Temperature Probe Buffer Types for Pharmaceutical & Biotech Temperature Monitoring Applications

We often receive questions about damping methods for temperature probes. Vaisala provides aluminum cylinders designed for our temperature probes to reduce false alarms when the air temperature changes in a room or chamber.

In some temperature monitoring applications there are normal fluctuations that only cause product issues if the temperature change is maintained for a long duration.

Short-duration changes have little effect on product. And the usual practice is to set the monitoring system thresholds to the product’s storage specifications. When using a sensitive temperature probe, a buffering mechanism will prevent short excursions from creating false alarms.

But how did we create our cylinder probe buffer? How do we know it equates to other industry standard and accepted devices?
There is a simple answer and a complex answer. For the simple answer, we understand the common operating environments use by our customers for monitoring samples, drugs and vaccines (2 – 8°C). We also understand the thermal properties of common buffers (40 mL glycol 20%), and the thermal properties of metals.

For our thermal buffer blocks, aluminum was the material of choice because of several characteristics that make it effective: it’s cost-effective, light, strong, malleable and therefore easily manufactured, corrosion resistant with satisfactory thermal and electrical conductivity properties.

This temperature graph shows one of many tests once we created the aluminum cylinder buffer. The red line is a temperature probe inside a 40ml of glycol bottle of glycol, a standard liquid media used to buffer a temperature sensor, delaying its reaction to a temperature change.
The green line is the buffer. We were able to construct the cylinder based on two known facts about aluminum: its thermal capacity and specific heat.

Let's take a science refresher for the second, and more complex answer…

There are two important thermal property values to understand: Specific Heat and Heat Capacity.

Specific heat is the amount of heat ΔQ needed to raise the temperature T of a given mass, m, by a ΔT.  So S=ΔQ/(mΔT).  Temperature is a measure of molecular motion; the more molecular motion, the greater the temperature. Molecular motion requires energy, and greater energy given to matter increases temperature because energy can be in the form of heat. Therefore increased Heat   = increased Energy, less Heat = less Energy.

To obtain the specific heat value for a type of matter, we must determine the temperature loss or gain of a material according to loss or gain of heat energy.

The mass can be liters or milliliters, grams or kilograms etc., and temperature is usually in Celsius. Specific heat is per unit mass, so the specific heat of 40 mL of glycol is equal to the specific heat of a liter or glycol. The next value to understand is the Heat Capacity of a type of material (C). But, C is easiest to understand in relation to S. The relationship between Specific Heat and Heat Capacity is:

 c = (m)(S)

To see this applied in research on thermal buffering methods, we invite you to read the research documented in a paper published by NCSLI. They tested 20% concentrations in several aqueous media in different vial sizes, but found no significant differences.

Instead, their tests determined that the location of the probe inside the chambers had a larger impact. Meaning, whatever buffer you use for whatever probes you use, probe placement within a given volume is crucial. Placing the buffered probe in the geometrical center of the monitored space will be usually be the most representative of the temperature of the space.

We had an issue with our probes... we could place them in glycol, sand, glass beads, or another commonly used media. But our customers told us the beads and solutions were bothersome. Spills happened all the time, especially in dynamic clinical environments. And we knew that much from experience, and didn't want to use liquid media for our own testing processes.

Based on an understanding of our own probe performance, along with the known values of specific heat and heat capacity of aluminum, we designed aluminum cylinders to approximate a 40 mL bottle of glycol – the most commonly used probe buffer we had encountered.

Creating a purpose-built buffer for our thermistor probes put us in control of variables such as the fit of the probe inside the cylinder. If the hole is not perfect fit according to the length and diameter of the probe, the thermal buffering is affected; the probe may simply end up sampling air temperature, thereby negating the effect of the buffer. As a result, we don't recommend using our thermal blocks with non-Vaisala EPT probes.