Differential vs. Single-ended Measurements

Apogee SO series oxygen sensor

A couple of years ago, while talking to a customer who had a limited number of input channels on his datalogger, I suggested measuring an oxygen sensor using a single-ended channel instead of a differential channel. He questioned the suggestion, thinking that a differential signal can only be measured using a differential channel. I used this opportunity to educate him about the difference between differential and single-ended measurements. An oxygen sensor can indeed be measured using a single-ended channel, by wiring the low output of the sensor to ground rather than the low input on the differential channel, but there are some trade-offs. This blog post outlines the trade-offs between single-ended and differential measurements, and describes when differential is necessary and when single-ended can be used.

A differential voltage is “floating”, meaning that it has no reference to ground. The measurement is taken as the voltage difference between the two wires. The main benefit of a differential measurement is noise rejection, because the noise is added to both wires and can then be filtered out by the common mode rejection of the data acquisition system. Differential measurements should be used if the sensor is in a noisy environment or for sensors with output voltages susceptible to noise interference. For example, we recommend that the thermopile output from Apogee SI-100 series infrared radiometers should always be measured differentially because the small voltages are susceptible to noise.

A single-ended measurement is taken as the voltage difference between a wire and ground. The noise is only on the positive wire, and as a result, it is still measured along with the output voltage from the sensor. Some sensors, for example amplified versions of Apogee SP and SQ series sensors, only have a single output and must be wired into a single-ended channel. A sensor with a differential output can be wired for single-ended by wiring the low side to ground. This is usually done to reduce the number of channels needed to measure the sensors. It should be noted that some sensors (for example, the thermopile output from Apogee SI-100 series infrared radiometers) can output a negative voltage which means that the data acquisition system needs to be able to measure negative voltages.

It should also be noted that taking a single-ended vs. differential measurement might also be based on the data acquisition system or the cabling. Shielded twisted-pair cable is also very effective at reducing noise in the signal other types of cable might not have the same effect. The data acquisition system is also important to consider if it doesn’t have a very uniform ground the signal could be biased. In most cases, modern data acquisition systems have improved in this area and it isn’t as much of a concern.

It should be stated that differential measurements should always be used if there are enough available datalogger channels or if the data acquisition system cannot measure negative voltages. Table 1 (below) shows which measurement method should be used with specific Apogee Instruments sensors. Again, it should be remembered that any in the differential measurement column can be measured single-ended if the conditions warrant it.

Table 1: Apogee Instruments Sensors Output

Differential	Single-ended
SI-100 series - thermopile (single-ended measurement strongly discouraged)	SI-100 series - sensor body temperature
SP-100 series	SP-200 series
SO-100/200 series	SF-110
SQ-100/300 series	SQ-200 series
SU-100	ST-100

 

 

Skif Smith
Electrical Engineer

New Product: Radiation Frost Detection Sensor (SF-110)

At the beginning of each growing season, as leaves and buds begin to emerge, crops will be susceptible to frost damage. On clear, calm nights, leaf and bud temperature can drop below freezing even if air temperature remains slightly above 0 °C (see Figure 1 below). This is called a radiation frost and is due to the lack of air mixing (wind) near the surface, and a negative net longwave radiation balance at the surface (more longwave radiation is being emitted from the surface than what the surface is absorbing from the clear sky). Under cloudy and/or windy conditions, radiation frost events do not occur.



Growers have options for mitigating damage caused by radiation frost, for example, heaters and wind machines to heat and mix the air within the crop canopy. Although these preventative measures are effective, they are also expensive to operate, requiring significant amounts of fuel and/or electricity. Historically, growers have depended on air temperature measurements and weather forecasting to determine the need to initiate frost protection measures. Air temperature can be misleading because the same atmospheric conditions that cause radiation frost also cause crop temperature to drop below air temperature, but not always by the same amount [1]. Forecasting requires use of a surface energy balance model with multiple measurement inputs [2] [3]. An estimate of crop temperature, based on a direct temperature measurement, is a simpler, more straightforward method of accurately determining when crop temperatures drop below the freezing point.

Apogee Instruments is pleased to announce the release of our radiation frost detection sensor, the SF-110. The SF-110 is a combination of two temperature sensors (precision thermistors) in a single housing. One sensor is designed to mimic a plant leaf and the other a flower bud. The SF-110 provides close approximations to leaf and bud temperatures and can be used for prediction of frost on leaves and buds.

 

 
The temperature measurement range of the SF-110 is -40 to +70 °C with an accuracy of ± 0.1 °C from 0 to +70 °C. However, the sensor is intended for applications in cropped fields and orchards when temperatures will be near freezing, and where air temperature measurements are not a good predictor of frost formation (Figure 1). The SF-110 offers a simple and effective method of helping growers determine when to initiate expensive frost protection measures, thereby saving crops in addition to operating costs. For more information on the SF-110 please click here.

Figure 1: Leaf and bud temperature approximations measured with an Apogee SF-110 compared to air temperature (top panel) and wind speed (bottom panel) on the evening of October 26, 2011. Leaf and bud temperatures were both below air temperature after 6 PM (hour 18) and reached the freezing point well before the air.

[1] “Frost/Freeze Protection for Horticulture Crops”, URL: http://www.ces.ncsu.edu/depts/hort/hil/hil-705.html 

[2] Kala, J., T.J. Lyons, I.J. Foster, U.S. Nair, 2009. Validation of a simple steady-state forecast of minimum nocturnal temperatures. Journal of Applied Meteorology and Climatology 48:624-633.

[3] Lhomme, J.P. and L. Guilioni, 2004. A simple model for minimum crop temperature forecasting during nocturnal cooling. Agricultural and Forest Meteorology 123:55-68.

 

Jacob Bingham
Customer Support and Technical Manager

New Product: Horizontal FOV Infrared Radiometer (SI-1H1)

Apogee Instruments is pleased to announce the release of our horizontal field of view (FOV) infrared radiometer (IRR). Some of the features and uses are listed below.

SI-1H1

The SI-1H1 IRR is different from any other IRR that Apogee has manufactured in the past. It was designed with a unique rectangular FOV, intended specifically for far-field applications. Unlike previous IRRs, the SI-1H1 has two different half angles: 32° horizontal and 13° vertical; which will be described in more detail below. The sensor has an accuracy of ± 0.2 °C just as the SI-111 and SI-121 IRRs.

Field of View (FOV) 

Field of view (FOV) is typically reported as half-angle. Previously, all Apogee IRRs had an axisymmetric FOV, which depended on the model number. For example, the half angles for the SI-111, SI-121, and SI-131 are 22°, 18°, and 14°, respectively. As seen in the photo above, the SI-1H1 sensor has a rectangular aperture, creating an asymmetric FOV, with a horizontal 32° half angle for the lateral direction of the rectangular geometry and a vertical 13° half angle for the narrower dimension of the rectangular aperture. The figure below demonstrates these two different FOVs (full angle) from cross-sections of the SI-1H1 sensor.

Better Measurements 

The question arises: Why a rectangular slit instead of the usual circular aperture? When using an IRR, one must make sure that only the desired target is in the FOV, in order to make an accurate temperature measurement. For example, a problem some crop scientists face is accidentally having more than their crops in the FOV of the IRR. This could include the sky and/or other background objects (see figure below). Unfavorable images caught in the FOV of the IRR alter the desired temperature measurements and yield incorrect results. By adding a horizontal “filter”, the likelihood of capturing the background is dramatically reduced in certain applications. As expected, the aperture has baffles in order to deter reflected radiation from influencing measurements. Furthermore, the IRR can be mounted at greater angles of inclination from the ground. The SI-1H1 isn’t limited to cropped fields, since unique circumstances could require its use in many applications. In order to calculate the target area (based on the installation directions given below), please click here for a preconfigured Excel spreadsheet.
 

Installation in the Field 

With the horizontal aspect of this sensor, installation is critical and involves more than mounting the sensor at the correct inclination angle. We recommend the AM-210 Mounting Bracket which allows users to adjust the angle of the sensor with respect to the target area. The AM-210 accommodates the radiation shield designed for all Apogee infrared sensors.

Once the desired angle has been set, the sensor needs to be rotated along its main axis so that the rectangular portion of the sensor is aligned to the horizon. This orientation will minimize the likelihood of capturing sky or other background interference.

If you are not using the radiation shield or the AM-210 mounting bracket, the SI-1H1 has the standard mounting hole (¼ - 20), which is aligned with the horizontal aperture. If the sensor is installed against a completely flat vertical surface, using the mounting hole, the rectangular aperture will be aligned with the horizon.

Apogee is proud to offer another great product to our customers. We are continually focused on product innovation so you can “Make Better Measurements.” To learn more about the new SI-1H1 sensor or to place an order, click here.

 

Adam Del Toro

Mechanical Engineer

Better Know a Distributor - Conviron

The ‘Better Know a Distributor’ series highlights other companies that distribute and resell Apogee products. 

Conviron has been in business for almost fifty years and is known worldwide for its quality service and products. The following excerpt is Conviron’s company profile:

Established in 1964, Conviron is the world leader in the design, manufacture and installation of controlled environment systems for plant growth research. Headquartered in Winnipeg, Canada, Conviron employs a global sales, distribution, and service network. Our products can be found in more than 80 countries worldwide, with projects ranging from single-chamber installations to large-scale, multi-chamber plant growth facilities designed and supplied entirely by Conviron. Our innovative design and manufacturing expertise has established Conviron as the industry leader with products that are proven, reliable and robust. As an ISO 9001 company, our products meet universally recognized quality and safety standards. 


As a fully integrated supplier of controlled environment systems, our services encompass the entire project life-cycle - from project consultation to manufacturing, installation, commissioning, and on-going maintenance and service. Our specialized equipment includes reach-in chambers, walk-in rooms and research greenhouses that precisely control light, temperature, humidity, carbon dioxide and other gases, as well as other environmental conditions. With a staff that includes a highly qualified design group of specially trained engineers, technicians and controls experts, Conviron is well equipped to supply both standard and custom applications for our clients.
www.conviron.com

Conviron has utilized Apogee quantum sensors in their cutting-edge environmental growth chambers for nearly 12 years. Throughout that time, Conviron has not only provided Apogee with a high level of independent validation of our quantum sensors, but they have also acted as an indicator for when certain lighting technologies start to become obsolete while others take hold of market demands. An example of this comes from a couple of years back when Conviron had made the switch from using T12 cool white fluorescent lighting in their growth chambers to the more efficient T5 lamps. This led to the question of whether we could supply our quantum sensor with an electric calibration under T5's rather than our standard T12 calibration. After favorable testing and data comparison with Conviron, we ultimately made a system-wide changeover to T5's for all of our electric calibration quantum sensors. It was later announced that the magnetic ballasts used in many T12 fixtures would no longer be produced for commercial or industrial applications, and as of July 2012 many T12 lamps will be phased out of production completely.

Apogee is happy to work with a knowledgeable and progressive company like Conviron. We are proud of our products used in Conviron chambers and we are proud of the contribution they make to science and research across the globe. For more information about Conviron’s products you may visit their website at: www.conviron.com.
 

 

Jacob Bingham
Customer Support and Technical Manager