NISTIR 7900

Thermal Analysis of a Small 

Pharmaceutical Refrigerator for 

Vaccine Storage

Michal Chojnacky

Wyatt Miller

Gregory Strouse

http://dx.doi.org/10.6028/NIST.IR.7900
NISTIR 7900

Thermal Analysis of a Small 

Pharmaceutical Refrigerator for 

Vaccine Storage

Michal Chojnacky

Wyatt Miller

Gregory Strouse

Sensor Science Division

Physical Measurement Laboratory

http://dx.doi.org/10.6028/NIST.IR.7900

November 2012

U.S. Department of Commerce 

Rebecca Blank, Acting Secretary

National Institute of Standards and Technology 

Patrick D. Gallagher, Under Secretary of Commerce for Standards and Technology and Director 
1

ABSTRACT

Each year, billions of dollars of vaccines are stored in refrigerators at the facilities of a variety of 

medical providers. Many vaccines must be maintained in the range 2 °C to 8 °C to retain product 

potency. As an extension to our study of household and pharmaceutical-grade refrigerator 

models used for this purpose [1,2], we have evaluated the performance of a small, under-thecounter pharmaceutical refrigerator model for vaccine storage use. The test unit [capacity =

0.138 m3 (4.9 cu ft)] proved fully adequate at maintaining vial temperatures within the desired

2 °C to 8 °C range, independent of how the refrigerator was loaded and under variable use and 

environmental conditions.

1. EXPERIMENTAL METHOD

In this study, we have evaluated the performance of a small, under-the-counter pharmaceutical 

refrigerator for vaccine storage use. This study supplements our previous refrigerator studies of 

dormitory-style, freezer-less, dual-zone and large pharmaceutical grade refrigerator units [1,2]. 

We have followed the same approach as in our preceding studies, installing 19 NIST-calibrated, 

Teflon-insulated, Type T thermocouples in various locations inside the refrigerator to determine 

the spatial temperature distribution throughout the unit’s interior. As before, thermocouples were 

attached to refrigerator walls or storage trays, hung in the air, mounted inside of glycol-filled 

bottles, and taped to actual vaccine vials. We also placed five digital data logging thermometers 

in various locations inside the refrigerator. The thermocouple and data logger installation pattern 

is shown in Fig. 1 below. Table 1 summarizes the equipment used for this study.

Fig. 1. Thermocouple and data logger installation pattern.
2

Table 1. Equipment used for the study.

Equipment name: Specifications:

Under-the-counter

Pharmaceutical Refrigerator

 Fully automatic cycle defrost refrigerator (115 V)

 Height: 0.508 m

 Width: 0.508 m

 Depth: 0.533 m

 Capacity: 0.138 m3 (4.9 cu ft)

 Positive, forced air circulation

 Electronic control system and digital thermometer

 Factory pre-set temperature set point (4 °C)

 Out-of-range temperature alarm

Plastic trays  Stacking plastic storage trays/ bins

19 thermocouples  Type T (copper-constantan)

 0.5 mm diameter, 1.83 m length

 Teflon insulation

 Calibrated at 0 °C

 Uncertainty (k=2): ± 0.12 °C

Data logger A  Rugged high-temperature logger

 Metal housing with integral 2.6 cm rigid probe

 Temperature range: –40 °C to +150 °C

 Temperature resolution: 0.05 °C

 Calibrated accuracy: ± 0.5 °C (from manufacturer)

 Memory: 32,767 readings

 Battery life: 1 year

 Body dimensions: 12.2 cm x 2.6 cm dia.

 Probe dimensions: 2.6 cm x 0.5 cm dia.

Data logger C  Plastic-housing temperature and humidity logger

 Temperature range: –35 °C to 80 °C

 Temperature resolution: 0.5 °C

 Accuracy: ± 0.3 °C (manufacturer rating)

 Memory: 16,328 readings

 Battery life: 1 year

 Nominal body dimensions: 10.2 cm x 2.3 cm max dia.

Data logger D  Data logger with flexible temperature probe

 Temperature sensors in readout unit and in a flexible 

probe

 Temperature range (readout): –40 °C to 85 °C

 Temperature range (flexible probe): –15 °C to 105 °C

 Temperature resolution: 0.1 °C

 Accuracy: ± 0.5 °C (manufacturer rating)

 Memory: 2000 readings per channel

 Battery life: 2 years

 Nominal body dimensions: 7.6 cm x 7.5 cm x 2.3 cm

Data logger E1, E2  Data logger with two flexible temperature probes

 Temperature sensors in flexible probes

 Temperature range: –15 °C to 105 °C
3

 Temperature resolution: 0.1 °C

 Accuracy: ± 0.5 °C (manufacturer rating)

 Memory: 2000 readings per channel

 Battery life: 2 years

 Nominal body dimensions: 7.6 cm x 7.5 cm x 2.3 cm

Data logger F  Wireless data recorder with external data logger

unit/temperature sensor

 Data logger features flexible temperature probe and 

readout unit

 Temperature range: –60 °C to 155 °C

 Temperature resolution: 0.1 °C

 Accuracy: ± 0.3 °C from -20 °C to 80 °C (mfc rating)

 Memory: 16,000 readings

 Battery life: 6 months

 Nominal body dimensions: 6.2 cm x 4.7 cm x 1.9 cm

 Wireless data transmission distance: Up to 100 Meters

(Line of Sight and No Obstructions)

Thermocouple measurement

system

 6.5 digit multimeter with scanner

Following data logger and thermocouple installation, the small pharmaceutical refrigerator was 

subjected to a number of use patterns designed to replicate the range of vaccine storage 

conditions that regularly occur in Vaccines for Children (VFC) provider offices. A complete 

listing of the different trials and order in which they were performed is contained in the 

measurement matrix (Table 2) below. In our previous studies, we found changing the vaccine 

storage method (e.g. plastic trays vs. cardboard boxes, or a mix of both) has minimal or 

indeterminate impact on a refrigerator’s ability to maintain vaccine storage temperatures. As a 

result, we have eliminated packing method variation trials from this study.

Table 2. Matrix of measurements performed for the study.

Trial Load Density

Low Medium High

Measurement Parameters

1 x Normal

2 x Normal

3 x Normal

4 x Normal

5 x increase room temp

6 x periodic door opening

7 x power off

8 x Normal

9 x increase room temp

10 x periodic door opening

11 x power off
4

In general, each trial was started during the afternoon, and measurements were recorded 

overnight and into the next morning. On average, we recorded 16 to 24 h of data for each trial, 

although some trials were continued over the course of several days. The only exception to this 

was the door opening trials, which only lasted 4 to 5 h. The thermocouple measurement system 

records readings once every 10 s. The individual data loggers can be set to various reading rates. 

In general, the loggers were set to record once every minute, except for logger F, which recorded 

at 30 s intervals. During the door opening trials, we increased the reading rate to once every

10 s to 42 s (depending on the capabilities of the logger).

2. INTERPRETATION OF RESULTS

2.1 Effect of loading density

In trials performed using this refrigerator model, vaccine storage density did not appear to have a 

major impact on the refrigerator’s ability to maintain the desired temperature range. The 

photographs below (Fig. 2) show the various levels of packing density tested.

Fig. 2. Typical packing patterns.

Average thermometer temperatures remained very stable throughout the study, showing minimal 

temperature variability in response to different loading densities (see Fig. 3). With the exception 

of 2 thermocouples (TC3 – wall, TC17 – floor), average thermometer temperatures shifted by 

less than 1 °C for the duration of the study, despite being subjected to low, medium, and high 

density packing loads. All of the thermometers maintained average temperatures well within the 

prescribed 2 °C to 8 °C range across all trials.

The two thermocouples that display slightly atypical behavior (3,17) were both installed in 

locations that we have consistently seen to be poor indicators of properly stored vaccine 

temperatures – that is, on the refrigerator wall (3), and placed in direct contact with the 

refrigerator floor (17).
5

Average temperature, °C

6

5.5

5

4.5

4

3.5

3

2.5

2

1.5

1

1 (wall mounted glycol)

2 (LH wall)

3 (RH wall)

4 (top back wall)

5 (air)

6 (vial - floor)

7 (air)

8 (air)

9 (air - top)

10 (vial in mfc packaging)

11 (vial in mfc packaging)

12 (vial - top)

13 (syringe in mfc packaging)

14 (inside tray)

15 (back of tray)

17 (glycol - floor)

18 (glycol - mid)

19 (glycol - top)

20 (vial - mid) 

LA (mid, shelf) 

LC (top, shelf)

LD (glycol - top, tray) 

LE1 (glycol - top, shelf) 

LE2 (glycol - mid, tray)

LF (glycol - mid, tray)

Figure 3. Average temperature of pharmaceutical refrigerator for all sensors, as a function of 

loading pattern. Labels on the abscissa correspond to entries in the measurement matrix of 

Table 1.

Figure 3 shows the average reading for each thermometer across all of the closed-door 

pharmaceutical refrigerator trials, as well as the periodic door-opening trials. Other than the two 

aberrant thermocouples discussed previously (3,17), it is readily apparent that the main body of 

the refrigerator as well as the vaccine vials themselves stayed within a tightly-controlled 

temperature range, regardless of packing density variations. The high-performance features and 

small size of this unit minimize the presence of spatial temperature variations within the 

refrigerator’s interior. Nearly all of the sensors recorded average temperatures within a band of 

about 1 °C regardless of where they were located (e.g., top shelf, bottom shelf). For the duration 

of our testing (51 days), the vertical shift (e.g., drift) of this distribution was less than 0.5 °C. As 

such, it is clear that the small pharmaceutical refrigerator maintains stable set point control 

across the variable packing densities likely to occur in a vaccine storage system, and is more than 

sufficient for normal, refrigerated vaccine storage.

One feature which is not visible in the average temperatures summary graph (Fig. 3), is that 

while all of the mean thermocouple sensors remained within the specified 2 °C to 8 °C bounds, 

the steady state temperature oscillations of several air and wall TCs consistently dip below 2 °C 

during normal refrigerator operation. These oscillations occur in response to the refrigerator 

compressor timing and periodic circulation of cold air needed to maintain a stable set point 

temperature. Figures 4a and 4b below show the thermocouple response to steady state 

refrigerator temperature oscillations. These graphs are representative of the standard temperature 

response observed during normal, closed-door operation throughout the course of our study.
6

Fig. 4a. (Left) One hour sample of normal, closed-door refrigerator operation with medium 

density load, all thermocouples shown.

Fig. 4b. (Right) Same one hour sample of normal, closed-door refrigerator operation with 

medium density load, but only thermocouples attached to vials or inside glycol-filled bottles are 

shown.

The effect shown in figures 4a and 4b highlights the importance of choosing the right location 

and installation method for vaccine temperature monitors. If we examine figure 4b, we can 

immediately see that all of thermocouples attached to vaccine vials remained well within the 

correct storage temperature range, as did the thermocouples kept inside glycol bottles, which 

give a good approximation of liquid vaccine temperature response. However, in figure 4a, we see 

that a number of thermocouples are registering minimum temperatures close to 0 °C every few 

minutes. Figure 5 below shows the same sample of data, but only thermocouples hanging in air 

and thermocouples in close proximity or direct contact with refrigerator walls are shown.

Fig. 5. One hour sample of data, thermocouples attached to/in close proximity to refrigerator 

walls or hanging in air are shown.

Clearly, thermometers placed in these locations (air, walls) experience much greater temperature 

fluctuations than the actual stored vaccine vials. This is consistent with the results we have seen 
7

in our studies of other refrigerator types. While the small pharmaceutical refrigerator exhibits 

extremely stable set point control and maintains stored items with minimal spatial temperature 

variation, the air inside the unit and the interior walls are subject to much larger temperature

fluctuations. Relying on either of these locations as an indicator of stored vaccine temperature is

a mistake, and will surely result in “false positive” out-of-range temperature alarms.

2.2 Effect of door opening and closing—normal use simulation

The results of the periodic door-opening trials also support the efficacy of the small 

pharmaceutical refrigerator model. The refrigerator’s ability to maintain temperatures of stored 

vaccines and correctly installed thermometers was not impacted by our repeated, 15 s door 

opening pattern designed to simulate normal refrigerator use. As we saw during our closed-door 

refrigerator tests, thermocouples hanging in air or close to the refrigerator walls recorded out-ofrange temperatures. Periodic door opening exacerbated this effect by causing large spikes in air 

temperature (Fig. 6).

Fig. 6. Door opening trial using a medium density load, all thermocouples shown.

However, these spikes in air temperature did not impact the refrigerator’s ability to properly 

maintain stored vaccine temperature. All of the remaining thermocouples (in trays, attached to 

vials, in glycol-filled bottles) and each of the electronic data loggers continued to record 

temperatures within the specified 2 °C to 8 °C range throughout the door-opening trials. To 

isolate the effects of door opening on vaccine vials and thermocouples kept in glycol, the data 

from these selected thermocouples only is shown in figure 7 below.

-2

0

2

4

6

8

10

12

14

16

13:05 13:20 13:35 13:50 14:05 14:20 14:35 14:50 15:05

Time, h:min

Temperature, °C

1 (wall mounted glycol)

2 (LH wall)

3 (RH wall)

4 (top back wall)

5 (air)

6 (vial - floor)

7 (air)

8 (air)

9 (air - top)

10 (vial in mfc packaging)

11 (vial in mfc packaging)

12 (vial - mid)

13 (syringe in mfc packaging)

14 (inside tray)

15 (back of tray)

17 (glycol - floor)

18 (glycol - mid)

19 (glycol - top)

20 (vial - mid)
8

Fig. 7a. (Left) Door opening trial. Only thermocouples monitoring vial temperatures or inside 

glycol-filled bottles are shown.

Fig. 7b. (Right) Temperature response of same selected thermocouples, recorded 17 h later and 

with refrigerator door kept closed.

Figure 7a shows the selected thermometer responses to repeated door opening. For comparison, 

figure 7b shows the closed-door, equilibrium temperature behavior of the same thermometers. 

While it is clear that the door opening does have some effect on stored vial temperature, this 

effect is storage method and location dependent, and ultimately is not significant enough to result 

in thermal excursions outside of 2 °C to 8 °C. Only thermocouple 12 (attached to a vial) shows 

marked temperature spikes in response to door opening. However, this particular vial had been 

removed from its original cardboard packaging and was kept in an open tray. Removing vials 

from their packaging is not recommended by vaccine manufactures or in VFC guidelines, as the 

packaging helps to preserve temperature stability of the vial by adding a protective layer of 

thermal mass. Thermocouples 6 and 20 were also attached to similarly unpackaged vials, and as 

a result, show much larger temperature variations than the monitored vials and syringes kept 

inside their original boxes (TCs 10, 11, 13). Comparing the door opening data to the closed-door 

data, we can see that the small pharmaceutical refrigerator’s performance is such that the impact 

of door opening on vial temperature is minimal. Even in the case of improperly stored vials (e.g., 

removed from packaging, placed on refrigerator floor), the small pharmaceutical refrigerator 

successfully maintains proper vaccine storage temperatures during normal door-opening use 

patterns.

Table 2 below summarizes the results of trials in which the refrigerator door was left ajar for 1 h, 

showing the amount of time elapsed once the door was opened until the monitored vials 

exceeded the allowed maximum temperature of 8 °C. These trials were conducted to maintain 

consistency with our previous refrigerator study methods. However, it should be noted that the 

specific small pharmaceutical refrigerator unit that we evaluated features a magnetic self-closing 

door. In reality, it would be quite difficult or impossible to “accidentally” leave the door of this 

particular unit ajar. For the purpose of these measurements, we intentionally propped the door 

open for 1 h.

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

7

13:05 13:20 13:35 13:50 14:05 14:20 14:35 14:50 15:05

Time, h:min

Temperature, °C

6 (vial - floor)

10 (vial in mfc

packaging)

11 (vial in mfc

packaging)

12 (vial - mid)

13 (syringe in

mfc packaging)

17 (glycol - floor)

18 (glycol - mid)

19 (glycol - top)

20 (vial - mid)

2

2.5

3

3.5

4

4.5

5

5.5

6

6.5

7

8:00 8:15 8:30 8:45 9:00 9:15 9:30 9:45 10:00

Time, h:min

Temperature, °C

6 (vial - floor)

10 (vial in mfc

packaging)

11 (vial in mfc

packaging)

12 (vial - mid)

13 (syringe in mfc

packaging)

17 (glycol - floor)

18 (glycol - mid)

19 (glycol - top)

20 (vial - mid)
9

Table 2. Time after opening door until vial temperature exceeded 8 °C.

Name and Location High Density Mixed Medium Density Mixed

(mfc = manufacturer) (minutes) (minutes)

vial 12 (in tray, top shelf) 7 did not exceed

vial 11 (inside mfc packaging) 20 did not exceed

vial 10 (inside mfc packaging) did not exceed did not exceed

vial 20 (in tray, middle shelf) did not exceed did not exceed

vial 13 (syringe in mfc packaging) did not exceed did not exceed

vial 6 (in tray, floor level) did not exceed did not exceed

For the most part, the refrigerator continued to maintain storage temperatures below 8 °C despite 

the continuous influx of ambient air through the open door. Drawing additional conclusions 

based on these results is difficult, especially given the fact that it is unlikely for this type of 

scenario (door “accidentally” propped open) to occur when using this small pharmaceutical 

refrigerator unit.

2.3 Effect of Increasing Room Temperature

To determine whether room temperature fluctuations impact the pharmaceutical refrigerator’s 

ability to maintain its set point temperature, we performed three trials in which the room 

temperature was varied between 20 °C and 25 °C. We can analyze the results by comparing the 

room temperature probe readings from logger LD to the refrigerator probe readings from LD, 

LE1 and LE2. This analysis is summarized in table 3 below.

Table 3. Refrigerator probe temperature change relative to room temperature change (°C)

Logger Name High Density Medium Density

LD 0.03 0.03

LE1 0.02 0.03

LE2 0.02 0.02

Table 3 shows the temperature change recorded by each of the three refrigerator data logger 

probes scaled to the change in ambient temperature recorded by the main unit sensor, which was 

kept outside the refrigerator. So, for example, during the High Density trial, logger LE1 recorded 

a temperature increase of 0.02 °C for every one degree increase in room temperature. All three 

data logger probes recorded a temperature change of 0.03 °C or less per degree rise in room 

temperature. This effect is so small that it could be a product of extraneous factors rather the 

changing room temperature. If we assume these changes are in fact due to room temperature 

variation and that the relationship is linear, we would need to vary environmental conditions by

33 °C just to see a one degree change in internal refrigerator temperature.

The small pharmaceutical refrigerator’s performance in these trials indicates that it should be 

able to withstand large ambient temperature changes while maintaining proper vaccine storage 

conditions in its interior. However, it must be emphasized that because our study only monitored 

the effects of a 5 °C room temperature change, we may not have captured additional thermal
10

effects that could occur during much larger room temperature changes. The use of a continuous 

temperature monitoring system will allow workers to immediately identify any problem 

temperature effects resulting from different environmental conditions.

2.4 Effect of a Power Outage

To observe what occurs following a power outage, we performed two trials in which the closed 

refrigerator was unplugged and monitored overnight. In both trials, all of the thermometers 

recorded a fairly steep increase in temperature that slowed down slightly over time. All of the 

thermometers, including those attached to vaccine vials, exceeded the allowed maximum 

temperature within 1:16 h:min for the medium density trial, and within 2:25 h:min during the 

high density trial.

To determine the required response time to preserve vaccine viability during a power outage 

situation, we examined the amount of time elapsed from the point that the refrigerator was 

unplugged until the thermometers attached to vials exceeded 8 °C. This information is reported 

in table 4 below.

Table 4. Time after power-off until vial temperature exceeded 8 °C.

Name and Location High Density Mixed Medium Density Mixed

(mfc = manufacturer) (h:min) (h:min)

vial 11 (inside mfc packaging) 0:35 0:33

vial 10 (inside mfc packaging) 0:53 0:48

vial 12 (in tray) 1:01 0:49

vial 6 (in tray) 2:09 1:00

vial 20 (in tray) 2:14 1:03

vial 13 (syringe in mfc packaging) 2:25 1:16

These results are similar to those obtained in power outage trials of the large pharmaceutical 

refrigerator unit [2]. During a power outage event, both the small and large pharmaceutical grade 

refrigerators warmed up to temperatures above 8 °C much more quickly than the household 

grade units that were tested. In the case of the large pharmaceutical grade refrigerator, we 

attributed this effect to the unit’s sliding glass door, which provides minimal thermal insulation 

in comparison to the standard opaque doors featured in household refrigerator models. However, 

the small pharmaceutical refrigerator features an opaque door rather than the sliding glass type. 

It’s possible that the unit’s small size simply results in faster total equilibration time because 

there is a significantly less thermal mass to contend with in a 4.9 cu ft unit than in a 14 cu ft or

16 cu ft model. In this case, the reduced size means that the small pharmaceutical unit warms up 

to ambient temperature relatively quickly during a power outage event. To combat this potential 

weakness, users who opt to use this type of refrigerator model ideally should also select a robust 

continuous temperature monitoring system with wireless alarm capabilities. In the case of a 

thermal excursion, staff members receive electronic alarm notifications by email or phone. If a 

power outage occurs outside of normal business hours, workers receiving immediate alarm 

notifications may be able to implement a contingency plan in time to save the stored vaccines.
11

Ultimately, it’s important for users to be aware that vaccines stored in a small pharmaceutical 

refrigerator may be more susceptible to damage from a power outage than if stored in some 

other, larger type of refrigerator. Other performance considerations may still make the small 

pharmaceutical unit an attractive choice to certain types of users, so individual provider offices 

should evaluate their needs to determine what sorts of risks are most critical to their situation. If 

the office features a robust, back-up generator system, then power-outage susceptibility may not 

be a significant concern. In this case, the small size and excellent temperature set point control 

may outweigh the potential risk. By contrast, a different office with different circumstances may 

recognize that frequent power outages in their area pose a significant risk to stored vaccines, and 

therefore would choose to use a unit that demonstrates better insulation and resistance to external 

heating.

Recovery from an outage in which the small pharmaceutical refrigerator is allowed to reach 

ambient temperature requires ~ 3 h, regardless of refrigerator loading density. The refrigerator’s 

small size means that the unit rapidly cools down and equilibrates to its pre-outage state.

2.5 Defrost Cycle in the Small Pharmaceutical Refrigerator

According to the manufacturer, defrosting of this unit is primarily accomplished by circulating 

air during off-cycle periods, and under normal conditions, the associated temperature increase is 

virtually unnoticeable. This is consistent with our findings, as we did not observe any 

temperature fluctuations occurring in response to a defrost cycle. However, the manufacturer 

notes that an occasional warm-up of 2 °C is possible under very high loads, heavy use, and/or 

extreme ambient conditions.

2.6 General Stability of Vial Temperature and Suitability of Small Pharmaceutical

Refrigerator Model for Maintenance of Vaccine Storage Conditions

Our results indicate that the small pharmaceutical refrigerator model is suitable for vaccine 

storage. Vaccine vial temperatures remained within the specified 2 °C to 8 °C range throughout 

all of the packing variation trials, as well as during the 15 second door opening trials. 

Furthermore, average vial temperatures varied by less than 0.5 °C throughout this entire range of 

testing. The small pharmaceutical refrigerator exhibits excellent long-term temperature stability, 

even under varying conditions. The unit also displays superior spatial thermal uniformity to that 

of household, consumer-grade refrigerators. With the exception of the interior walls and floor, 

average temperatures of stored vials and thermometers remained within a temperature band of

1 °C, regardless of exact location inside the unit.

2.7 Proper Locations for Vial Storage in the Small Pharmaceutical Refrigerator

Throughout the course of the closed-door pharmaceutical refrigerator trials, vaccine vials 

generally remained within the desired temperature range regardless of their location in the 

refrigerator. Only one monitored vial (6) recorded small thermal excursions of less than half a 

degree below 2 °C on two occasions. However, this particular vial was kept on the floor level of 

the unit. While the unit’s user manual does not specify whether storage on the floor is acceptable, 

we have seen other manufacturers advise against this practice. In our previous studies, we
12

observed that items stored in direct contact with the refrigerator floor frequently experienced 

aberrant temperature conditions as compared to items kept in the center of the unit. In addition, 

vial 6 was unpackaged from its original cardboard box, a practice that is discouraged in VFC 

guidelines and which we have observed to be problematic in previous refrigerator studies. All 

other vials remained within the specified temperature range and recorded zero excursions for the 

duration of testing, regardless of location or storage method. This refrigerator model is purposebuilt for vaccine storage. As a result, it features enhanced temperature set point control and more 

a sophisticated ventilation system than is included in most household refrigerator models.

Based on our studies, vaccine storage anywhere inside the small pharmaceutical refrigerator unit 

will result in appropriate storage temperatures provided that several storage conditions are met. 

These conditions hold true for ANY refrigerator model or type:

• Store vaccines on shelves in center of the refrigerator

• Do not place any vaccines in direct contact with refrigerator walls

• Do not store vaccines on the floor level

• Keep vaccines in original cardboard packaging, as supplied by manufacturer. If plastic 

trays are used to contain and organize vaccines, place vaccine boxes in trays. Do not store 

individual vials in the trays.
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3. SUMMARY

SUMMARY OF REFRIGERATOR PERFORMANCE 

IN RESPONSE TO TESTED CRITERIA

Criteria Impact on Refrigerator Performance

Loading density  No impact on performance

Opening/ closing 

refrigerator door

 Good temperature stability preserved throughout door opening trial

 All vials remained within proper storage temperature range throughout 

trials

 Most vials experienced only minimal temperature changes in response to 

door opening

 Only one vial, which was unpackaged and kept in an open tray, showed

significant temperature spikes ( ~2 °C)

Door left ajar

 Most vials remained within 2 °C to 8 °C even with door propped open for 

up to 1 h

 Unit features magnetic, self-closing door, so this scenario is unlikely to 

occur in real life

Increasing room 

temperature

 Room temperature changes have very little impact on refrigerator’s ability

to maintain internal temperature stability

 To cause internal refrigerator temperature to change by 1 °C, ambient 

temperature must vary by > 33 °C

Power outage

 Depending on vial location and storage method, and it takes 33 min to 2 h

25 min for vials to exceed 8 °C after power off

 Unit’s small size = less thermal mass, faster equilibration to ambient

temperatures during an outage

 Power outages pose a greater risk to vaccines stored in small refrigerators, 

as they may be exposed to out-of-range temperatures much sooner than

vaccines kept in a full-size unit

Defrost cycle  Automated defrost cycle

 No discernible effect on refrigerator chamber temperature

General stability of vial 

temperature

 Excellent

 Vaccine vials remained within specified temperature range during regular 

operation, regardless of packing density or storage location (except floor),

as well as during door opening trials

 Average vial temperatures varied by less than 0.5 °C throughout entire 

range of testing
14

ACKNOWLEDGMENTS

We thank the Centers for Disease Control and Prevention for providing financial support for this 

project. Additional thanks to Dawn Cross and Wyatt Miller, who assisted with the Vaccines for 

Children clinic tours.

REFERENCES

1. Chojnacky, M. J.; Miller, W. W.; Ripple, D. C.; and Strouse, G. F., “Thermal Analysis of

Refrigeration Systems Used for Vaccine Storage,” NISTIR 7656, 2009.

2. Chojnacky, M. J.; Miller, W. W.; and Strouse, G. F., “Thermal Analysis of Refrigeration

Systems Used for Vaccine Storage,” NISTIR 7753, 2010.

3. Day, G., Rossetto, B., and Ho, J., Testing of Purpose-built Refrigerating Vaccine Storage

Cabinets, Test Research Report E175VC-01, Marrickville, New South Wales, 2009.

4. Page, S. L., Earnest, A., Birden, H., Deaker, R., and Clark, C., “Improving vaccination cold 

chain in the general practice setting,” in Australian Family Physician 37, No. 10 (2008).

5. Proceedings of the National Vaccine Storage Workshop, edited by A. Langley, and S. Grant., 

Queensland Health, Brisbane, 2004.

6. “Performance characteristics of domestic fridges for vaccine storage,” E2E IT Solutions Pty.

Ltd. Available from: http://www.temptrack.com.au/documents/Fridge Performance Report

(Rev A).pdf

7. Vaccine Storage and Handling Toolkit, Centers for Disease Control. Available from:

http://www2a.cdc.gov/vaccines/ed/shtoolkit/