Most freezer applications in the pharma/biotech world require temperature measurements ranging from the freezing point (32°F [0°C]) down to the cryogenic range (-320.8°F [-196°C]). Resistance temperature detectors (RTDs) are well designed to handle these low temperatures and the high accuracy requirements of these applications. Platinum RTDs, where platinum is used as the sensing element, provide temperature stability, a high degree of repeatability and good resistance versus temperature linearity.
To maintain product quality and integrity, industrial freezers must be kept within a narrow temperature range. In addition, a record of the storage temperature — and any temperature deviations — must be maintained, verified and available for reference. Pharmaceutical and biotech products are heavily regulated, and this traceability is critical for audits, quality control and product viability.
You may ask, does the temperature sensor supplied with the freezer do all of that? Most of the time, the answer, unfortunately, is no.
It is true that most freezers have a built-in temperature sensor for control purposes (a setpoint) and a display for visual reference. The supplied temperature sensor, however, seldom is accurate enough to be trusted to verify that the freezer cavity has reached and maintained the target temperature. Validating the temperature with a higher accuracy secondary sensor ensures that the condition indicated on the controller is achieved and maintained within the freezer.
Secondary verification and subsequent calibration are ways to make sure that the built-in sensors are working correctly. By installing a secondary, high accuracy RTD in the freezer, the owner/operator can know the actual temperature with confidence . Also, the secondary sensor can be removed and recalibrated periodically to provide another level of verification.
Using a Calibrated RTD and Transmitter
High accuracy — ±0.9°F (±0.5°C) or better — often is a requirement in freezer applications. Using a platinum RTD is the first step to achieving this level of accuracy. But, what if you need a more accurate measurement?
For improved accuracy, a transmitter can be added and matched with a calibrated RTD. Using a multipoint calibration over the range of use, the sensor’s individual temperature coefficients can be programmed directly into the transmitter. This eliminates much of the interchangeability error associated with RTDs.
Verifying the temperature with a higher accuracy secondary sensor can ensure that the reading indicated on the controller is achieved and maintained within the freezer. Source: Burns Engineering
In addition to increasing accuracy, such an approach converts the RTD’s resistance reading to a standard 4 to 20 mA signal that can be integrated with a chart recorder or control system. While adding a transmitter increases the initial cost of the measurement device, it is recommended when highest level of accuracy is needed. Where product loss or recalls can result in significant financial losses, using a transmitter can be viewed as economical insurance.
Overcoming Operational Challenges
Other challenges exist when measuring temperature in cold environments due to the harsh conditions to which the sensor is exposed.
The sensor should be designed for the moisture and associated frost buildup present in cold environments. Ideally, the entire sensor should be sealed and waterproof. The materials of construction should be selected based on their ability to handle temperatures as low as -320.8°F (-196°C) without cracking or deteriorating due to the expansion and contraction that occurs at low temperatures.
Why is frost buildup a problem? A look at how RTDs are built will demonstrated the need. In a typical RTD, the internal platinum sensing element is insulated electrically from the exterior metal sheath (usually stainless steel). The lead wires are connected to the sensing element before the sensing element and lead-wire connections are sealed with an epoxy or similar material. Once the RTD is installed, a small electric current is transmitted via one lead wire through the sensing element. The current returns via the other lead wire and the resistance is measured.
Any leaking or shunting of that current will reduce the measured resistance and result in an inaccurate temperature measurement. Likewise, any type of frost buildup on the sensor body can eventually work its way into the sensor. Once inside, the moisture allows the current to leak and dissipate, resulting in a lowered resistance reading and measurement. Eventually, the moisture buildup will cause the sensor to fail.
A waterproof RTD designed for freezer use can be immersed in a water bath or installed in a freezer without frost buildup. Source: Burns Engineering
A waterproof RTD can be immersed in a water bath or used in freezer environments without concern of frost buildup. By contrast, a standard epoxy-sealed RTD without additional waterproofing is not capable of maintaining a dry environment inside the RTD if it is exposed to liquids or if pressure changes that force moisture inside the sensor.
Another challenge that often is seen in freezers is determining a way to measure or track the temperature of the product itself. If the product is in liquid or solid form, its temperature will change more slowly than the ambient temperature measured inside the freezer (an air-temperature measurement). If the freezer is opened to introduce or remove product, for example, the air temperature will increase temporarily.
Freezers like those used for vaccines have an alarm to alert operators if the temperature inside the freezer changes. No one wants an alarm to go off every time the freezer door opens, however, if it has no real effect on the product itself.
In applications such as these, two approaches are possible. The first is to dampen the sensitivity of the sensor. Often, this is done by inserting the sensor into a plastic thermowell or glycol-filled bottle. The second is to program a delay into the transmitter to account for the few seconds that the door might be opened.
A look at a typical pharma/biotech freezer application can be used to illustrate. In this case, the company needed to measure the actual temperature in its -112°F (-80°C) freezers. Different RTDs were installed in the various freezers, and the company engineers wanted to standardize on a single model. RTD selection criteria included:
- Ease of installation.
- An accuracy of ±0.36°F (±0.2°C).
- The ability to use a transmitter.
- A means to protect the RTD.
The sensor manufacturer recommended a waterproof platinum RTD with a small profile (1.25" in length and a diameter ranging from 0.25 to 0.125"). Features include ease of installation and the ability to be calibrated and matched to a transmitter.
For the pharma/biotech freezer application, the custom design included a housing for the matched transmitter. The sensor was installed into the freezer by drilling a hole in the freezer wall and sealing around the sensor cable. The transmitter was mounted on the freezer wall exterior.
In one application, a hole was drilled in the freezer wall to allow the sensor and cabling to pass through. The transmitter was mounted on the exterior wall. Source: Burns Engineering
In conclusion, improving temperature measurements in pharma/biotech freezer applications is achievable by keeping a few considerations in mind.
Platinum RTD sensors offer repeatable and stable measurements at the low temperatures found in freezers. When designed with the environment in mind, RTDs provide years of trouble-free service.
To get the most accurate temperature measurements, follow these tips:
- Calibrate the sensors and match them to a transmitter.
- Know what you are measuring (product vs. freezer air).
- Use secondary sensors to verify and monitor actual temperature and setpoints.
Dampening or offsets can help limit false alarms, provide peace of mind and impart confidence in your measurements. PC
Jerico Sanchez Hulstrand is an applications engineer at Burns Engineering, Minnetonka, Minn. For more information from Burns Engineering, call 800-328-3871 or visit burnsengineering.com.