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Old 12-10-2013, 06:43 PM   #1
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Lightbulb Test Report: Absorption Refrigeration Protection (ARP) device

I'll apologize in advance if this sounds like a sales pitch: it is not. It's a report of what I learned while participating in a field test of a new device designed to protect RV absorption refrigerators. In doing so, I became a lot smarter about how these fridges work, and came to respect the engineering that went into designing the ARP device. So onto the report...

RV fridges made in the last 20 years claim to be fairly tolerant of being off-level and generally are spec'ed to allow up to 3 degrees side-to-side and 6 degrees front to back. That's from the perspective of the fridge, which is normally sideways in the RV, so it's 3 degrees tilt fore & aft in the RV.

Recently, though, I've been helping Paul Unmac test his invention, a patented device called the ARP that will control the heater (boiler) in an RV absorption fridge in a much narrower range than the factory control board. Paul believes this will extend the life of an RV fridge as well as preventing fires in failed cooling units. One of the test beds is a skeleton Dometic 6 cubic foot fridge set in a portable frame that can be carried around (for demos) and tilted in any direction at will. With an ARP installed and its optional data collection package connected, we can watch how the boiler reacts when the cooling unit is tilted.

The results amazed me! Merely tipping the fridge a few degrees to one side (by sliding a board under one edge), the boiler temperature immediately soared! And I do mean soared - it climbed over 100 degrees F, in less than two minutes and showed no signs at all of stopping. We quickly re-leveled the cooling unit to avoid damage - Paul didn't want to risk the unit he is using for demonstrations at RV shows this fall and winter - and the temperature began to fall again nearly as quickly. I'm sorry I failed to get a picture of the display graph as the temperature climbed, but I was literally too amazed to click the shutter!

Why does the temperature climb? Basically because the ammonia refrigerent solution stops condensing back to a liquid and flowing back into the boiler at the end of the cycle. Without liquid in the boiler, the temperature shoots up quickly, guaranteeing that the cycle won't restart on its own becasue it is now too hot to ever cool sufficiently. It's a vicious cycle that quickly gets out of control after even a brief hiccup.

RV gurus have been telling folks that their fridge is OK if they can walk around comfortably, but now I'm not so sure. According to Paul's research, high boiler temperatures cause the internal rust inhibitor (sodium chromate) to crystallize and lose it rust preventive qualities. Loss of the sodium chromate increases allows the extremely corrosive ammonia to attack the steel tubing and eventually cause a leak. And a leak is both a failed cooling unit and a fire risk. The sodium chromate crystals (they yellow stuff you see in a failed cooling unit) is also the main ingredient in a clog that can block flow through the condensor and evaporator tubing. Without laboratory testing I can't guess how much sodium chromate is lost, how quickly, and how much temperature rise is needed to cause it, but I think there is sufficient cause for concern.

Another thing I have learned by having the ARP data collection package installed on my own coach is that the boiler temperature can swing widely while driving. Mine actually was getting about 25 degrees (F.) cooler while underway at interstate speeds, and that is enough to cause the temperature in the fridge to rise (which I have observed). I can also see the boiler temperature move quickly when I pull into a rest area, slow down in traffic, or stop at the campground office to check in. The changes are immediate and dramatic. Paul says that climbing a highway grade also causes wide swings, sometimes hundreds of degrees. I surmise that wind is blowing down from the roof vent while driving ay highway speeds and that cools the whole process off. I have since added insulation around my boiler area but haven't been out on the road to see if it helps stabilize the temperature.

Paul has also measured temperatures on fridges installed in slide-outs and found then running substantially hotter than non-slide fridges. Slide-mounted fridges have the upper vent in the side and do not provide as much air flow over the coils as roof-mounted vents. He recommends adding a good sized fan at the bottom of those fridges to improve cooling unit performance as well as extending its life.

As part of the testing I arranged for about 20 others RVers to install an ARP and give feedback. All are working fine and there have been no complaints of any kind, either problems with the ARP or impact on normal fridge operation. Since then Paul has been selling the ARP at RV shows and online via his website. I don't know sales numbers, but it's into the hundreds.

There is more information on the ARP controller and absorption fridge characteristics on Paul's web site, ARPrvSafe

I have no financial interest in this product but I did receive a free controller and data collection package in exchange for my help in testing it.
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Old 12-10-2013, 06:44 PM   #2
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Arrow Questions & Answers

I've been asked a number of questions about absorption fridges and/or ARP. Here's a few of them, and the answers I have learned over the past several months:

1Q. What temperature does a fridge boiler normally operate at?
1A. It varies by brand and model, but it's about 350 F.

2Q. How does ARP manage boiler temperature?
2A. A temperature sensor on the boiler tube reports actual temperature. If the temperature rises more than about 30 degrees F., the heat source (gas or electric) is shut off until the boiler returns to its normal operating temperature. Once the temp is in range again, it is turned back on. If this cycle repeats 5 times in a short timeframe, it is assumed there is a problem, perhaps a failed cooling unit, and the boiler is not restarted. The ARP controller display shows that the fridge has been stopped.

3A. Doesn't this boiler shutdown shutdown cause the fridge to get warm inside?
3Q. Actually not. As long as the temperature is above the boiling point of the ammonia solution in the unit, the cooling cycle continues. Once the temperature drops below the boiling point, it is safe to restart the boiler. Therefore there is little or no loss of cooling in that relatively short time period. Further, the cooling unit doesn't work any better once the boiling point is exceeded, so running it hotter has no benefit at all.

4Q. Does everybody need an ARP on their fridge?
4A. I honestly don't know. I think it's a lot like asking whether everybody needs a surge protector on their shore cord. You might run for years without one and be fine, but you might also find out later that you should have had one.

5Q. Does it work on LP gas mode or electric? Are both modes at risk for overheating?
5A. The ARP sensor measures boiler heat and doesn't care what the source of the heating is. The electric and gas heaters put out roughly the same btus of heating, so either one can cause an over-temperature condition.

6Q. Why don't the fridge manufacturers do what ARP does?
6A. Good question. I suspect they feel the failure rate is low enough that they don't need to do it. They tell you not to operate off-level for long periods and maybe they figure that short periods don't cause enough damage to be a problem to them. And once the fridge is out of waranty, do they really care? Nobody on the outside knows how often RV cooling units fail, nor are there statistics on the causes. We do know that replacement cooling units is a thriving industry, though.

7Q. Does ARP take the place of the Norcold recall sensor?
7A. No, the Norcold module should be left in place to meet Norcold's requirements. However, with the ARP in place, the Norcold module should never see a temperature high enough to trigger it.
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Old 12-11-2013, 05:28 AM   #3
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Gary, Thanks for posting and your testing with Paul. As I have mentioned in other threads, it's important for people to understand exactly what the ARP is and isn't.

It is a much more precise monitoring and control method from several different key areas of the cooling unit. It is a more finite control on units being operated too far out of level. It is a much needed, and quite frankly should be OEM on any refer from any mobile absorption type refer manufacturer. With that being said, people also need to understand that, just like the little black box recall (OTS) installations, the ARP is not the be-all end-all for potential catastrophic failure.

For example, I install many recall kits on customer's units and after speaking with them many get the warm-n-fuzzy feeling that all is well, even if they operate the unit way out of level, neglect routine maintenance such as burner inspection. I even recommend that when they return in the Spring for de-winterization that they allow us to pull the refrigerator out and visually inspect the tubing for damage or rust, insure the fans (if equipped) on the condenser are operating properly, and removal of the insulation around the boiler area to visually inspect for any signs of chromate seepage or excessive rust. On any unit it takes roughly only 45 minutes for us to pull one out perform the inspection and re-install it. Amazingly, the majority of people refuse the service as they feel that the simple idea of the OTS recall kit solves all their problems. Sad .

As you and I well know, with the increase in temp comes the increase in pressure. Additionally, we should all realize that there is an equal probability of a catastrophic release of refrigerant that is not heat related such as a failure in a weld, stress crack or rust-thru. The ARP will not detect this immediately. A pressure switch installed within the system will.

As we discussed in another thread, it is entirely possible that even a pressure switch would not 100% prevent ignition of the refrigerant as there could still possibly be enough heat present to ignite it even if the pressure switch shuts down the boiler immediately. However, just like with the ARP, it will remove the current heating source (LPG or 120VAC heating elements) from the equation immediately - and that is most important.

I am still absolutely baffled why none of the manufacturers have incorporated a much more precise control of temps such as offered by the ARP and included pressure switch safety systems.

While neither the ARP or a pressure switch (alone or in combination) will prevent all possible situations of catastrophic failure they will most certainly help to eliminate many more than the OEM manufacturers are protecting against currently.




P.S - when the ARP unit hits the market for public purchase, I will most certainly entertain installing one on mine
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Old 12-11-2013, 06:01 AM   #4
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Gary, is there any indication (other than the 5 cycle shutdown) that the ARP has triggered?
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Old 12-11-2013, 06:14 AM   #5
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Thumb - great info guys. Good show.
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Old 12-11-2013, 06:31 AM   #6
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Great info. This could be huge. Manufacturers have been telling us for quite some time if the RV feels level, that is close enough for the fridge.

The price isn't bad considering what the ARP does. If a manufacturer decided to license the device and use it on all their products, the cost could be even lower with increased volume. I hope Paul has applied for a patent.
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Old 12-11-2013, 08:44 AM   #7
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Quote:
Gary, is there any indication (other than the 5 cycle shutdown) that the ARP has triggered?
First, the status light changes to indicate that a shutdown has occurred. In addition, ARP keeps an internal log of the over-temp events and the event count is displayable on its little LCD panel. The event count is available at any time, but you have to be proactive and check it occasionally if you want to see if an over-temp condition has happened in the past without causing a complete shutdown. You can also display the other values that ARP has observed or is using to manage the boiler temp, e.g. current boiler temp. There is an ARP user manual on the ARPrvSafe website - you can read the details there.
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Old 12-11-2013, 08:50 AM   #8
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The price isn't bad considering what the ARP does. If a manufacturer decided to license the device and use it on all their products, the cost could be even lower with increased volume. I hope Paul has applied for a patent.
US Patent #8,056,360 has been granted.

I think the price is reasonable for an add-on device. But as you say, the fridge manufacturers could implement the same function in their control circuitry with the simple addition of a decent quality temperature sensor on the boiler tube. They already have the controller and a display panel on the front of the fridge, so the cost increment would be quite small. However, I know Paul has approached them and neither Norcold nor Dometic have shown any interest in implementing the ARP concept.
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Old 12-11-2013, 08:53 AM   #9
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P.S - when the ARP unit hits the market for public purchase, I will most certainly entertain installing one on mine
An ARP can be ordered from the website today. The list price is shown as $120 but a promotional price of $85 is in effect at this time.
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Old 12-11-2013, 09:10 AM   #10
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As you and I well know, with the increase in temp comes the increase in pressure. Additionally, we should all realize that there is an equal probability of a catastrophic release of refrigerant that is not heat related such as a failure in a weld, stress crack or rust-thru. The ARP will not detect this immediately. A pressure switch installed within the system will.
I don't know the probability of the various possible causes of cooling unit failure, so can't judge if they are "equal" or not. However, I agree that temperature and pressure go hand in hand.

It seems to me that a drop in pressure, resulting from a mechanical leak, is going to disrupt the basic boil & condense cycle such that liquid ammonia no longer returns to the boiler reservoir. As soon as that begins to happen, the boiler temperature shoots up dramatically, as evidenced by the test data obtained from ARP test installations. I think the ARP temperature management algorithm would respond quickly, though perhaps not as quickly as a direct pressure sensor might. The evidence that I have seen so far shows that boiler temperature is a symptom as well as a cause, i.e. an increase in boiler temperature accompanies other changes. For example, blocking the air flow over the cooling unit tubing also causes the boiler temp to rise quickly and the ARP reacts to that.

I don't think Paul Unmack would ever claim that an ARP can detect or prevent all possible failure modes. I certainly would not, but I'm not his agent and cannot speak for him.
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Old 12-11-2013, 09:15 AM   #11
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Originally Posted by Gary RVRoamer View Post
An ARP can be ordered from the website today. The list price is shown as $120 but a promotional price of $85 is in effect at this time.
If I followed the level/un-level narrative correctly, it would appear it would be prudent to turn off an absorption reefer while traveling up/down inclines (hills, bridges, etc.).

If I still had an absorption reefer I'd order an ARP in a heartbeat!!
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Old 12-11-2013, 09:39 AM   #12
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Gary, thanks for the info and links.
After all the fire/recall, re-recall, problems with fridges. This product, particularly at this price, is a must have.
Shame on the manufacturers for not upgrading to this level of protection.
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Old 12-11-2013, 10:03 AM   #13
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How easy is it to install? Can a moderately mechanically inclined individual do it?

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Old 12-11-2013, 06:05 PM   #14
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If you can do basic 12v wiring, it is usually pretty easy. The only potential wrinkle is getting access to the boiler output tube to mount the temperature sensor. In most fridges it's not difficult, while others can require some effort. It can vary with the RV as well. Mostly you can get at the upper boiler area quite easily from the outside access panel, but in a few the fridge is offset enough to make it awkward to work.

On the ARP website there are five (5) photo essays showing the install on different fridge models, plus there are detailed diagrams and instructions in the ARP User Manual, also on the website.

ARP SAFETY DEVICE
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