STA Compact Cooling System
Tutorial on how the STA Compact cools the product drawer and heats the measurement assembly.
Transcription of STA Compact Cooling System
Welcome back to Bio Med Buddy. Today we're going to discuss how the STA Compact cools a product drawer, and heats the measurement assembly. The product drawer temperature is maintained at 16 degrees C, plus one per service manual. The measurement assembly is maintained at a temperature of 37 degrees, plus or minus .5. That is the field service engineer's spec. The factory spec is .2. I will mention the head three temperature is also maintained at 42, plus or minus .5. We'll talk about that at a later time.
The cooling system is consisting of Peltier devices, heat sinks, some foil, help dissipate the heat, some spacers, springs, screws to hold the Peltiers in place, to assure they are not damaged by over pressure. A PC board on the product drawer and the measurement assembly. A temperature regulation board. If you open the front of the instrument it's on the right hand side. A cooling pump, cooling reservoir, and a radiator with a fan attached to the back. Peltiers used for the product drawer and the measurement assembly are the same. A Peltier device heats on one side, cools on the other. Glycol, or water, starts out from the reservoir bottle, goes through a stainless steel tube out to the cooling pump. From the cooling pump it goes to the measurement block to the two Peltiers there, then out through the cable chain, out to the product drawer, where it goes through four Peltier devices.
Then out to the radiator which has a fan on it to help dissipate the heat. Back to the clean reservoir. You'll be able to see that return. If you look closely at the top of a bottle, you usually can see it. I will tell you at this point, there should be no air bubbles in that line. Air bubbles are an indication of a leak. Common spots for leaks are, in back of the cooling pump there is a fan. Sometimes it rubs on the tubing and causes a leak there. Second place is underneath the product drawer. (Slide Change 2:22) Probably number one. The tubing gets old, and it breaks there for some reason. Third common
place is in the table chain itself. I will tell you that depending on the position of the drawer, sometimes that's intermittent, and sometimes it's not. In other words, you may not see the leak if you open up the product drawer, but it may be leaking inside the instrument. Generally, you can see a leak. Take the cooling pump out of the back of the instrument, take out the product drawer, you're probably going to see your leak.
Second common problem is the cooling pump. The diaphragm tears. If you lift up the top of the diaphragm, underneath you'll see that it will go halfway, it will be torn almost in half. Common, very common. That's changed on the PMs at least once a year for that reason. As a way of checking the cooling pump, you can take a piece of tubing, put it on one side, run it through the bottle; meaning, you just take the bottle off, take it right in the back, and put the other piece of tubing to the other side. It's a quick way of checking whether the cooling pump is working. (Slide Change 3:27)
It's just as easy to just take the cooling pump and take it apart and check and see if the inner diaphragm is torn. Rebuilding the cooling pump, there are four Philip head screws that hold the pump assembly together. Those screws are usually provided in the kit.
There is an orientation. Little marks on the pump head. (Slide Change 3:48) These marks should all line up facing the rear of the motor. (Slide Change 3:54) It's pretty hard to put these together wrong, if you take just a little care. There is a gasket, I generally do not replace that gasket. You would have to take a razor blade and scrape that off, being careful not to damage the pad. Generally I never really see any damage to it, so I've not seen the need in my experience. Then you unscrew the diaphragm, careful not to lose any of the washer underneath it. When replacing that diaphragm, make sure you tighten it in there. Do not use any type of lubricant on any of the O rings, or on the lock tight on diaphragm.
I will mention that there is a white plastic nut that they say not to turn because it has factory settings. I would tell you that yeah, I don't turn it, I don't see the need; however, if you really want to get increased pressure and flow through it, you can turn it. I'll tell you, if you go that route, one of the problems is, you'll over turn it, and your pump will stop working. I would stay away from the white plastic nut. (Slide Change 5:00) Let's talk about the electronics a little. There is a board on both the product drawer and the measurement assembly. On that board you'll see that the Peltiers solder on to. They are supplied with 24 volts DC. They are simple series loops. If you have your 24 volts for both the product drawer, and another 24 volts for the measurement assembly, your power supply is good for them. I have to say, generally there that's not your problem area, it's the Peltiers device themselves. You do not have to change all four of them. You can actually go down and change the one that is defective.
Bio Med Buddy will be providing these on an individual basis. Again, the 24 volts is supplied, and you have the Peltiers in series. Essentially, the Peltiers take 12 volts. There's 12 volts across each Peltier. Pretty straightforward, simple. Nothing complex here. The control for both the measurement block and the reagent block is from the temperature regulation board. Again, open the front of the instrument and you'll see it on the right hand corner. Please note that the adjustment for your measurement assembly is P2, and your adjustment for your product drawer is P1. You will see a yellow LED.
That is for your product drawer, and you'll see a green LED. That is for your measurement assembly. If they flash, that basically means that they're at a stable spot. A common, it's pretty straightforward for me to troubleshoot a temperature issue, which, by the way, they can be somewhat intermittent at times, is to check the flow.
Number one; you have to have flow. Then check to see if the yellow LED is on constantly. If you have flow, the yellow LED is on constantly, I can almost guarantee you've got bad Peltiers, or a bad Peltier. The measurement block is very stable. If you have a problem with the measurement block temperature, now that would be unusual. It's the reagent block that causes your problem. It does not require that the cooling bottle be filled. In fact, there can be awful little in it and still work just fine. The measurement block generally is not affected by how much cooling reagent you have in the system, just the reagent block. You can fill that reagent bottle all the way to the top. It's a sealed system, although some does evaporate from it. A common problem also with this system is that the operator will allow the cooling reservoir to become extremely low, and essentially empty. Sometimes you have to purge that pump. Essentially if you have to purge the pump, the only reason to do that is if the reservoir essentially got empty, and you got a lot of air in the lines.
The reason I mention that is because if you do it once, and it corrects your issue, you are stating to yourself or the operator that they did not put solution into the bottle. It's common for you to do that. It works for a while, and you don't have the proper flow. There are specs for that flow, and I would tell you that if you're getting some flow, probably OK. It takes very little flow for that system to work, but there are specs for it. Additional comment on the Peltier devices. I have taken them out without any heat sink on them, put my fingers across them, and be able to tell the hot and cold side of them. I have heard people say that you have to have a way of dissipating the heat or you'll cause damage to the Peltier devices. I have not found that to be the case personally. I will tell you in fact, that I have seen many instruments, the Peltier devices not work for a long period of time, and to cause no known problems. I have seen cooling pumps fail and not be repaired by the customer for over a year, and have no problems once you repair the pump, of all the Peltiers coming back and work, for what it's worth. There are temperature probes, they're misters. I believe they're LM35s. I can look that up later, very seldom do they give you a problem. Again, it's back to mainly on the reagent block. When they do, what I've seen is the reagent block is very cold, as cold as the instrument will get it, and yet it's calling for, saying that it's not down to temperature. That's a very rare problem.