Liquid sensors require a media-compatible, solid-state pressure sensor. The pressure range of the sensor depends on the height of the column or tank of fluid that must be sensed. Described here is a way to sense water height in a tank or column using EG&G IC Sensors’ Model 90 stainless steel diaphragm, 0-15 psig sensor.
Because large chemical or water tanks are typically located outside in “tank farms”, it’s insufficient to provide only an analog interface to a digitization system for level sensing. This is because the very long wires required to interconnect the system cause IR drops, noise, and other corruption of the analog signal. The solution is a system that converts the analog-to-digital signals at the sensor. In this application, a “liquid height-to-frequency converter” was implemented.
The analog front end of the system includes the LT1121 linear regulator for powering the system (Fig. 1). The LT1121 is a micropower, low-dropout linear regulator with shutdown. For micropower applications of this or other circuits, the ability to shut down the entire system via single power supply pin allows the system to operate only when taking data (perhaps every hour), conserving power.
The LT1121 (U3) converts 12 V to 9 V to power the system. The 12 V may be obtained from a wall cube or batteries. The LT1034, a 1.2-V reference, is used with U1D, 1/4 of an LT1079 quad low-power op amp, to provide a 1.5-mA current source to the pressure sensor. The reference voltage also is divided down by R4, R5, R6, and the 10k potentiometer. It’s used to offset the output amplifier (U2A) so that the signals don’t swing around the supply rails.
Op amps U1A and U1B (each 1/4 of an LT1079) amplify the bridge’s pressure-sensor output and provide a differential signal to U2A (an LT1490). U2A must be a rail-to-rail op amp; the system’s analog output is taken from U2A’s output. The pressure change is independent of diameter of the water column, so that a tank of liquid would produce the same resulting output voltage as a column of the same height.
The remainder of the circuitry allows for transmission of analog data over long distances (the circuit was designed by Jim Williams) (Fig. 2). The circuit takes a dc input from 0 V to 5 V and converts it to a frequency. For the pressure circuit in Figure 1, this translates to approximately 0 Hz to 5 kHz. The voltage-to-frequency converter in Figure 2 has very low power consumption (26 µA), 0.02% linearity, 60 ppm/°C drift, and 40 ppm/V power supply rejection.
When operating, C1 switches a charge pump, consisting of Q5, Q6, and the 100-pF capacitor, to maintain its negative input at 0 V. The LT1004s and associated components form a temperature-compensated reference for the charge pump. The 100-pF capacitor charges to a fixed voltage. Consequently, the repetition rate is the circuit’s only degree of freedom to maintain feedback. Comparator C1 pumps uniform packets of charge to its negative input at a repetition rate precisely proportional to the input-voltage-derived current. This action ensures that circuit output frequency is determined strictly and solely by the input voltage.
Figure 3 shows the output frequency versus column height for two different Model 90 transducers. Note the straight lines, which are representative of excellent linearity.