In the past few years, it has become increasingly common for hospitals, clinics, healthcare and life science organizations, and other businesses to use an electronic temperature monitoring system to safeguard their products and satisfy regulatory demands. You may know that you need a monitoring system, possibly with alarming capabilities, but aren't sure how to select the best one to meet your needs. To complicate matters, there are literally dozens of different types of temperature monitoring systems with different features and a wide range of prices.
Whether you're tasked with recommending what to buy, a purchasing agent, or the ultimate end user, you can ensure that you're getting the right system by learning a bit about the most important parts to focus on. This basic tutorial covers the six parts of a typical temperature monitoring system to help you know what to look for.
Consider each of these six factors when specifying/selecting a temperature monitoring system:
Temperature Probe or Sensor — The type of temperature probe type will affect the measurement accuracy and the temperature measurement range. Common sensor types include thermocouple, RTD, and thermistor.
Thermal Buffer — A thermal buffer helps smooth rapid temperature fluctuations at the sensor due to compressor cycling, door opening, or loading/ removing products. Thermal buffers come in the form of Nylon block, bottle filled with ethylene glycol, and bottle filled with glass beads.
Temperature Measurement Device — The heart of the system, it connects to the probe to measure and possibly record the temperature. There are many kinds of these including a standalone monitoring device with local memory to store measured data, a networked/LAN or WiFi measurement device with or without local memory, and a wireless measurement device using proprietary communication protocol with base station or gateway, again with or without local memory.
Data Storage — While all monitoring applications require some type of immediate data reporting, most also include recording values for historical purposes. The location and amount of memory determines how much historical data will be available. Memory can be internal memory, local base station or gateway, local PC, or cloud-based service.
Software — Of course, any system will require some software to control the operation of the system. Software functions include configuration, charting, alarm management, data retrieval, and reporting.
Alarming — Most users want immediate notification of temperature excursions outside of the safe operating range. Alarm delivery methods include visual indicator, audible alarm, email message, SMS-text message, and phone call.
1. Temperature Probes
Temperature is among the most common measurements across a broad variety of industries including food, medical and life science, pharmaceutical, machine/ equipment monitoring, environmental monitoring, and practically every other field. Temperature monitoring systems capture temperature data via a sensor such as a thermocouple probe. Since temperature sensors are designed for such a wide variety of needs, it's important that you decide on the type of sensors or inputs you'll use.
The three most common temperature sensors used with temperature monitoring systems are thermocouples, thermistors, and RTDs. Thermocouples are the most common temperature sensors. They have the widest measurement range and are typically the least expensive but also have limited accuracy — typically ±1-2 °F (±1 °C). RTDs have higher accuracy than a thermocouple, on the order of ±0.2 – 0.5 °F (±0.1 – 0.3 °C). RTDs have a narrower operating range, with a maximum temperature of 150 – 600 °C, depending on the material and construction. Thermistors offer even more precise measurements, ± 0.1 °C or better, but have a very nonlinear response and therefore require a more advanced measurement system. They also have a more limited operating range than RTDs or thermocouples.
It is worth noting that most sensor manufacturers can embed the temperature sensor in a variety of probe types. From stainless steel probes to probes suitable for immersion in liquids and magnetic surface contact probes, you can find what your application requires.
Thermocouples are the most widely used temperature sensor and also one of the least expensive sensors available. They are widely used where cost, simplicity, and wide operating range are paramount and where extremely high accuracy is not required. A thermocouple is two different metal wires of very specific alloys fused together at a single point. A thermocouple produces an output voltage (typically in the millivolt level) proportional to the temperature. The measurement system samples the voltage created by the thermocouple junction and then applies a calibration equation to convert the voltage to temperature. The monitoring system also incorporates a cold junction reference to compensate for any offset voltage that occurs at the connections between the thermocouple wires and the measurement device itself. Because of variations in the composition of the thermocouple wire, typical thermocouple accuracies are on the order of 1 to 2 °F, although special composition wires with reduced errors are also available.
Consider thermocouples when you just want a low-cost device that's easy to use. Care should be taken with the environment in which you are recording the temperature. Because of their wide operating range, thermocouples can be used in almost any temperature monitoring application, from liquid nitrogen cryostats to metal heat-treating ovens. Due to a thermocouple's low-level voltage, there can be adverse effects in electrically noisy environments, especially when the sensor wire length is long.
An RTD sensor provides a change in resistance that is related to temperature. They offer more accurate readings than thermocouples but have a narrower operating range. The most common RTD consists of a fine platinum wire wound around a cylinder — nickel and copper wire are also used. The resistance vs. temperature curve has a very specific slope and the RTD is made so that it has a specific resistance at 0 °C, with 100 Ω being the most common value.
To measure temperature, the monitoring system will source a known current through the RTD and measure the resulting voltage, from which it can calculate the resistance using Ohm's law. Finally, using the slope of the resistance vs. temperature curve and the 0 °C resistance, it can calculate the temperature. RTDs are typically more stable and accurate than thermocouples but at the expense of a more limited operating range. Consider RTD sensors when you need high-precision measurements for a narrow temperature window. They are ideal for temperature monitoring systems for freezers and refrigerators.
Thermistors are similar to RTDs (they're sensors whose resistance changes with temperature) but their resistance change is highly nonlinear. Like RTD sensors, they take more accurate readings than thermocouples. Because of this characteristic, thermistors can offer very accurate temperature measurements, down to an accuracy of 0.01 °C but only over a very limited temperature range (typically 0 °C to 100 °C). Like RTDs, thermistors are designed to have a specific resistance at 0 °C (2252 Ω is a common value) and each family of thermistors has a specific resistance vs. temperature characteristic that the measurement system must be able to accommodate. Consider using thermistors when you need to record at the highest accuracy, have a limited measurement range, and are using a temperature monitoring system that can accept the nonlinear resistance curve; for example, skin temperature measurements.
2. Thermal Buffers
Thermal buffers are thermal masses (materials and liquids) that are attached to the temperature probe to increase the time constant (slow the response time) of the temperature probes in order to more closely match the temperature of the material being stored. This has the major benefit of making the reported temperature more closely mimic the actual temperature of your refrigerated product. Glycol bottles, nylon blocks, and vials full of glass beads are common types of thermal buffers used in cold storage applications.
A common example is a probe that is measuring the temperature of a refrigerator used to store vaccines. These probes have a much faster response time than old-fashioned mercury thermometers. Whenever the door is opened, warm air from the room displaces the cold air in the cavity. A bare probe can respond to this change and a rise in temperature will be detected by the monitoring system. If the door is only open a brief period of time, the temperature will decrease back to the nominal temperature of the cavity within a minute or two; however, during the brief temperature “spike,” the temperatures of the vaccines do not exhibit the same spike in temperature due to their own thermal mass. By using a thermal buffer surrounding the temperature probe, the air temperature spike will be “buffered” so that the probe will not experience the same jump in temperature. Due to CDC recommendations, thermal buffers are becoming standard in hospitals, clinics, and pharmacies as well as laboratories and even cold chain settings. By using a buffer, you can eliminate the temperature spikes in the data from the monitoring system caused by opening the fridge or freezer door.
In an experiment, bare probes were shown to display temperature fluctuations that are greatly reduced through the use of various types of thermal buffers. Even the compressor cycling of your storage unit can cause false alarms and pose a major inconvenience along with widely varying temperature data that does not reflect the actual product temperature. Figure 1 shows that the bare probe's readings were extremely variable compared against the buffered probes. In fact, if this had been an actual medical monitoring application, the bare probe could generate false alarms simply due to the normal cycling of the refrigeration compressor. If the limits are set too tightly, even a small variation in the cycling can trigger an alarm. Since stabilizing temperature readings is so critical, you can avoid nuisance alarms and get much more accurate data by using thermal buffers on all your probes.
3. Measurement Device
The heart of the system is the actual temperature measurement device. These come in many forms, from simple single-channel devices with a USB interface to multi-channel intelligent data logging systems. The measurement device connects to temperature sensors, digitizes the temperature value, performs any local alarm evaluation, and records the reading's memory or transmits it to a server in the case of a network-based system. The measurement device can be battery-operated or may have options for external power. They may have fixed input types and include the sensors or they may have universal inputs with screw terminal connections to let the user attach their choice of sensors. The least expensive measurement devices feature a single input type (only one type of measurement per device) and a fixed number of inputs, i.e. no expansion. No matter the type of measurement device, there are a few characteristics that need to be considered to help you make the right choice.
Sampling Rate. After determining what temperature range you need to log and where you need to record it, it helps to decide how often you need the temperature monitoring system to take a measurement. You might need second or sub-second sampling for an industrial process or you might only need to take a reading once every 30 minutes or every hour just to keep tabs on a long-term ultracold storage environment.
Most monitoring systems can handle recording at rates up to about 1 Hz (once per second). If you need a faster sample rate, be aware that as the speed of the system increases, the price does as well. Also, make sure that the recording rate you are specifying is appropriate; for example, using a K-Type thermocouple, the sensor/ sample may take several seconds to register a change in temperature. Recording such a temperature at 5 Hz would provide redundant or useless data.
While monitoring devices usually consume very little power, if the unit operates solely on batteries, you'll want to look at the battery life, which varies considerably based on the manufacturer, model, and how often it's configured to make a measurement.
Measurement accuracy is another important factor to consider. Most temperature monitoring devices are accurate enough to cover typical applications; for example, if you're monitoring room temperature, a system that's accurate within a degree or two should be enough. But if you are monitoring a vaccine or other refrigerated sample, you might need a high-accuracy model accurate to within a half-degree or better.
One of the biggest differences between devices from different manufacturers is whether the temperature monitor is designed to be used as a standalone or if it has to be connected to a PC or network and if so, what the communications interface is that connects the temperature monitoring system to the PC or network. Communication can be done in many different ways including serial or RS-232 interface, USB interface, Ethernet interface, wireless connection including Wi-Fi and proprietary RF links, or cellular 3G or 4G/LTE.
Standalone Temperature Monitoring Systems. Many temperature monitoring systems can operate in standalone mode, meaning that they don't require a PC or other devices to record temperature and process alarms. These devices commonly have an LCD display showing current temperatures with an indictor or LED to alert you when the temperature is out of spec. Some devices, such as standalone data loggers, are very durable and will continue to reliably operate for years, while other types like cold chain recorders are designed as low-cost, single-use devices.
Standalone devices typically have internal batteries providing months to years of operation but be aware that sample rate is inversely tied to battery life. These devices usually have built-in non-volatile memory that ensures that recorded data is still safe if the battery fails or power is lost. Units with a display will often have an indicator to warn you when the battery's getting low. There are three types of batteries: rechargeable, non-rechargeable user-replaceable, and non-rechargeable non-replaceable (single use).
Finally, there is the question of how to connect to the monitoring system to make configuration changes or download stored data. Today, USB connection is the most popular choice but other options include serial (RS-232), Ethernet, WiFi, and Bluetooth.
In contrast to standalone temperature monitoring devices, more advanced models have the ability to automatically send their data to a PC, server, or the cloud. They can connect to a LAN using an Ethernet or WiFi interface to automatically send data. Cloud-based systems provide the advantage of managing data from long distances; for example, you can view current temperatures anywhere, anytime using a standard Web browser on a PC or mobile device. Depending on the manufacturer, cloud-based systems can also send warning e-mails, text messages, or voice notifications whenever values go outside safe windows.
Wireless Temperature Monitoring Systems. Wireless technology is quickly becoming the standard in many applications including temperature monitoring; life science and healthcare applications are major markets. These systems are very effective for temperature monitoring and alarming in refrigerators and freezers, cryostats, storage areas, and incubators. Key features of wireless monitoring systems include wireless range, data update rate, and cost that are based on the wireless technology employed.
Wireless systems are ideal when:
You have a number of distributed points where you need to measure temperature.
It would be difficult or expensive to run wires from your measurement points back to a central location.
Data needs to be collected and transmitted from a truck or other vehicle while it is in motion, preventing the use of wired sensors.
Data and/or alarms need to be collected from a site that is difficult to access or does not offer regular Internet connectivity.
Many manufacturers now provide systems that use remote devices to collect the temperature measurements at the point being monitored and then automatically send their readings via a wireless communications link to a base station or wireless gateway. From the base station/gateway, downloaded data can be sent by e-mail to specified addresses or over the network to a local or remote server including cloud-based services. Moreover, the base station can be set up to monitor for warnings and send alarm messages. Systems that automatically transfer their readings save the time and trouble of traveling to each device to retrieve the data or check status.
There are many other options for the actual wireless link including standard protocols such as Zigbee and proprietary wireless systems. These systems normally operate on one of the unlicensed frequency bands such as 932 MHz (US) and 2.4 GHz. Depending on the device and frequency, the wireless range can be from 50 to 1,000 feet. Many systems offer wireless repeaters to extend the wireless. In some cases, the physical layout can make wireless system deployment difficult. Consider whether the units would have clear line of sight to a gateway or a repeater or if their communication would be obstructed by walls or objects.