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?

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.

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.

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.

If it's an Arjo you might need to drain the water tank. I'm told it's in the manual.

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.

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

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.

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.

What was the original problem you were trying to solve before the switches were bumped?

Well, as much as I hate to admit it, there wasn't a problem initially. This whole ugly scenario got started because another identical unit's automatic door was malfunctioning (still is, intermittently, but that's a whole other story) and after much troubleshooting and advice from the local tech it was suggested that swapping I/O boards from a working unit might show where the trouble lies (we didn't have any spare boards and we were more than a little hesitant about using a working washer's board - oops). That didn't solve the problem, the boards were swapped back, and almost 45 minutes later the second unit started having issues. We initially thought it had to do with removing the circuit board and checked, re-checked, and tripled checked the connectors and dip switch positions before thinking that a coincidental malfunction happened elsewhere and a little bit of bad luck was involved.
For myself personally, I see this as a huge learning opportunity since much of this work is still fairly new to me. For the others involved I don't know if they see a sunny side to this experience. To be fair to the other guys, I haven't worked on these bed pan washers until recent, but I know that the others have spent a great deal of time trying to keep them running.