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Need help making a CAPD/Dialysis bag warmer Answered

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We are doing a lot of traveling right now to a bunch of different hospitals and I need a heating device I can plug into the car or preferably run off a battery. I have a hard platt suitcase which will hold 2 bags and am toying with the idea of just insulating it and putting a heating pad in there to plug into an inverter. The problem there is I don't always have access to an outlet in the hospital, and you tend to look very much like a terrorist and scare the holy jeebus out of people strolling around the hospital with this wired suitcase thingy. I am unaware of  IED or ordinance requiring an electrical outlet, but hospital security knows all about them and are just itching to go all *observe and report* on you. 

I picked up one of those black and decker heat/cooler's at a yard sale with the intent to pop it open and see if anything can be repurposed, but I am not real confident on how to cover the elements so they don't melt the bags. 

Temperature regulation is really important, and I really don't have the foggiest idea how to best do this. It's a 4.4lb bag of fluid, so it would have to heat 2 of them without turning them into boiling lava. It also can't be so weak that it takes 12 hours to heat the bag. It would need to get to 37*C/98.6*F (or near to it in any event) in about 4 hours.

This is the real McCoy I am trying to rinky dink replicate: http://www.kidneystuff.com/warmers.html
Keeping her alive ain't cheap so I simply don't have the 400$ to hand over for it. 

Nutshell: I need help with (everything :P )
1. Heating element
2. How to best secure bags 
3. Best/most efficient way to power it
4. What material and in what fashion to insulate the case.
5. How to regulate temperature.

I'm sure I missed some things, but really any and all advice would be more than greatly appreciated. 

28 Replies

jnelson44 (author)2013-07-01

Orphicdragon-you could always carry the 4 L bags for her-you could secure them by wrapping them around your mid section-to get them warm you could use a instant hot pack( ThermaCare for example) a hand warmer pack in the same insulated bag as you put the dialysis bag.-according to my dialysis support group, you would need about 3 or 4 to get a large bag warm. Or you can heat hem by putting in the back window of your car and let the sunlight do the job. Body heat works but takes about 8 hours.

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Dr.Bill (author)2011-10-15

So how did it turn out ?
Did you use the bulb idea the PD Units suggest ?

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frollard (author)2010-11-11

Really the microwave is a great option - since you're dealing with a fluid, 'hot spots' aren't a worry. literally 2 shakes and the fluid would disperse any heat evenly. As for finding a nuker in a hospital, agreed, that might be tough.

As others state, a kapton heater element would be good because its flexible. Immersion heating is good for efficiency but is messy if it has to be portable.

The beauty of this design is you're always dealing with the exact same amount of fluid, 1 or 2 Litre IV bags if I'm not mistaken, and the delta T will be very similar each time presuming you start at a similar 'storage' room temperature. 2 4.4lb bags would be 4 litres. Heat capacity of the fluid is then known, and energy required to heat it is also a simple calculation: say it goes from 20 degrees room temperature to 37 degrees body temperature, thats a 17 degree difference C. Energy required is 1 Joule per Gram/DegreeC (I love metric) so

thats 4000 grams * 17 degrees C = 68kJ of energy. (at perfect transfer efficiency and no leakage, we'll account for that later)

Why is that useful? Because you said you want it to happen over a given amount of time, a few hours, lets say 2 hours.
turns out, 1 watt = 1 joule/second. (did I mention I love metric?)
We need to deal with 68000 joules, and we have 60*60*2 seconds (7200 seconds) to do it...68000/7200 = 9.44 watts.

Well insulated, a 10ish watt heater would do the job in about 2 hours. A simple 2 hour timer could be used to run a heater to get the bags up to temperature, and then it would be a matter of using them. YAY!

Now to design the heater: First you need to know your power source. A convenient reusable high-load power source would be either an SLA (Sealed Lead Acid) battery, or NiCad or NiMh batteries. I'd recommend D size Nimh if they exist as they have a great power/weight ratio, and good recharge lifespan.

Depending on your battery you'll determine your battery Voltage. We know the 12 volts jumps to mind as a good solution because its so common (12 volt batteries) and easy to find fixed power supplies for. Choose something different if you prefer, just adjust the calculations accordingly below:
We know we need about 10 watts. Watts are volts times amps.
thus: 10 watts = 12 * I (amps) I = 10/12 == 8.333 amps.

A smart fellow, Ohm, came up with a law to determine what resistor (heating element) we'll need. Volts = Amps * Resistance (Ohms). in this case, 12 = 8.333 * R. R = 12/8.333 = 1.44 Ohms.
If you're using nichrome or some other variable heater, just measure its resistance and find the length that equals about 1.44 ohms, and hook it up to your battery with a switch and fuse in series.


So now we know the bits we need, its time to build!

Insulate insulate insulate! Get some of that pink styrofoam stuff from a hardware store, and use it to line the case. Build the circuit and test to make sure it does what you want, and blamo, heater case!

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kill-a-watt (author)frollard2010-11-11

Good reply, but you assume too much with the 12 volts.

Most autos go up to about 14.4 volts when they are running. I'd use this as the lowest voltage in your calculations. If it's lower than this it just heats up slower.

I'd also use power resistors rather than a custom length of nichrome wire. Say 4 of them in parallel at 3 watts each minimum.

So 10 watts / 14.4 volts = 0.69 amps. (you are off by one decimal place)
14.4 volts / 0.69 amps = 20.9 ohms total

For this, use 4, 80 ohm, 3 watt resisters in parallel. Or two 40 ohm, 6 watt resisters. Or one 20 ohm resistor that's around 12 watts

Here's a 40 ohm, 10 watt resister at a single unit price point of $1.71


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kill-a-watt (author)kill-a-watt2010-11-11

Thinking a bit further, a car cigarette lighter plug will source 10 amps easy (way more than you need)

Two of the above resistors should get warm, but not hot. You should be able to get the plug, the resistors, and maybe a discarded foam shipping container (commonly used to ship diabetic supplies) all for under a sawbuck.

Know anyone with a soldering iron? This should be an easy assembly, but you will need to time it by yourself manually. I hope that's not an unrealistic expectation.

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orphicdragon (author)kill-a-watt2010-11-14

I can manage a soldering iron. Won't be pretty, but it will work. :)

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Dr.Bill (author)orphicdragon2011-10-15

PD Units say to use 60 watt bulbs. If you want you can use a 50 watt bulb with the 12 volt system. Bottom line is, you don't want it too complicated.

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kill-a-watt (author)orphicdragon2010-11-18

Here's a 40 ohm, 25 watt resister for $3.78 each


while this is rated even higher than the 10 watt one linked above, it has the advantage of mounting holes. Two of these should be easy to attach to a piece of aluminum (1/8 inch thick scrap should work).

Upon further reflection, I'll bet the device you are trying to make is maybe $20 in parts, $80 in profit, and $300 in liability insurance, just in case they get sued.

If you have someone program a $2 microprocessor to switch a 75 cent SCR to thermostatically switch on and off the heating element, don't tell them that they're making a medical device.

The lower two portable heaters on the page you linked to has a 20W (1.6A) heating element. So the device is expecting to be supplied with 12.5 volts by the regulator. To match this heating element exactly, I'd use one resistor at 7.8125 ohms, or 8 ohms should be close enough. One of these $4.63 resistors,


mounted with the Al plate, with the thermostatically switched power supply should work exactly the same. If you use this one, make sure you have the thermostat! It should heat things faster than the two resistor version above.

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kill-a-watt (author)kill-a-watt2010-11-11

OK, I finally looked at the link you provided. The plastic cases are called "pelican cases" and they're not cheap. It looks like the power resisters are mounted onto a aluminum sheetmetal piece to spread the heat (simple and a good idea). A small microcontroller probably bit-bangs the heating element on and off. There's a 110v AC adapter and a cig lighter plug, and you need the latter one only. It looks like the heater is 20 watts for the smaller ones

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frollard (author)kill-a-watt2010-11-12

Mine was thought to centre around an actual 12 volt supply. You're right a car would output 14+ volts, but I never mentioned a car other than suggesting the SLA battery :)

Everything I did was an approximation, and I didn't use significant figures because...I don't care :D I see the oops, it should be less than an amp, not 8 amps :D

Thanks for the suggestions for power resistors - convenient way to get just the right heater.

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Dr.Bill (author)frollard2011-04-22

Some hospitals use a 60 watt light bulb in an incubator kind of thing.

No need to be too high tech.

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frollard (author)frollard2010-11-11

*there are a couple assumptions I made there which may or may not affect the outcome significantly:

1: Heat capacity of your dialysis liquid . I'm presuming its very similar to water (1 joule/gram degree c). May be more or less, adjust accordingly.

2: Efficiency of your insulation. Doesn't help if the bag is leaking the heat out the other side as quickly as you're adding it - aim a bit higher for the wattage to account for this, and run the timer shorter if you go too high.

3: I do nothing about maintaining the temperature. There are no instructions for that as it makes the system significantly more complex, either engineering some intelligent analog (passive) components that adjust the heater to maintain temperature, or an active solution like a microcontroller that talks to sensors and adjusts the heater accordingly. That said, between an Arduino, a few sparkfun sensors (temperature, etc) and you've got a 30 dollar solution to the accurate heating problem.

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orphicdragon (author)frollard2010-11-14

Thanks so much for all the math and the response in general between you and kill-a-watt I think I may have a workable plan. :D

I have to actually re read and make sure I understand everything I read first...

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frollard (author)orphicdragon2010-11-14

Good deal! I hope it works out well for you! Medical tech can be gratuitous especially for 'quality of life' goodies.

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orksecurity (author)2010-11-11

As you say, temperature regulation is CRITICAL. As is electrical safety; when you're dealing with subcutaneous, the body's ability to resist electrical shock is TREMENDOUSLY reduced.

To address the latter, I would highly recommend not going anywhere near an outlet when the device is connected to a human.

For the former... Frankly, I don't feel comfortable offering advice. My personal reaction is that a proven-safe and proven-reliable solution is worth the $400. In anything less than a life-and-death solution, I wouldn't want someone's life and health relying on my handiwork.

(I've worked in a hospital's electronic repair shop. I know just how cautious they are about anything that comes in contact with a patient.)

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orphicdragon (author)orksecurity2010-11-11

The heater and her body never come into contact. This is CAPD, which isn't the same as the automated PD most are familiar with. The CAPD system is basically gravity fed. The bag is warmed then hung on an IV pole where she uses a special cap to connect her catheter. The dwelled fluid drains via gravity into an empty bag on the floor, then when full the warmed back is unclamped and she fils again. Very low tech, no plugs or machinery of any sort :)

What they advise you to do is microwave the bag. I find this extremely stupid advice given the uneven heating and the potential for REALLY hot spots in the bag. When I do microwave a bag for her I end up shaking it around for 5 minutes out of paranoid worry a hot spot will fry her peritoneum. It's also surprisingly difficult to find a microwave to use in the hospital.

Like I said it would be great to just buy the thing, but I just can't spend the money. She has so many complications that finding a donor is going to be ridiculously difficult, I am registering her in multiple hospitals in multiple states to widen the donor pool and participate in multiple paired donor programs. I can't justify $400 for what is essentially not necessary. Cramps and pain on filling sucks and I do all I can to remedy it, but her life has to come before her comfort at this point. :(

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Dr.Bill (author)orphicdragon2011-05-25

Gravity fed Peritoneal Diaylsis is not rocket science !

A 12 volt 60 watt bulb can be had from an RV store and can be powered off your cigar lighter in your car.
Temp. Regulation;
Get an indoor outdoor thermometer drill a hole and put the sensor between the bags.

If you want to use a 120 volt 60 watt bulb, get a 150 watt inverter from Sears and set it up the same way as above. Cigar lighter will power the thing ok.

Don't go over 60 watts. Hospitals suggest 60 watt bulbs.
NO MICROWAVE OVENS ! The chance of making the bags TOO HOT are real good.

And please try to Relax. This is not rocket science. I've been doing it for my wife Myrna for a long time now.

Looks like you use the Frisinous system there

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orphicdragon (author)2011-04-25

Haven't forgotten about this, just been swamped with more crap than I can handle and am only halfway through a build. >.<

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knektek (author)2010-12-11

Peltier heaters are direct drive from the battery or inverter. You could add a switch to do that and another one to turn it on and off, a potentiometer could be added to control temperature. This setup could be used with any heat source. Peltier units are not the best, but induction heaters are much better, but that would mean ALOT of heat.

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seandogue (author)2010-11-11

I'd think immersion in a heated water bath would be the best way to go, although your space requirements are a bit tough for a battery supplied device. I'd think that Kapton or silicon coated heaters could be used for heating, along with a small pump for circulating the bath fluid. Temperature controller is pretty easy, since there are many off-the-shelf thermostatic circuits that are good to 1degree or better. Use RTD sensor to measure the temperature, in cclose proximity to the bag, either taping it to the bag itself or positioning it close. Since it's immersed, the bath temp is the control...eventually equilibrium dictates the fluid temp will be the same as the bath temp.

Perhaps one of those 1 gallon portable coffee urns or a larger portable "keep it hot" containers could be modified to support the idea, but I'm not sure that the throat width would work for inserting or removing the bag(s). Unfortunately I suspect it wouldn't for most coffee urns. I'd be inclined to look at the camping industry for solutions to that aspect...

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seandogue (author)seandogue2010-11-11

One other note. an immersion bath allows for contact with nearly all of the fluid bag, speeding up (and spreading out) the thermal load.

It might be most effective to modify a beach cooler, one of those little 1 gallon jobs that you can carry a six pack in?

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orphicdragon (author)seandogue2010-11-14

While you are correct, immersion is a no no, which I should have mentioned but when you see something 4 times a day for 4 years it becomes sort of invisible.

"Peritoneal dialysis solutions may be warmed in the overpouch to 37°C (98.6°F) to enhance patient comfort. However, only dry heat (for example, heating pad) should be used. Solutions should not be heated in water due to an increased risk of infection. Microwave ovens should not be used to heat solutions because there is a potential for damage to the solution container. Moreover, microwave oven heating may potentially cause overheating and/or non-uniform heating of the solution that may result in patient injury or discomfort."

Source: http://www.drugs.com/pro/dianeal-low-calcium-peritoneal-dialysis-solution.html

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seandogue (author)orphicdragon2010-11-14

Ahh... makes sense.

I had thought of the immersion because of possible overheating with microwave or dire t contact with a heating element, but that warning about immersion makes quite a bit of sense (and I should have thought of that!).

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salec (author)2010-11-14

If it is for personal usage I'd keep it simple, using well proven and available technology: strap it near my body, by some sewn pocket "jacket" or something:

1. Heating element - human body
2. How to best secure bags - carry them close to the body
3. Best/most efficient way to power it - metabolism (food + breathing)
4. What material and in what fashion to insulate the case. - usual clothing, sweaters, coats.
5. How to regulate temperature. - metabolism (body thermo-regulation)

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orphicdragon (author)salec2010-11-14

Lol, I wish I could just strap it to her and call it even. The bags weigh 4.4lbs each and are roughly the size of a large lays potato chip bag. She is 4'11" weighs 100lbs and her stroke 3 years ago left her with permanent weakness on her right side. So eco friendly and easy yes, practical for my purposes not so much.

The visual image of her trying to walk around with it was pretty awesome though. Had a whole Peter/ Meg Griffin thing goin' on. :D

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kylara70 (author)2010-11-11

2 small bags or long socks filled with rice or beans, microwave them, then immediately pack the bags with the dialysis liquid in your suitcase. The beans will transfer warmth to the dialysis fluid over time with no hot spots. This would be something you could do in advance of a trip.

Why would you need to take the B&D heater apart? Does it get too hot? If so, you could keep the rice bags or bean bags in the heater and then just transfer them when needed.

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orphicdragon (author)kylara702010-11-11

The problem with the b&d is the shape. It's built to be a center console in the car. I probably should have mentioned that seeing as how not many folks are mind readers these days. :)

I'm thinking the socks with beans/rice would be my best bet for keeping it warm in the hospital. I could pull the bags from the heater and drop 'it in an insulated messenger bag. That would mean no mad dash or trying to remember where the heck I parked and how to get back to where I just left her.

It's hard to do things *in advance* because she needs 4-5 bags per day, so I sort of have to cycle through, pull one out, put a new "cold" one in. So with just the socks I would be set for the first two, but later in the day would probably run into issue. Still a stellar idea to keep one with me warm.

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