Flow Cell Meter MP20 uses ATMEGA128

Today, I’m working on a QED Environmental Systems, Inc. Micro Purge Flow Cell Meter MP20. This particular meter is active in field work and expected to be in use very soon. I have to be somewhat careful while rendering the repair of this instrument because it contains calibration data and must pass a great deal of tests.  I can’t hack this device but i can tell you a little about the device internally. The first thing that caught my attention is that the device seems to have a rattle about it. Something inside is moving around, it almost sounds like sand. Don’t worry about this noise because it is simply two small pouches of desiccant. You’ll have to deal with the lousy pictures. Generally desiccants are also indicators. Most that I’ve seen either turn blue or red. These el-cheapo packs tend to approach a yellowish tint. Since I’m at work, and have access to some really neat toys here, I’m going to try to remove the moisture from these desiccants.  The idea here is to weigh the packs on an analytical balance and then put them in a moisture drying oven for several hours. Then they are removed and placed in a special desiccator box that is free from moisture(we don’t need any moisture going back in our experiment!). After the packs have cooled to room temperature I will once again get a weight. I will then plug both numbers in to a formula which calculates the moisture content of a sample. With any luck I will find a positive number and have good reason to believe that I had removed the moisture from the desiccant(in a not so technically correct term. “regenerate”). Off to the lab to get a weight and warm these guys up… Ok, I weighed each packet individually. Prior to weighing I marked each packet so that I could easily Identify the packets.

  1. 1.2784 gram
  2. 1.3103 gram

I placed both packets in a drying oven set to 104c with typical volume flushes/min. While my experiment bakes,  so to speak,  I’m free to inspect the rest of the MP20 and take pictures.  Before I begin taking pictures I already see a problem in the FPC. More on that later. I notice a rather large battery backed circuit inside the instrument. Three wires extend from this circuit. Namely, Black(ground), Yellow/w(data), Red/w(Vcc). I can just make out the screen print letters “RTC” under the left side of the battery. Obviously, this is real time clock circuit.

Next is the main PCB with the AVR. The name of this board is HYDROLAB, REVB. It has ab ATMEGA128, external flash memory AT29C512, an OKI M6775 lcd controller and misc analog and digital support circuits.

Back to the FPC problem I mentioned earlier. This instrument is used in a very rough environment. It gets banged around, dropped, rained on, and probably even dropped in water every now and then. To combat this the manufacturer uses o-rings in all the proper places. Where the FPC connection is sealed externally and internally in the battery compartment, and sealed inside the main board housing. The power wires are sealed in the battery compartment and inside the main board housing. I have to give them credit, they really did try to keep the water out.. but it did not work. Water worked it’s way in along the power wires. It was not much water. But it just so happens that there is a label on the FPC which also just so happens to touch the wires as they enter the main pcb area. A small leak developed, the label acted like a sponge and held a small drop of water. In time, the water permeated the FPC seal. These seals must be very porious, because this is a very common failure mode for FPC circuits. I say this is common because I see it all the time second only to mechanical fatigue of the plastic where buttons touch the circuit. I had no choice but to take a bad picture here. Any other lighting or angle would not show the water mark on the label.

Here is the other side of the FPC which shows the water damage to the actual FPC circuit trace. The circuit material always looks like this from water damage. This particular example is not in very bad shape compared FPC’s from other instruments which appear as if the entire circuit had simply burned up. I suppose in a way this is true, the circuit is probably oxidizing, ie, rusting. Which is really a very slow combustion process. I’m focusing on this area of the FPC because a button stopped working. It may very well be a mechanical failure. But I look for the easiest fix first.


The next step is to determine circuit and button configurations. I tested the FPC in circuit because the printed conductor is very thin and easily damaged with browsing probes.  The FPC connector makes a convenient lug to attach a micro grabber.

 After testing everything I’v determined that the damage is in fact mechanical in nature, and the water damage is not a major player of malfunction at this time. The next step is to slightly warm the casing and remove the button overlay and adjacent FPC sublayer. This can be a tricky process, depending on the adhesives used and the type of plastic. There are risk factors involved in this process and it would be very naive to attempt this without first consulting management. After all, the manufacturer may off to economically repair the FPC. Or they may supply a replacement panel, or circuit.  The only thing left to do is weigh back the desiccant.

The initial desiccant weights were:

  1. 1.2784 gram
  2. 1.3103 gram

The ending weight after drying in a laboratory oven for 6 hours, and resting in an intermediate desiccator for 15 mins:

  1. 1.0689 gram
  2. 1.0943 gram

If this experiment is to be trusted, both desiccants should be very close in moisture content. However, If i find that there is a large deviation then i must disregard the results.

The normalized formula for moisture content is:

[1-( mass after drying/mass before drying)]*100

I find that:

  1. [1g-(1.0689g/1.2784g)]*100 = 16.39% moisture
  2. [1g-(1.0943g/1.3103g)]*100 = 16.48% moisture

I’m satisfied with the results and there is nothing else i can do until i meet with management to discuss options.



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