Measuring Temperature in Fridges for Year Round Fresh Produce

A temperature-controlled supply chain makes it possible to have roses in January or fresh salmon in August. Year round, fresh produce like seafood, vegetables and flowers move through a continuous network of refrigerated warehouses, trucks and cold rooms, from farm to retailer.

This whole system relies on maintaining stable temperatures. In contrast to frozen produce, which is usually kept at 20 °C (68 °F) or more below freezing, fresh produce must be maintained within a narrow temperature range. Normally this is 0 to 5 °C (32 to 41 °F). If the cooling system fails in a freezer, it can take many hours for the temperature to increase above freezing and for produce to spoil. But fresh produce, being maintained within much tighter limits, spoils a lot faster.

To counter this, it is vital to monitor temperatures continuously. In many environments though, it is simply not practical to run long lengths of cable taking signals from thermocouples to a main control room. In such situations wireless temperature monitoring is an extremely cost-effective alternative. However, freezers and cold rooms present specific challenges for wireless communications. As anyone who has ever lost a cellphone signal while in an elevator knows, metal boxes can obstruct radio signals. This article from OMEGA Engineering discusses the advantages of wireless temperature monitoring and the challenge of obtaining radio frequency signals out of a metal enclosure.

Principles of Wireless Temperature Monitoring

While thermometers are suitable for visual temperature readings, instrumentation demands the use of resistance temperature detectors (RTDs) or thermocouples. Usually these are linked directly to the display device, but that becomes difficult when long cable runs are required. In those situations, the answer is to employ a wireless temperature transmitter.

Wireless systems are easy to set up. Thermocouple wires are run into a battery-powered transmitter. This broadcasts a signal more than the 2.4 GHz frequency to a receiver mounted up to 122 m (400’) away. Line-of-sight is chosen for highest range and signal strength but the system can transmit through obstructions with a reduced range.

Using a USB connection to a PC, the receiver transmits the temperature plus connector ambient reading, RF transmit strength and battery condition to data acquisition software.

If data must be transmitted over a longer distance a receiver/repeater can accept data and retransmit it to a receiver up to 5 miles away.

Wireless repeater/receiver system

Wireless repeater/receiver system

Wireless thermocouple connector system

Wireless thermocouple connector system

Why Do It Wirelessly?

It is hard to beat the sturdiness of a hard-wired set up, but there are good reasons for selecting a wireless system.

  • Allows monitoring over long distances without long cable runs. It is suggested that thermocouple wire and extension wire be kept to under 30.5 m (100’) to avoid loop resistance becoming too high or measurement errors because of electrical noise.
  • No power supply required. Some wireless transmitters use batteries.
  • Cold room conditions should be maintained 24/7 and it is not practical to expect someone to make standard checks with a thermometer
  • Rapidly identify anomalous operating conditions, decreasing the risk of spoilage if the cooling system fails
  • No wires to run, which can be expensive
  • Flexibility: If a cold storage area is moved or reconfigured wireless transmitters can be redeployed rapidly and with nearly no cost
  • Real-time monitoring, unlike temperature logging which just provides historical data
  • Ability to program alarms if temperatures outside of present limits. With some systems it is possible to have an SMS or email alert sent should an issue arise.

Limitations

Wireless temperature transmitters have one weakness: many cold rooms and freezers are constructed with steel walls. These obstruct radio signal transmissions. Such an enclosure is referred to as a Faraday Cage, named after the English Scientist Michael Faraday who first demonstrated the occurrence. A Faraday Cage is a metallic enclosure that prevents electric charge from either leaving or entering. Instead of penetrating the cage, electrical charge, whether from radio transmissions or another source like lightning, spreads over the surface (this is why one is safe within a car during a thunderstorm). The same effect applies to signals within the cage, which cannot get out for the same reason.

Solutions

Providing the wireless system installer is aware of the Faraday cage issue, implementing alternative transmission techniques is typically direct. Two techniques are:

  • Position the transmitter by a window (a door will work too if it is of nonmetallic construction)
  • Thread the thermocouple wires via a small hole in the wall, mounting the transmitter on the outer side of the cold room

Wireless Temperature Transmitters

Compact battery powered transmitters can accept a wide range of thermocouple inputs to profit a range of applications. The frequency of data transmission is user programmable. For situations where an RTD is chosen, a transmitter takes a three-wire RTD sensor input and broadcasts data the same way.

Wireless transmissions go either to a receiver linked to a PC or onto a network or the internet via a receiver.

Data Outside the Box

The cold chain distribution system lets consumers enjoy fresh produce in nearly any location and at any time of year. This network of trailers, refrigerated warehouses and storage rooms rely on temperature monitoring systems providing prompt alters to any issues that could cause spoilage.

Wireless monitoring offers better flexibility than wired systems together with the ability to transmit data over distances of miles. However, as many freezers and cold rooms are built with steel paneled walls there is the issue of getting a wireless signal out from a Faraday Cage.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.

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