The UTA is a special purpose amplifier that converts the microamp-level current output of LI-COR light sensors to a corresponding signal voltage. The UTA can be configured at EME Systems or by the end user for any one of the three standard LI-COR sensors, and for any one of four popular output voltage ranges, through the manipulation of two dip switches. The UTA provides a simple interface between LI-COR sensors and voltage input data loggers, chart recorders, HVAC, and greenhouse control systems.
LI-COR sensor Typical full sun response UTA output ( user selectable )
LI190 PAR sensor 10µA @ 2000 µE/m2s 1, 2, 5, 10 Volts out @ 12.5µA in
LI200 Pyranometer 100µA @ 1000 W/m2 1, 2, 5, 10 Volts out @ 125µA in
LI210 Photometer 40µA @ 100 klux (=9290 ftcd) 1, 2, 5, 10 Volts out @ 50µA in
Alternate gain settings are available on special order. One of the most popular alternate settings is a 2.5V output for use with Onset HOBO data loggers. The calibration tag provided by LI-COR with each sensor, in conjunction with the gain setting of the UTA, can be used to compute the light levels incident on the sensor with a high degree of accuracy.
On the UTA shown in the photo, the input cable from the sensor will enter through the gland nut to the left, and the output cable to the equipment will exit through the gland nut on the right. The UTA is also available with a BNC connector on the input.* The calibration label on the outside of the enclosure is marked with the applicable gain setting. Wiring instructions and calibration notes are included with each UTA.
* Some customers have ordered the UTA without a BNC connector even though they have a "sa" model LI-COR that has a BNC connector interface. They have done this either on purpose, because they want to take advantage of the waterproof nature of the gland nuts (as seen in the picture above), or they have done it by mistake. In either case, the BNC connector can be cut off of the LI-COR sensor and thus a "sa" model LI-COR sensor can be turned into a "sz" model LI-COR sensor. If you would like to make this conversion, download the PDF instructions on how to remove the BNC connector from a LI-COR sensor.
The LI-COR sensor connects to the input, either to the BNC connector (SA series) or to the two screw-down terminals (SZ series). If the internal terminals are used, the coaxial cable from the sensor passes through a gland nut. The output of the UTA connects to your equipment via a three-wire cable, with +V (5 to 25 volts DC), signal, and common. The three conductors attach to the color-coded terminals inside the UTA enclosure and the cable passes through a gland nut. In electrically noisy environments, it may be desirable to use shielded cable. The enclosure has mounting holes outside the gasket for securing the enclosure to a backplane if desired.
In order to convert the UTA's output voltage into the appropriate units of light, you will have to program your equipment to multiply the UTA output voltage times the conversion factor printed on the calibration tag that accompanies each individual LI-COR sensor, divided by the UTA's transconductance gain (Volts per µAmp). Refer to the calibration tag on your LI-COR sensor for the sensor conversion factor for your particular LI-COR sensor. If you configure the UTA jumpers yourself, please refer to the table below to find the transconductance gain corresponding to their positions.
Table of LI-COR sensor types vs. Transconductance gain for various switch settings. The calibration setting for each UTA is indicated on a label attached to the outside of the enclosure.
Switch 1 | 1 Volt fs 2 Volts fs 5 Volts fs 10 Volts fs }Switch 2 position
LI-190 | 0.080 V/µA 0.160 V/µA 0.40 V/µA 0.80 V/µA \
LI-200 | 0.008 V/µA 0.016 V/µA 0.04 V/µA 0.08 V/µA }UTA gain
LI-210 | 0.020 V/µA 0.040 V/µA 0.10 V/µA 0.20 V/µA /
Example 1: Suppose you will be using your UTA in conjunction with a Quantum PAR sensor (LI-190), and that the Quantum sensor calibration tag states a conversion factor of 187.5µE/m2s per µA. Suppose your data logger has a full scale input of 1.28 volts. You choose the 1.0 volt output setting for your UTA, with a transconductance gain of 0.08 V/µA (1st column, 1st row in the table). The conversion is: light level in µE/m2s = (UTA volts) * (187.5 / 0.08) = (UTA volts) * (2343.75 µE/m2s per volt) A 0.836 volt output signal would indicate 1959.4 µE/m2s.
Example 2: Suppose you will be using your UTA in conjunction with a Pyranometer sensor (LI-200), and that the Pyranometer sensor calibration tag states a conversion factor of 9.8 W/m2 per µA.. Suppose your meter has a full scale input of 5.0 volts You choose the 5.0 volt output setting for your UTA, with a transconductance gain of 0.04 V/µA. (3rd column, 2nd row in table). The conversion is: light level in W/m2 = (UTA volts) * (9.8 / 0.04) = (UTA volts) * (245 W/m2 per volt) A 5.0 volt output signal would indicate 1225 W/m2.
Example 3: Suppose you will be using your UTA in conjunction with a Photometer sensor (LI-210), and that the Photometer calibration tag states a conversion factor of 2.63 klux/µA. Suppose your recorder has a full scale input of 2.5 volts You choose the 2.0 volt output setting for your UTA, with a transconductance gain of 0.04 V/µA ( 2nd column, 3rd row in table). The conversion is: light level in klux = (UTA volts) * (2.63 / 0.04) = (UTA volts) * (65.75 klux per volt) A 1.25 volt output signal would indicate 83.4 klux. If you need units in foot-candles, 1 foot-candle=10.764 lux. Note: If you need more sensitivity at low light levels, choose a higher UTA transconductance gain. A more thorough description of the calculation process is described in he user's manual, available for download in PDF format, below.
It is also possible to calibrate UTAs to match an individual LI-COR sensors. For example, you might want the output of a UTA to be exactly 4.0 volts at 1000 watts per meter squared input. The special calibration done in the hardware, so that the individual calibration factor does not have to be entered in software. Please consult the factory or the instructions that come with the UTA amplifier.
<