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Factors In The Placement Of Temperature Sensors In Medical Refrigerators

Apr 29, 2014

The Wireless Temperature Monitoring (WTM) devices have arrived, now what do I do?

Okay, the long search for the optimal WTM system has been completed. The wireless technology works in all locations to be monitored. AC and/or battery powered devices are specified and arrive. Now what does one do? In this ongoing series about NYC Hospital Queens’ experience in selecting and installing a WTM system to monitor temperature sensitive medications (e.g. vaccines) and blood in hospital refrigerators (Link to Article) some consideration as to the placement of temperature sensors in the hospital’s medical refrigerators is given.


vaccine monitoring       blood transfusion bags

Diligent temperature monitoring vaccines and blood for transfusions is critical to ensuring product quality, efficacy and safety and in meeting regulatory reporting requirements.

Link to Image Sources:  Left: Vaccine Vials  Right: Transfusion Blood Bags


Sure, the sensor will go into the refrigerator to monitor the storage temperature of the medications, vaccine, blood or similar temperature sensitive products. Once installed they will be turned on and begin collecting data. But what is actually sitting in the refrigerated space and what is the optimal placement inside the refrigerated space to insure temperatures are being maintained properly? As in considering the issues so far discussed, wireless technology, electrical power source, etc., an understanding of what placing sensors in various locations will have on the temperature readings is needed.


First, what does the device look like? Was it planned that the temperature monitoring device sit wholly inside the refrigerator or just the sensor (temperature probe)? Some devices have integrated sensors and must sit inside the space to be monitored. Others use wired sensors where the sensor is placed into the refrigerator and plugged into the WTM transmitter that sits outside the refrigerated space. Which configuration to use would have been determined during the testing phase. Let’s look at both options.


wireless temperature sensors

Temperature@lert’s Z-Point battery powered wireless sensor node contains one integrated temperature sensor and two ports for wired sensors, therefore can monitor up to three refrigerators with one device.  Left: Z-Point with wired sensor and Right: Without wired sensor. Each has one integrated sensor in the body of the WTM device.


WTM devices that contain integrated sensors have the advantage of being able to be place into refrigerator. If they are battery powered there is no outside connection needed, so no need to have a wire going through a hole in the door gasket or refrigerator wall. However, if the devices are AC powered a power cord will need to pass from the inside of the refrigerator to the nearest electrical outlet, generally the same outlet used by the refrigerator. (Note: Medical grad refrigerators and freezers often have wall penetrations/holes to introduce sensors, power cords.)  


cellular monitoring vs wifi monitoring


Temperature@lert Cellular (Left) and WiFi (Right) Editions employ wired sensors. The devices are AC powered (power supply not shown) and sit outside the refrigerated space, insuring optimal signal strength and range. Each device can support up to four sensors making them very cost effective where several refrigerators are clustered together.


Devices that use wired sensors, meaning the sensor is at the end of an electrical cable like a phone or computer cable, are often placed outside the refrigerator. This helps with wireless connectivity because the wireless signal will not have to pass through the refrigerator’s signal robbing metal enclosure. The sensor is passed through a hole in the door gasket or wall and the other end is plugged into the appropriate slot in the WTM device.


This is a good thing because wires through gaskets generally compromise the refrigerator's door seal and let cold air escape and warm air enter. Wires through the walls have the same issue but leakage can be minimized with a gasket material around the penetration after the wire is in place.


Cellular Edition mounted on wall between two freezers supports two wired and two Z-Point wireless sensors in a Bio/Pharmaceutical Facility.   Graph (right) shows a Sensor Cloud report for one sensor fifteen (15) minute sampling intervals.over a three hour period.


Cellular Edition mounted on wall between two freezers supports two wired and two Z-Point wireless sensors in a Bio/Pharmaceutical Facility. Graph (right) shows a Sensor Cloud report for one sensor fifteen (15) minute sampling intervals.over a three hour period.


Whether or not the entire WTM device or a wired sensor sits inside the refrigerator, proper placement of the sensor is important in order to maintain critical medication temperatures. Temperature@lert has published several pieces about sensor placement in refrigerators. The most important lesson is that the middle of the middle shelf is likely the ideal place for the sensor. Door openings and the difference between the coolest air at the bottom of the compartment compared to the top of the compartment will be minimized with middle shelf locations. And placing away from the air outlets is also a good practice since cold air from the compressor can lead to lower temperature readings than would otherwise be seen.  Additionally, an earlier piece in this series described the use of buffer vials that help prevent “false” alert messages when temperature spiked happen due to door opening and closing. Temperature@lert has just released it’s Virtual Buffer Vial feature that uses software to mimic the physical buffer vial. Links to Temperature@lert paper: Sensor deployment factors


Temperature@lert wireless Z-Point sensor installed inside medical refrigerator communicates with Cellular gateway to Sensor Cloud, does not depend on site wireless network availability and can operate for up to 24 hours, longer with optional UPS (Uninterruptable Power Supply).  Alerts, escalation settings, data logs and report templates reside on redundant Sensor Cloud servers, providing fault tolerant temperature monitoring for the most critical applications.


Temperature@lert wireless Z-Point sensor installed inside medical refrigerator communicates with Cellular gateway to Sensor Cloud, does not depend on site wireless network availability and can operate for up to 24 hours, longer with optional UPS (Uninterruptable Power Supply). Alerts, escalation settings, data logs and report templates reside on redundant Sensor Cloud servers, providing fault tolerant temperature monitoring for the most critical applications.


Everything is in place, plugged in and ready to go - almost. The next pieces in this series will describe the process of setting up the user’s account and alert limits and the types of alerts one can receive. The series will wrap up with a piece that asks the question, “Okay, I have all this data at my disposal and I will be alerted when problems occur, now what?” Stay tuned.


Temperature@ert’s WiFi, Cellular and ZPoint product offerings linked to the company’s Sensor Cloud platform provides a cost effective solution for organizations of all sizes. The products and services can help bring a laboratory or medical practice into compliance with minimum training or effort. For information about Temperature@lert visit our website at http://www.temperaturealert.com/ or call us at +1-866-524-3540.

free temperature monitoring ebook



Written By:

Dave Ruede, Well-Versed Wordsmith

Dave Ruede, a dyed in the wool Connecticut Yankee, has been involved with high tech companies for the past three decades. His background in chemistry and experience in a multitude of industries such as industrial chemicals and systems, pulp and paper, semiconductor fabrication, data centers, and test and assembly facilities informs his work daily. Well-versed in sales, marketing, management, and business development, Dave brings real world experience to Temperature@lert. When not crafting new Temperature@lert projects, Dave enjoys spending time with his young granddaughter, who keeps him grounded to the simple joys in life. Such joys for this wordsmith include reading prize winning fiction and non-fiction. Although a Connecticut Yankee, living for a decade in coastal California’s not too hot, not too cold climate epitomizes Dave’s favorite temperature, 75°F.

Temperature@lert Dave Ruede

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