types, usually color coded, and are polarity sensitive. Fairly linear and most commonly used in process control applications like this.

Thermistors change resistance significantly however are useful over a narrow temperature band.

All three sensors require wiz bang electronics to get a useful signal out of them.

Does the temperature sensor connection have polarity or color markings on it? If so, that probably means it is set up for a thermocouple. Try swapping the leads and see if the temperature reading changes. Verify the color code on the replacement sensor wires matches the original color code.

Verify that a sensor lead has not been swapped with a shielding connection.

Good luck.

Joined: Feb 2013
Posts: 264
Potseal Offline OP
Member
The temperature sensor that connects directly to the I/O circuit board has a metal tubular end with two wires of different colour ending in a plastic connector that can only connect one way. The sensing end is inserted into a metal shaft that is apart of the heating element which is near the bottom of the water tank and includes another temperature probe that is part of a safety/temperature limiting device.


A malfunction at the junction
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Dwayne
Joined: Jul 2004
Posts: 9,673
Likes: 7
G
Member
Sounds like a thermocouple. The other two are not polarity sensitive. Is there a calibration pot or some software calibration routine? I have used a lot of digital equipment and all of it was only as accurate as the last calibration.
When I do water sampling for the state, I am required to calibrate the machine before I start and run the calibration again after. It is amazing how many times the "after" test fails.


Greg Fretwell
Joined: Mar 2004
Posts: 947
T
twh Offline
Member
If it's an Arjo you might need to drain the water tank. I'm told it's in the manual.

Joined: Jul 2004
Posts: 783
L
Member
The other thing that it MIGHT be, is poor thermal contact between the sensor and the sensor well.

Did all of the previous sensor come out?

Typically thermocouples fail either by being an open circuit or picking up stray voltage by shorting to the sensor well.

What are the colors of the two wires? Thermocouples use standard color codes.

Joined: Jul 2007
Posts: 1,335
S
Member
Correct me if am Larry or anyone, if they know the name and model of the probe, can't they ohm out the probe and calculate the temperature. At least that can narrow it down to the probe, connection or what ever is converting the probe reading to a temperature readin

Last edited by sparkyinak; 04/03/17 11:12 PM.

"Live Awesome!" - Kevin Carosa
Joined: Dec 2002
Posts: 205
G
Member
If you replaced the probe without success its probably the circuit board, but it will most likely be the A to D convertor rather than a digital issue. Have a good look at resistors around the input area onto the board. There may even be an associated calibration potentiometer which would be worth checking.

Joined: Feb 2013
Posts: 264
Potseal Offline OP
Member
First, as always I appreciate all the feedback.

Second, you are not going to believe what solved this programming issue. There are dip switches on the I/O circuit board that, naturally, have to be in certain positions. These were NOT overlooked. The switches were looked at several times but no one physically touched them because they appeared to be in the correct position. During the troubleshooting process connectors were removed and plugged back in. It must have been during one of these occasions that a switch was bumped ever so slightly out of position. It took a call from a tech in Germany to suggest that switches be physically moved back and forth to insure they were FULLY into position and that solved the problem.


A malfunction at the junction
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Dwayne
Joined: Mar 2004
Posts: 947
T
twh Offline
Member
What was the original problem you were trying to solve before the switches were bumped?

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