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RTU Temperature Sensors - History & Buying Guide

By Andrew Erickson

August 4, 2021


There's no more common sensor on our RTUs than temperature sensors. In fact, the majority of our builds have one built right on the board to give you a good ambient reading of the temperature in your site.

A site without a temperature sensor suffers expensive equipment damage due to fire.
As you can see in this diagram, failing to monitor your remote temperatures properly is a recipe for financial disaster - or worse. If your system is involved in public safety, including any phone/radio system, lives could be put at risk.

Client Conversation: Many Different Types of Temperature Sensors

I recently had an email exchange with a DPS client. He had questions about the specific architectures and I/O we use for measuring temperature. I think that conversation will be useful for you as you research your options for remote site temperature measurement.

My client asked me:

"If the temp sensors are D-wire (Dallas One-Wire), then why do they require an analog input? One-Wire is digital. Are you using the analog inputs as bi-directional digital comms channels to communicate digitally with the sensors? I've seen similar things being done with Arduino controllers, so I guess it is common practice."

I responded:

"The D-wire temperature sensors do not require an analog input. The analog value is digitized at the sensor as you described. The NetGuardian 420 has analog inputs, but you will not need them for D-Wire sensors."

Next, my client asked:

"Ok, that answers most of my questions but what about the 4 dedicated analog inputs for temp & batt volts?"

I cleared everything up by explaining:

"The 6 traditional analog circuits are in essentially the same state on the NetGuardian 420 as they always were: 2 are internally tied to power inputs to monitor them, 1 is tied to internal temperature sensor, 1 is tied to pluggable port (looks like headphone plug) for traditional external analog sensor, 2 are available for your general use via back-panel connector.

This has not changed in the D-Wire era. The internal temperature sensor remains on the 420, and many people choose to use only that sensor if they're not very worried about temperature. Although it cannot be placed away from the 420, it's a good general ambient temp reading. The D-Wire port has had the effect of making our traditional external temperature probe less common. If people want an external sensor, they'll tend to use a D-Wire sensor.

As a result of the above, you'll have 2 general analog inputs for any purpose AND an internal temperature sensor on the 420 AND an external D-Wire temp sensor. You could add many more D-Wire sensors of various types later if you like and add them to your daisy chain."

As you can see, there are many different kinds of temp sensors you can use. This is the result of different approaches across time (and among manufacturers).

How Temperature Sensing Evolved at DPS (and in the general remote monitoring industry)

Temperature sensors have evolved across time, generally trending toward being better and cheaper. Where it was once common to have just a single ambient sensor in a room, you can now install sensors at troublesome "hot spots" to track those temperatures specifically.

Let's take a look at key developments of sensor and related RTU developments at DPS. I think of us as a bit of a pioneer in this space in some cases, but you should know that many of these evolutions reflect the broader monitoring industry.

Milestones in the evolution of temperature measurement:

  1. The simplest temperature sensors involve a physical change that can be visually inspected. Mercury thermometers are common. Also in this category are metallic spirals that expand contract and move a needle gauge. These function locally, but they don't have an application in remote monitoring.
  2. The next step in sensor evolution is a discrete temperature sensor that can be remotely detected. This might locally measure an analog, but the reporting is purely binary. Your home thermostat is a good example. It measures an analog temperature, but it only latches a relay when temperature crosses an important value that you determine. This activate your air conditioner or heater. Imagine instead that this relay is connected to an RTU/PLC discrete input. That's how you can detect a "high temp" or "low temp" alarm from your central office.
  3. Even better than discrete measurement is analog measurement, which will send you "how much?" information on a continuous spectrum. Instead of just "high temp", you'll know "48 degrees Celsius", for example. These tend to measure temperature as a change in electric voltage across a temperature-sensitive material. You'll commonly get an output to your RTU/PLC in the 0-5 VDC or 4-20mA range, which you'll need to pick up with an analog channel.
  4. The final evolution is a sensor with a CPU and digital communication. These are becoming more and more common as microprocessors continue to evolve and become viability for smaller and smaller applications. A digitized sensor value is immune to line interference, so you get better readings. You can also daisy-chain sensors instead of home-running everything, which cuts down on your install labor and expense. Some sensors are even IP enabled, acting as their own RTU. Data can then be sent back via LAN to your SNMP manager or SCADA HMI or other central server.
A modern D-Wire (one-wire) sensor that converts its temperature reading to a digital message
This modern D-Wire sensor supports digital communication across hundreds of feet and a convenient daisy-chain architecture.

Where Should You Place Your Temp Sensor(s)?

Location, location, location. It's as true for sensors as it is for business. Where you put your sensors matters.

You can have an RTU with a built-in ambient sensor. Where you place that one is pretty obvious, although you do have the freedom to choose a location, usually in a 19" or 23" equipment rack, for the RTU itself. Although you might see some heat from the RTU itself, and your critical equipment might be farther away and not directly measured, this sort of sensor is WAY better than nothing.

You can also have an external sensor with some location flexibility. In my email exchange above, you saw me talking about our external probe with a 7-foot (about 2 meters) lead on it. That connects via a stereo-type plug to the NetGuardian 832A G5 and several other NetGuardian models. The same placement freedom is possible whether you're using analog (like the 7-foot probe) or digital (like the D-Wire sensor) communication to the sensor itself.

What D-Wire gives you, in terms of placement flexibility, is the ability to customize your wire length. D-Wire supports runs of potentially several hundred feet/meters, depending on the sensor and RTU. You can also daisy-chain multiple sensors without having to run them all back to the RTU.

D-Wire also means that you can "place" a temp sensor by virtue of installing a combo unit that measures multiple different things. The two most common that DPS sells are: Temp+Humidity & Temp+AirFlow. These two sensors are excellent for assessing either weather conditions and possible water accumulation - or the performance of your HVAC system.

Let's now consider the key elements to consider when choosing a temperature sensor for any remote location:

How to Choose the Right Temperature Sensor

Of course, everyone know that a temperature sensor measures the nearby heat level. There's a lot more to consider than that, however.

Factors to consider when shopping for temp sensors:

  1. Temperature range measured: Most sensors can monitor a wider range of degrees than you usually need for telecom-related monitoring. If you can get good readings all the way down to freezing and up to about 60 degrees Celsius, you have plenty of granularity to know the environment surrounding your equipment. If temperatures get outside this range, the needle will be "pegged", so you'll still know that the site is very hot or very cold. If you have special requirements, like very hot industrial processes or "cold chain" freezers for food, you may need to choose and purchase a more specialized sensor. Make sure it still has a compatible output to your RTU or PLC.
  2. Operating range: Even if you get outside a sensor's rated measurement temperature, you'll still get a reading (either the max or the min) if you're within the sensors operating range. If you get outside this range, your sensor will fail like any other piece of electronic equipment. Choose appropriately when shopping.
  3. Measurement method: Most modern temperature sensors involve analog measurement. This achieved via resistance temperature detectors. These simply contain a material in the circuit that changes electrical resistance based on temperature. As temperature increases, a material with a negative temperature coefficient will show a decrease in electrical resistance. By comparing voltage output to the supplied reference voltage, you sensor can accurately estimate the surrounding temperature. The only other common method is a discrete temperature sensor, which often mirrors the function of a home thermostat. The measurement itself is analog inside the sensor, but the output is on/off (digital), usually at a specific "set point" temperature you've selected.
  4. Communication method: For many decades, output voltage was the standard way to send an analog from a sensor to a monitoring device (RTU, PLC, etc.). For low-level systems inside of computers/servers, this is still very common. More recently, new technologies like "D-Wire" discussed in the client conversation above have been released. These involve digital communication from the sensor that eliminate voltage degradation on long wire runs and the effect of signal noise. It's also possible to send digital data wireless in applications where you may not want to run wires, either to avoid install/trenching expense or to create an air gap around a propane tank or other explosion risk.

Temperature Sensors are Recognized as Mission-Critical by Major Companies and Government Agencies

I had an exchange with a government contractor via email recently about a bid from a big-name agency. This is the kind of household-name organization with ultra-strict security and uptime requirements. I could tell you more, but I'd have to... well... you know.

I'll tell you what: let me tell you half and then maybe just rough you up a little.

Here's an email exchange I had recently with a government contractor:

"Andrew, I'm having trouble finding the part number D-PK-MODBX-12016. Are you able to provide a quote for this item, Qty 3?"

DPS sells direct and frequently creates new products, so contractors often struggle when first calling their typical distributors. I replied:

"We sell the MODBX to a pretty select group of customers (it was developed for one in particular), so I have more insight into the use case than is probably typical for a manufacturer. I'm a bit concerned by your request for: D-PK-MODBX-12006. Although we initially sold that to our primary customer for the MODBX product, we later settled on a package that also includes two AC wall transformers and a temperature+humidity sensor. That package is: D-PK-MODBX-P0001.00003. I've emailed the end user to clarify, but there is a large time difference. I haven't heard back just yet. Do you have any insight into this? I'm happy to quote you whatever you'd like. It won't do either of us any good to quote something that doesn't do the job."

My client responded:

"Thanks for the information/insight! I've reached out to the end user and asked if they want to go with the previous MODBX requested or the new package that includes the wall transformers and sensor. As soon as I hear back, I will let you know how they want to move forward. Thanks again for your help!"

Then, later:

The customer came back and stated that they need the bundle part: D-PK-MODBX-P0001.00003."

Live Demo: D-Wire Temperature Sensors:

As you can see, temperature monitoring is taken very seriously in large, important organizations.

Get Help Planning Your Remote Monitoring Project

If you want to remotely monitor temperature at a remote facility, it's a fair bet that you want to (or should want to!) monitor other things at that site, too.

You have important equipment that deserves good monitoring. This can include self-reported device failures, but also environmental levels like humidity, air flow, hydrogen accumulation (from batteries), and water leaks.

You really need to be working with someone who has done this before.

Fortunately, DPS experts are available at no cost to you. Of course, we hope you take a look at our sensors and RTUs, but that's ultimately up to you.

Give me a call at 559-454-1600 to get started. You can also email me at sales@dpstele.com

Andrew Erickson

Andrew Erickson

Andrew Erickson is an Application Engineer at DPS Telecom, a manufacturer of semi-custom remote alarm monitoring systems based in Fresno, California. Andrew brings more than 17 years of experience building site monitoring solutions, developing intuitive user interfaces and documentation, and opt...