LP7 Liquid Nitrogen Level Transmitter
The Teragon LP7 cryogenic level transmitter provides a 4-20 mA current signal proportional to the height of the liquid nitrogen level. The transmitter uses a capacitive probe which is sensitive to the difference in the dielectric constants of liquid and gaseous nitrogen. This capacitive difference is measured and converted to a linear current signal proportional to the level height. The advanced electronic design of the LP7 results in superior long term accuracy and temperature stability. The 4-20 mA output provides excellent noise immunity, even when transmitting over long lead lengths. The transmitter allows wire runs of over 1000 feet with virtually no signal degradation. The LP7 operates using a simple twisted pair of wires and is readily interfaced to industrial process controllers, monitors and computer systems. The transmitter can be installed on a threaded boss using a tube fitting with an NPT adapter. As an option, the probe can be manufactured with a tube fitting adapter in place. The LP7 comes factory calibrated to your specified sensing length. If desired, the transmitter can be recalibrated in the field using its independent offset and range adjustments. Standard features of the LP7 include: Sensing Lengths from 4 in. to 9.5 ft. True 2-Wire Operation Wide Voltage Range (10-30 VDC) Reverse Voltage Protected Less than 1% Nonlinearity Compact Size Low Cost Easy installation 1 Year Warranty Made in the USA The LP7 is designed to be a simple, reliable, and economical means of monitoring liquid nitrogen. The LP7 can also be configured to operate with other cryogens and liquids (Ar, H2, CH4, Freons, CO2, LNG, etc.); please call for more information regarding your specific application. LP7 Specifications Excitation: 10-30 VDC, reverse polarity protected Output: 4-20 mA, two-wire Nonlinearity: Less than 1% full scale Sensing Lengths: 4 in. (100 mm) to 9.5 ft. (2900 mm) Wetted Parts: 304 Stainless Steel and PTFE Cabling: 3 ft. of 3 conductor, 22 AWG, red = V+, black = V-, and green = case Electrical Isolation: 1 MegaOhm min. from either V+ or V- to probe case 1 year warranty Specifications subject to change without notice. Download the LP7 User Guide             LP7 User Guide (66 KB)

Application Notes

The probe diagram shows the key dimensions of the LP7 transmitter. The probe is a cylindrical capacitor with a vent to allow the liquid to rise between the inner and outer plates. The Sensing Length is defined from the bottom of the probe to the vent. This is the customer-specified length which, when filled with liquid, results in a 20 mA signal. The vent should be located within the liquid nitrogen reservoir. The probe should not be mounted flush against the floor of the reservoir. A minimum clearance of 0.040" (1 mm) is required at the end of the probe to allow the liquid to enter.

The figures below illustrate two typical applications where the LP7 is used to monitor liquid level. The first approach uses a Teragon PM70 Digital Panel Meter which is powered from 115 VAC. The PM70 provides 24VDC excitation for the LP7, measures the LP7 current, subtracts the 4mA offset, scales the result, and then displays it with 7-segment LEDs. In the second approach, the LP7 is powered from 12VDC and the current is displayed with an ammeter. In both of these applications, the LP7 is connected using a simple twisted pair of wires. Normally the case (green) wire is left unconnected.

The LP7 current can also be measured by converting it to a voltage with a resistor as shown below. For proper operation, a minimum of 10 volts is required between the LP7's V+ and V- inputs. This places limits on the resistor value for a given loop voltage, as shown in the graph below. For a full-scale 20 mA signal, the maximum loop resistance is given by:

LP7 Options

The transmitter can be assembled with a coaxial BNC connector instead of the 3 foot cable. V+ is the central conductor and V- is the outer conductor. Threaded adapters are available in stainless steel with straight or pipe threads. If you require an adapter, please contact Teragon to determine the appropriate Insertion Length. The LP7 can be manufactured to operate with other cryogens and dielectric liquids including argon, hydrogen, methane, freons, LNG, and CO₂.

U.S. Pricing

Please specify the required Sensing and Insertion Lengths (see probe diagram). The minimum Sensing Length is 4 inches (100 mm). The Insertion Length must be at least 1 inch (25 mm) longer than the Sensing Length.

LP7 with Insertion Length up to 12 inches (305 mm): $1,020

For each additional inch (25 mm) of Insertion Length: $5

1/4 inch MPT adapter: $30

Prices are FOB San Francisco and are subject to change.