TV2 showing one lab fridge temperature

Three easy steps: Getting temperature sensors into a refrigerator

Three easy steps for getting a temperature sensor into a refrigerator.

All refrigerators have a temperature probe which is connected to the compressor to regulate the internal temperature of the refrigerator.  However, the need exists foImage result for refrigerator with Temperature sensor inside glycol bottler a second temperature sensor to trigger an alarm or keep a record of the internal temperature.  These temperature sensors can be either wired or wireless.  But whether they communicate with another piece of equipment via a wired or wireless connection the need to be positioned inside the refrigerator.  These sensors almost always have a wire which goes to a display or a transmitter.

So, how do you get the sensor inside the cold cavity with the wire leading to the display or a transmitter and not have the wire create an air gap as it is threaded between the gasket and the door frame.  The challenge is to get the probe inside the refrigerator and still maintain a good seal, so the refrigerator stays cold.

If you have large walk-in refrigerators you can drill a hole through the side or top of the unit, insert the wire and then fill the hole with some sort of thermal sealant. You can do the same thing with smaller refrigerators.  There is no reason you can not drill a hole in the refrigerator wall.  The only danger is that you could puncture a coil.  But the coils should all be located on the back of the unit.  Your drill bit will go through the outer metal shell, an inch or so of insulation and the inner plastic shell.  There are no cooling coils in the walls of a normal stand up refrigerator.  But, do check first to verify that the cooling coils are on the back of the unit.

Some smaller refrigerators, particularly scientific refrigerators, have a port through which the wire can be threaded.  If a port exists it will be filled with some sort of filler; molding clay, Styrofoam, plastic, etc…  Generally, the port will be on the back of the refrigerator but look at the back and the sides for a hole filled with some easily removed filler.  Once you have identified the port:


  1.  Remove the filler from the port;
  2. Thread the wire through the port, positioning the sensor near the center of the fridge;
  3. Reinsert the filler, pressing it around the wire.

If your refrigerator does not have a port through which the sensor can be threaded:

  1. Open the refrigerator door and thread the sensor wire between the gasket and the door frame near the upper hinge. Draping the wire over the top of the door hinge will help keep the sensor wire from moving around;
  2. Position the sensor near the center of the fridge. Numerous studies have shown that the temperature near the walls or front or back of a refrigerator is much warmer than the air in the center of the refrigerator;
  3. Tape the wire against the door frame so that it stays in place. Duct tape is best because it will adhere to the wire and door even given the cold temperatures of the refrigerator.

A small gap may be left between the door and the door frame, but it should be small enough that only a tiny bit of air can escape from the fridge into the outside air and the internal temperature should not be affected.  If you are worried about the resulting gap you could drill a hole in the gasket.  Refrigerator gaskets generally have an expandable bellows below an internal magnet which ‘snap onto’ the metal door frame to provide a seal against outside air.  Drill a hole in the bellows, underneath the magnet, and thread the sensor through the hole.  Then squirt some silicon in the hole so that it fills the hole in the gasket.  You can get a tube of silicon from any hardware store.

EU GMP Annex-1


Annex 1 of the EU GMP is a guideline and set of specific rules describing the European Union’s requirements for the manufacture of sterile medicinal products, including what we refer in the USA as “compounding pharmacies.” EU MP Annex 1 guidelines are applicable to all EU nation states in regards to pharmaceuticals bought, sold and manufactured –  including those imported from non-member nations. The latest revision will be released in 2019, and is expected to have a greater reaching impact on QA/QC and all laboratory activities in the EU and abroad.

So, what exactly is Annex 1 of the EU GMP, and what does it mean for pharmaceutical companies operating in the USA? For the most part, USP regulations in conjunction with 21 CFR 11 dependencies satisfy EU GMP, especially Annex 1, however, it is important to ensure manufacturing processes are not simply performed within standards and regulations, but monitored thoroughly throughout the process as well.


On 20 December 2017, the European Commission published the long-awaited draft of Annex 1 “Manufacture of Sterile Medicinal Products.” In fact, it was published nearly three years after it was first announced. Many see the change over the previous (technologically outdated) versions as having a focus on Quality Risk Management (QRM)

A key driver for the change, the concept of risk management is hard to miss in the new document:

  • 92 instances of the word “risk” (only mentioned 20 times in previous version) total times mentioned is 600, so “risk” is huge.
  • 15 references to QRM specifically

The 2018 (and presumably 2019) update contains substantial additional detail on virtually every topic in the 2007 version. In addition to those noted above as potential game-changers, compliance personnel can look forward to new levels of detail on such subjects as: Trending of environmental monitoring results (meaning the existence of a dependable chart recorder/data logger of pressure differential, temperature, and relative humidity)

Two key areas of focus should be viable and non-viable environmental and process monitoring and environmental control of pharmaceutical clean rooms as the essential part of the manufacture of a quality product. Simply adhering to standards without documented, digital storage of ongoing process controls is an exercise in futility.

We are somewhat partial since we engineer and develop instruments to monitor environmental conditions in cleanrooms, and for monitoring and logging data of control processes during pharmaceutical manufacturing. However, this partiality comes form countless instances of customers, and manufacturers reporting occasions where they assumed a process control was operating within optimal standards, only to find out later in log reports there was an anomaly which compromised the process. If end-use retail products are being manufactured – there is an acceptable risk for these incursions, and worst case scenario is a product mail fail or operate undesirably. In the case of a pharmaceutical product, it may be a person’s health and wellness which is ultimately compromised.

While there is no specific language to dictate specifics on “annex compliant” negative and positive pressure monitoring, as there is for acceptable micron size in particulate monitoring, it is important to note that the language does reference the requirement to maintain environmental control process monitoring, logging data throughout the manufacturing process.


There are many instruments on the market; some specialize in monitoring temperature. others focus on humidity. Some are standalone room air differential pressure monitors. The TV2 Cleanroom Monitor is the only instrument to perform all three actions with specifications which exceed EU GMP requirements, USP 787 requirements and provides data storage for one full year.

We are happy to consult with you regarding your temperature, relative humidity and room pressure monitoring needs – whether you re in the USA or adhering to new EU GMP Annex 1 compliance guidelines. We are here to help.

Here is a PDF of the EU GMP Annex 1





Zoom in of one pressure readout on partial screen of TV2

Negative Pressure in a Positive Pressure Cleanroom

Not too long ago we were asked by a pharmaceutical manufacture why our TV2 pressure monitor showed a negative pressure indication on the Max/Min display.

We investigated and found that if you quickly open the door into the room the pressure drops down into the negative range.  This had always been the case but he had never noticed it since he was using analog pressure monitors.  They did show the jump to negative pressure but you had to look quick since the needle swings happened fast.  The TV2 monitor, being digital, records and updates the high and low pressure, showing each in red if it is below the safety level.  So even if the room experienced negative pressure for a few seconds it is written to the display where it is very obvious.  In fact it jumps right out at anyone walking by.  That is, of course, the whole point, but this manufacturer was worried that an inspector would balk if the minimum pressure reading showed up in red.  Explaining that it was probably negative for a few seconds would not be enough to avoid ‘ding’ on the report.  Any indication of a negative pressure was a problem.  It was a problem easily fixed.  We simply set the display to not show the Max/Min.

However, this fix ignores the real problemAnytime the door of a positive pressurized  room is opened, all of that positive pressure air can and often does spill out into the hall where it is instantly mixed with the outside air and then sucked back into the room.  And, of course, it brings with it any and all suspended particles so that now the cleanroom is no longer clean.  Or at least as clean as it needs to be.

There are actually three solutions to this problem:  1.  Open the door verrrry slowly;  2. Install large capacity blowers that turn on any time a door is opened to maintain positive pressure; and 3.  Install a pressurized air lock so that pressure can return to positive before the door to the clean area is opened.  Each solution has its own drawback.  Option one is impractical and probably impossible to enforce.  Option two is expensive to install and may lead to increased maintenance costs.  Option three is the best solution.  In order to be effective personnel must pause in the air lock until it is re-pressurized and the in-rushing air has had time to be evacuated.  It is also important that the two doors not be opened at the same time.