June 2009


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Selecting Sensors for BAS

A tutorial on choosing the right temperature sensor for the right job


Steven R. Calabrese

Steven R. Calabrese
Control Engineering Corp.

Contributing Editor

Temperature sensors used in the Building Automation Systems (BAS) industry are typically nothing more than temperature controlled resistors, or “thermistors”. When you think of a resistor, you picture an electronic component with some colored bands wrapped around it, and a wire sticking out of each end. A thermistor is comparable in size to it’s electronic cousin, and similar in function, however instead of offering a fixed resistance, it’s resistance varies as a function of the temperature surrounding it. The resistance of the device is inversely proportional to the temperature being sensed, in that as the temperature rises, the resistance of the thermistor drops. Today’s thermistors offer tight tolerance and good stability and repeatability at a small price, making them extremely applicable to the HVAC industry and building automation and control.

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Below is a table showing how resistance varies with temperature for a typical thermistor used in the BAS industry. This particular sensor is characterized as having 10,000 ohms of resistance at 77 degrees Fahrenheit, or more simply, 10K ohm @ 77 deg. F. There are other types of thermistors with different temperature-resistance characteristics, but we’ll go with this one for our discussion.

Temperature (degrees F.)

Resistance (K ohms)





























This column is all about sensor selection, so we’ll be exploring the various applications and discussing which type of sensor to use in which application. Bear in mind that all of the following sensor styles utilize the basic thermistor as the actual sensing element.

Outside Air Sensors

Outside air temperature sensors are designed and constructed to withstand the rigors of outdoor life. The thermistor is protected from the elements, hidden inside an aspirated enclosure, aspirated being a fancy word for ventilated, meaning simply that the enclosure has some holes in it to allow the sensor to breathe (huh?). The electrical connection box is weather-tight, and the entire assembly is rated for outdoor installation.

Well I just got through saying that this column is all about selection, however I would be remiss if I didn’t at least touch on how and where to mount the outside air sensor. The assembly is to be mounted on the side of the building, preferably the north exposure, or in a location that doesn’t get much direct sunlight throughout the course of the day. An ideal situation is when the building has a mechanical equipment penthouse. The sensor can be mounted on the north outside wall of the penthouse, up around eye’s height or higher, to keep it isolated from accumulated rainwater, tampering, or damage from whatever. The north exposure is good, especially during the fall-winter-spring months when the sun is doing its thing in the southern portion of the sky from morning to evening.

Not to change the subject, but outside air sensors are also available in the combo temperature-plus-humidity variety. I only mention this because if you’re installing an outside air temperature sensor as part of a BAS, then you may as well go the extra yard and pop for the combo sensor, as outside air humidity plays an important part in temperature control, perhaps just as much as outside air temperature does, if not more in certain situations. Makes good “sense” to include it (bad pun), even if the specification doesn’t explicitly call for it.

Space Sensors

Space temperature sensors are offered in a vast variety of styles, from the decorative enclosure types with all the bells and whistles, down to the bare-bones stainless plate sensors with no features whatsoever. Most of these are available as “vertical mount”, meaning that the long dimension of the device is up and down, just like a single-gang light switch. There are, however, adaptor plates, or “goof plates”, that are available, for instance, to use in a retrofit scenario to cover a hole left from a horizontal mount thermostat. Or simply for when you “goof” and orient the wall hole incorrectly (that never happens, right?).

Common to all of these space sensor types is, of course, the thermistor. From there the options run as follows: setpoint adjustment level or buttons, unoccupied mode override pushbutton, communication jack, and space temperature indicator. If the sensor is a “smart sensor” meaning that is it specifically designed to “communicate” with a digital controller, then it may also be offered with an LCD readout that can display space temperature, setpoint, and even outside air temperature, provided that there is an OA temperature sensor hooked into the BAS somewhere along the line.

Selecting the appropriate sensor depends on your application and requires some insight. Unless specified for you, you will need to make an informed decision on what to go with. Decorative enclosure or low profile plate sensor? Is color important? How about visual indication? Does the occupant need to see what the space temperature is? Occupant adjustability important? What about after-hours use of the space? Do you need to provide an override button? How about a tamper proof plastic guard or cage guard?

These questions and others need to be answered before selecting the right space sensor for the application. So do yourself a favor and give it some thought, and don’t be afraid to ask some questions to help narrow down your choices. The last thing you want to do on a large project with dozens and dozens of these suckers, is to make the wrong selection, only to find out after they’ve all been installed and wired!

[an error occurred while processing this directive] Duct Sensors

We restrict the discussion on duct sensors to the two most popular varieties: probe sensors and averaging sensors.

Probe sensors are constructed using a thermistor and a rigid metal probe. The thermistor is stuck inside of the probe, toward the end of it, and the probe gets inserted into the duct. The probe sensor is a good general-purpose sensor for measuring airflow temperature in a duct. Probe sensors range in length from 4”, up to 18” and longer, if custom made. Select your probe sensor so that the end of the probe reaches upwards of halfway across the duct, as it is toward the middle of the airstream that you will see the highest velocity, and a safe bet to get your most accurate temperature reading. For VAV and fan powered box applications, an 8” probe is a good standard size to go with, unless you have some uncommonly large or small VAV boxes.

Averaging sensors are constructed using a bendable metal tubing. Traditional construction dictates the use of multiple thermistors, spaced equally within the tubing (one every two to three feet), and wired in a series-parallel configuration. However some manufacturers have introduced continuous sensing technologies that don’t use discreet thermistors. Anyway, averaging sensors pick up where probe sensors leave off, in terms of duct size. For ducts over four feet wide, your best bet is to go with an averaging sensor. The sensor mounts in a serpentine fashion, back and forth across the cross section of the duct and supported at each point it meets the duct. Rule of thumb states that you should have one foot of sensor length per square foot of duct. For example, a 58 x 20 size supply air duct is approximately 8 square feet, so select an 8’ averaging sensor.

Averaging sensors are also useful in the mixed air sections of air handling units, where there may be stratification, which is a fancy way of saying that the air hasn’t had a chance to thoroughly mix and be of uniform temperature. This is often the case where the outside airstream meets the return airstream. So use an averaging sensor in the mixed air chamber to ensure a good, average reading of the entire volume of air passing through the chamber.

All of these sensors will typically be equipped with a “wiring box”, if you will, or an enclosure that allows access to the sensor leads. The box has conduit knockouts for affixing flexible conduit, and can be a simple plastic enclosure, a metal 2” x 4” “handy box” as you would find behind a wall switch, or a ruggedized weatherproof enclosure for outdoor applications. Of course you can order the sensor without the wiring box, in which case the leads protrude from the end of the probe, however the box does make for a neater installation, and is mandatory if the project has a requirement for all wiring to be in conduit.

Pipe Sensors

Immersion sensors are similar in construction to duct probe sensors, and are available in different lengths from 2” up to 8”. These sensors are designed to be inserted into a pipe well, the well being made of brass, or more typically, stainless steel, and welded into the pipe such that the probe portion will be “immersed” in the liquid contained within the pipe.

Insertion length is generally dictated by pipe size. Two-inch and four-inch immersion sensors cover the vast majority of HVAC applications, although there may be instances where you would need something larger (big pipe!). Not much more to say about these, except that, upon installation, a small amount of thermal conductive grease should be smeared into the well before insertion of the sensor, to ensure good thermal transmission and accurate temperature measurement.

Tip of the Month: Need to measure temperature in pipes smaller than what an immersion sensor would allow you to? Not to worry. Use a strap-on sensor. Basically a thermistor impaled to the back of a bendable metal plate that can conform to the radius of the pipe, and held in place with a hose clamp, it’s good for those “here-and-there” small pipe applications, and also when you “forget” an immersion sensor, only to discover after-the-fact that it’s needed (oops!). As far as accuracy goes, it’s surprising to find that these strap-on sensors, when installed correctly, can be extremely accurate and reliable. So don’t sweat it if your hydronic system has just been filled and you find yourself needing another sensing point. As long as the consultant will approve of it, this one’s got you covered!

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