Temperature standards

[Karen Olsen]

We have criteria for our refrigerators and freezers which state that the top and bottom temps are within 1 degree for refrigerators and 2 degrees for the freezers. I can not find a reference for this standard anywhere. Can anyone help??

[Sandy Jo]

I have changed our acceptable criteria to within 2 degrees celsius for different thermometers for both refrigerators and freezers as described under QC of Equipment (Thermometers) in chapter 8 (p. 198) of current AABB TM. "A 2 C variation between calibrated thermometers allows for the variation that may occur between thermometers calibrated against the NIST thermometers."

Sandy R.

South Bay Hospital

Sun City Center, FL

[John C. Staley]

Just to stir the pot. As long as the refrigerators stay above 1oC and below 6oC in the top and bottom what difference does it make how close they are? My new freezer has a digital read out (which was calibrated and checked against an NIST thermometer of course) for both the top and bottom shelves. As long as both of them are reading below -20 I'm not concerned at all as to how close they are. They usually are within a degree or two. My only current requirement for the refrigerators is that the chart temps are within +/- 1oC of the top shelf temp. The perfect storage devise would have an alarm sensor top and bottom with digital temperature readouts tied to both. Then you have constant monitoring of everything. I find it interesting that most alarm sensors are in the top of refrigerators but the biggest posibility for damage to red cells occurs at the bottom where temps are coldest and freezing is more of an issue. I'm much more concerned that the temp at the bottom stays above 1oC than I am about the top staying below 6oC.

Now that was quite a ramble if I do say so myself. Hope it made some sense.

[Mabel Adams]

Would a larger-than-usual difference between top and bottom temps in a fridge clue in the more oblivious among us (like me) that the fan has stopped working in the fridge?

[Art]

At our blood center, we perform temperature mapping for refrigerators and freezers (large and small) at validation to ensure that all points in that unit are performing within specification. After validation, we go to one point continuous monitoring with a commercial system. I don’t believe there is a regulation for temperature mapping, but our European plasma buyer requires mapping every two years and I believe it will become a de facto standard soon.

[Lcsmrz]

The purpose of having upper and lower thermometers is to ensure that the entire cabinet is relatively uniform: that the circulating fans are working properly and not so full of products that the air cannot properly circulate.

I tend to agree with John. The temp difference between thermometers is only an indication of the consistency of the temperature of the cabinet. As long as they are within specs, who cares! The check of the difference only serves as a warning to follow-up with a visual check for circulation problems.

All of my devices have their coils at the top, next to the solution with the upper temp/alarm, graph, and outside alarm system probes. I find these probes to be quite accurate -- more accurate than the NIST thermometer used to check them! It's hard to get older techs to think metrology, though.

[TrevorUAH]

That section in the AABB TM regarding the 2 is intended as a check on the thermometers themselves, not the storage cabinet. Each thermometer should have an accuracy tolerance of +/- 1 oC. If the difference between the two is greater than 2 degrees, then you probably have a calibration issue. Later on in that section it does talk about multiple thermometers in larger storage units, and as mentioned the only thing that matters is that they are all within proper range, not how far apart they are. Having said that, we just buy all NIST traceable thermometers for all our storage units. Doesn't cost much more, and it remove alot of issues around calibration! I know the standards state that the lower limit is 1 oC, and we adhere to that very stricktly, but for interest sake, one of the researchers we work with has done extensive work in the area of cryopreservation of various products, and they have demonstarted quite well that packed cells are not adveresly affected (aka "frozen" ) until around the -4 oC mark.

[adiescast]

I took the reference to 1C agreement out of the daily temperature SOP here. Now I have it in the quarterly alarm check SOP and the annual thermometer check SOP. The chart recorder should be checked periodically for agreement with the thermometers to prevent drift. I agree with John in principle - if you are in range, it doesn't really matter. The problem comes when you go out of range. Did you go out of range because the temperature was out of range, or was one of your devices for measuring temperature inaccurate, or both? The agreement between the upper and lower measurement is important (as stated by others previously) to assure even temperature distribution throughout the unit. This is a CAP requirement (TRM.42600).

[rcurrie]

At least with some refrigerator/freezer equipment, a large difference between top and bottom temps can indicate that the insulation has settled, and it is time to add a new piece of equipment to the future capital equipment budget. When I review charts and temp logs, I look harder at the difference between the chart reading and the internal thermometer reading, as it is more important to make sure the chart is calibrated than it is to worry about top and bottom temp differences. I have found circulation problems and door sensor problems, though, when looking at top and bottom temp differences.

BC

[Eagle Eye]

we have a freezer with >5 C temp difference between top and bottom shelf thermometer. We try to defrost the unit every month.