Monitoring -80 to -20 Freezers

Background:

Virtually all medical facilities use “Ultra-Low” temperature freezers for tissue and pharmaceutical storage. These freezers tend to be mechanical, often operating at temperatures of -80°C. With the inbuilt motors and compressors, the freezers tend to be noisy. Due to this, multiple freezers are often co-located in closed “freezer” rooms to restrict the noise.  The ambient temperature in these rooms can increase as a result and may be as high as 30°C. This can then further stress the motors and compressors.

The Problem:

As such, failures are not uncommon and while most Ultralow Freezers can be professionally repaired in hours, the samples can be lost before a repair can be carried out.  The value of a single freezer’s content can be from $10,000 right through to unique or virtually priceless samples.

The Solution:

Grant customers using these freezers typically require one or both of the following types of monitoring: 

1.      Reports for regulatory purposes

2.      Alarming to prevent product loss.

In Ultra-Low Freezer applications, an RTD temperature probe can be inserted into the cavity of the freezer through an access port at the back of the freezer or through the door seal.  Ideally a hole will be drilled in the door seal and plugged with silicone sealant. However, the door seal may be simply closed on the probe wire.  In either case the hinge side of the door is preferred for the greater leverage on the seal and the relatively lack of potential disturbance on the hinge side. 

Note: that closing the door seal on the probe wire may allow a small air gap which may increase frost build-up inside the freezer.

Above are examples of typical -80^oC freezer installations.  The pod is mounted on the side or back of the freezer with the probe wire running though the access port on the back. Alternatively it can run though the door seal (on the hinge side). 

Grant recommends using external battery packs on wireless pods for long-term monitoring with battery power (1 year +).

The above graph is from the online software and shows actual Ultra-Low Freezer data for one day. 

Note: The average temperature changed during the day.  It appears as if someone accessed the freezer during the day, and either moved the contents, changing the air flow pattern, or accidentally or intentionally moved the probe itself (the cycling was right at the upper alarm limit).  See text for more information.

Monitoring freezers is not as simple as it may appear.  The temperature inside the freezer is not uniform.  Cold air enters the freezer from only one location and is measured by the thermostat in only one location, so a temperature gradient will exist.  There is no ONE, uniform temperature inside the freezer.

Also, the temperature indicated by the freezer digital display will probably not match the readings by the Grant temperature pod. There are two reasons for this. Firstly, manufacturers of the freezers tend to use very slow-response temperature probes so that door openings do not set off the alarms.  Secondly, the probes used by the manufacturer are often not located inside the freezer cavity. They are typically installed behind the back wall of the freezer (away from potential damage) and a “correction factor” applied electronically to the reading.

Due to the above, when calibrations are performed, the Grant probe and the manufacturer probe must be placed very close together and the temperature allowed to stabilize over sufficient time.

Door openings and defrosting cycles present another challenge.  The Grant monitoring systems allows “filtering” of data to avoid false alarms.  For example, if the data is alarm-checked every 10 minutes, it is advisable to set the alarm filter to 3 samples out of 3, meaning that three data points (at 10 minute intervals) in a row must be out of limits before an alarm is issued. 

Actual ultra-low freezer data showing door openings, although the internal temperatures did not exceed alarm limits (red horizontal lines).