Air Conditioning Laboratory Project 99. 10

Testing/Re-Design

Hardware (calibration):

Once we determined that the AC unit was functioning properly we were able to proceed with the testing of the other three components of the hardware. First a sample LabVIEW program was written. It is possible to test a program in LabVIEW by using its capability to read-in sample data. Usually it would be difficult to test the sensors and the acquisition board independently of each other. Fortunately the UD Mechanical Engineering department has a number of sensors that have already been determined to operate properly. With functional sensors we were able to determine the correlation between LabVIEW channels and inputs on the acquisition board. In LabVIEW a different ‘channel’ must be used for each input to the program from the acquisition board. By specifying a ‘channel’ the user is telling LabVIEW where on the acquisition board to ‘look’ for a certain input.

With the acquisition board and the LabVIEW program being operational we were able to turn our attention toward the sensors determining if they were operational and, if so, how to calibrate them. The pressure sensors were not an issue since the manufacturer calibrated them. They have a linear output of 1-5Volts with 1V being 0psi and 5V being 500psi.

The thermocouples, however, presented a bit more of a roadblock. The only metric/restriction on thermocouples was that they be the economical and function over the entire temperature range of the air conditioner. Therefore, we originally had purchased ‘Type-T’ thermocouples since they best fit our temperature range and were quite inexpensive. In trying to test them we immediately noticed that, although we had ‘Type-T’ thermocouples, the university’s acquisition board is equipped with ‘Type-K’ modules. The problem being that, since different ‘Type’s of thermocouples are specified for different temperature ranges, the very same temperature is converted to different voltages depending on the ‘Type’ of thermocouple. Since the manufacturer was not aware of a correction factor between ‘Type-T’ and ‘Type-K’ products, a purchase was required. We determined that buying ‘Type-K’ thermocouples was a more cost and time effective option than re-quipping the acquisition board with ‘Type-T’ modules.

If only life were so simple. We come to find out that the range on ‘Type-K’ modules is 0-5Volts and 0-500degrees Celsius. The problem here is that there are points in the air conditioning cycle that the temperature of the refrigerant drops below 0degrees C. It is interesting to note that if in the LabVIEW program it is specified that the expected voltage is ­­^–1-4V instead of 0-5V then the module is ‘tricked’ into accepting different voltages than usual. And no error occurs since the module is still being asked to accept voltages within a 5Volt range. Now the temperature range on the ‘Type-K’ thermocouples is effectively ^–100-400degrees C, as opposed to 0-500degrees C.

With this problem solved it became possible to calibrate the thermocouples. This is necessary since the correlation between temperature and voltage is not precisely linear. Calibration is done by placing the thermocouples in a medium of a known temperature; namely ice water at 0degrees C and boiling water at 100degrees C. The voltage is read at these two temperatures. With two points it is now possible to make a new linear correlation between voltage and temperature. This correlation is approximately 93% accurate and is thus acceptable given our target of 90%. However it is considerably more reliable than simply taking ^–1V to be ^-100degrees C and 4V to be 400degrees C, and it is accurate certainly enough for the purposes of our labs.

The mass flow sensor has it’s own read-out and thus does not need to be run through LabVIEW. And although it is calibrated for the mass flow of Nitrogen a correction factor for refrigerant was readily attainable. It was important, though, to have the mass-flow sensor placed in a location where no phase is homogenous, since phase change introduces error into the readings. Such a location is readily determined from an understanding of the air conditioning cycle.

The relative humidity (rh) sensor has two outputs, temperature and relative humidity. The temperature was calibrated using ice water and boiling water, the same as for the thermocouples. And stream was used as the calibration medium for the relative humidity. Actually, since this sensor uses temperature as one of its tools to calculate the rh, the steam was used to check the accuracy. And with a rh near 97% was obtained using steam, it was assumed that the relative humidity sensor was now properly calibrated.

The velocity sensor is hand held with its own digital indicator and does not need to be run through LabVIEW. It also has a temperature reading. It can be placed at the inlet or exit of the condenser or evaporator and give a reading of the maximum velocity of the airflow if held perpendicular to the flow.

The wattmeter is clamped to the end of the power cord and there is a digital read-out, revealing the power drawn by the air conditioning unit.

Labs:

We had students from the ME undergraduate thermodynamics class come to our lab in groups of 4 to perform any one of 4 labs that we had written. While they performed the labs we, as instructors, made notes regarding the duration of the experiment, how much time was spent waiting, and the nature of the questions that the students asked.

After the students performed the lab we had them fill-out a survey. (See Appendix H) On this survey we asked them to evaluate our lab in several key areas, we asked:

1. 
   Did you learn/see a number of fundamentals?
   
2. 
   Were the lessons/objectives clear?
   
3. 
   Did you feel ‘real world’ connections?
   
4. 
   Did the lab keep your interest?
   
5. 
   Was the lab hands-on/interactive?
   
6. 
   Was the procedure logical?
   
7. 
   Was the lab fun?
   

After evaluating this first round of surveys we were rated with a high percentage of 4’s and 5’s and no 1’s or 2’s (for complete results see appendix E). We received a number of comments about what we had done well; but we also received a number of suggestions. The suggestions included: better labeling the sensors and components of the air conditioner; more clearly identifying the relation between the location of each sensor and where its output was reading in LabVIEW. Also, we took note of the fact that the students seemed to learn the most about the air conditioner when they were discussing its functions among themselves.

Using their suggestions and our own observations we determined that it would be beneficial to put all the experiments into a lab manual and include an introduction explaining some important aspects of the labs. We also included a complete description of all the hardware aspects of the lab (for a complete copy of our lab manual see appendix F). Upon completion of the lab manual we brought all the students into the lab for a second round of testing. Again we asked them to read the lab manual as well as run an experiment after which they were to fill out a survey. The second round survey was identical to the first round survey except that was asked for an additional evaluation: “If you participated in the previous round of experiments, please comment on whether or not you found the changes beneficial.”

The second round of surveys yielded extremely positive results. The percentages of 4’s and 5’s rose significantly and all comments were positive. And every single person who participated in the first round of experiments found our changes to be beneficial (for complete results see appendix E).