From beer brewing to vaccine stability: Adding a digital temperature controller to your vaccine refrigerator

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Praxis

From beer brewing to vaccine stability

Adding a digital temperature controller to your vaccine refrigerator

Peter Hutten-Czapski

Your practice nurse walks by with a cooler full of vaccines that she is returning to public health. “The refrigerator’s temperature went out of range last night ... again.” You adjust the temperature on the refrigerator but the ther- mostat just does not allow for fne enough adjustments.

Stability data demonstrate that freezing (<0°C) most refrigerated vaccines destroys them.¹ Additionally, while all vaccines cumulatively lose potency with time, the loss in potency is accelerated in a vaccine-specifc man- ner when they are stored above the recommended tem- perature.¹ Thus, frozen vaccine is usually discarded, and vaccines that have been exposed to temperatures above recommended ranges should be marked and, depending on the particular preparation and the extent of exposure, used before other stock.

For these reasons suppliers use purpose-built vaccine refrigerators to keep stock between 2°C and 8°C. Some pharmaceutical-grade refrigerators² have features such as digital temperature control, forced air circulation, and alarms, with a rated temperature stability of ± 1°C. Such refrigerators, however, are expensive.

Home beer makers also have strict temperature re- quirements for their products. After watching a YouTube video on how to wire a digital temperature controller added to a domestic refrigerator for brewing use,³ it seemed reasonable to add a digital temperature control- ler to a refrigerator used for vaccine storage.

Materials

The following materials are needed:

• 4-ft³ cooler-only refrigerator (about $200);

• 3/16-inch flat screwdriver;

• 14-gauge power cable;

• STC-1000 digital temperature controller (purchased online for about $20);

• plastic or metal enclosure (about $8);

• drill and file;

• wire strippers;

• soldering iron and tin; and

• digital maximum-minimum thermometer (about $20) to measure vaccine temperature with the probe sub- merged in a vial of glycol, as indicated by the Centers for Disease Control and Prevention.⁴

Considerations

Put your refrigerator on bypass. Using the temperature controller as a bypass requires setting up the controller

to intermittently power an electrical plug for a domes- tic refrigerator when the air probe temperature is higher than the set point. The refrigerator in turn is set to run at its coldest setting and is plugged into the controlled plug.

We took an enclosure (in this case a recycled com- puter power supply), gutted its original contents, and cut a rectangular hole to receive the controller. Then wiring was attached to the controller from the incoming power cord to a switched plug (Figures 1 and 2).

The calibration of the temperature probes was tested against ice water as a reference.

The digital thermometer probe was run past the gas- ket at the top of the refrigerator door and placed in the centre of the refrigerator measuring the air temperature.

The fnished assembled bypass is shown in Figure 3.

Figure 1. The wiring of a bypass temperature controller

Figure 2. Schematic and pinout of a bypass temperature controller

Enclosure wiring and controller pinout

110-volt AC U

Chassis ground

Power cord to refrigerator STC-1000

1 2

3 4 8 7

Probe

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Figure 3. Assembled bypass controller

Optional improvements. We put a standard 12-volt computer case fan (about $20) inside the refrigerator to reduce temperature variation. We found that the vari- ation between the bottom and top of the refrigerator decreased from 5°C to 2.5°C with fan operation at a cost of wider temperature swings with door-opening events.

We use water bottles in nonusable high- and low- temperature areas of the refrigerator (crisper, door, and top and bottom shelves) to add thermal ballast. The water bottles displace 10% of the rated refrigerator vol- ume and slow internal heating to 0.04°C/min when the refrigerator turns off owing to power failures; this extends the time that the refrigerator keeps vaccines under 8°C during power failures to more than an hour (about $4 for 10 L of water bottles).

Clinic’s experience with the technique and settings.

We keep our temperature controller set to 3.8°C (air) with cooling cycling after a 1°C change. Over a month’s time, this kept the vaccine thermometer between 3.5°C and 7.2°C. (With the refrigerator’s original mechani- cal thermostat, the previous month’s vial temperature ranged between 2.4°C and 7.7°C.)

We had not experienced any cold-chain failures from the time of installation of the digital temperature con- troller to the time of manuscript submission to Canadian Family Physician.

Conclusion

Adding a digital thermometer controller to your refrigera- tor is a practical approach to controlling temperature vari- ations in domestic refrigerators, potentially to the level of pharmaceutical-grade tolerances. Those who are uncom- fortable with their wiring skills can consider prewired digi- tal bypass controllers that are available (about $170).

Dr Hutten-Czapski is a practising rural physician in Haileybury, Ont.

Acknowledgment

I thank Dr David Page for his input throughout the study and for reviewing the manuscript draft.

Competing interests None declared References

1. World Health Organization. Temperature sensitivity of vaccines. Geneva, Switz:

World Health Organization; 2006. Available from: http://apps.who.int/iris/

bitstream/10665/69387/1/WHO_IVB_06.10_eng.pdf. Accessed 2017 May 8.

2. 360 Medical [website]. Products: 2~8°C pharmacy refrigerators, model: HYC- 610 2015. Schomberg, ON: 360 Medical. Available from: http://360medical.

ca/pharmacy-refrigerators-HYC-610.html. Accessed 2017 May 8.

3. How to wire the STC 1000 temperature controller [video]. YouTube; 2012. Available from: www.youtube.com/watch?v=30TvX1Zz1-Y. Accessed 2017 May 8.

4. Beckenhaupt P. Guidelines for storage and temperature monitoring of refriger- ated vaccines. Atlanta, GA: Centers for Disease Control and Prevention; 2012.

Available from: www.cdc.gov/vaccines/hcp/admin/storage/toolkit/tool kit-resources.pdf. Accessed 2017 May 8.

We encourage readers to share some of their practice experience:

the neat little tricks that solve diffcult clinical situations. Praxis articles can be submitted online at http://mc.manuscriptcentral.

com/cfp or through the CFP website (www.cfp.ca) under “Authors and Reviewers.”