Liquid Level Height Sensing UsingSelected Honeywell Board MountPressure Sensors: MPR, ABP, andTBP SeriesA Technical Note
This technical note demonstrates how to calculate the liquid level height in an unpressurized or pressurized container by using a board-mounted pressure sensor to measure the hydrostatic pressure.
EQUATIONS AND CONSTANTS
The full equation for pressure liquid level height versus pressure is:Liquid Level Height = Pressure Exerted/(Density * Gravitational Constant)However, for most applications at lower altitudes the simplified equation may be used:Liquid Level Height = Pressure/Specific Gravity, or H = P/SG where:H = Height, in inches, of the liquid being measuredP = Pressure, in inches of water (inH2O), of the liquid being measuredSG = Specific gravity constant of the liquid being measured (See Table 1.)Table 1: Approximate Specific Gravity Constants of Common Liquids1
|Specific Gravity Constant
|Water at 4°C
|Water at 20°C
|Ethyl alcohol at 20°C
|Isopropyl alcohol at 20°C
|Seawater at 25°C
FINDING LIQUID LEVEL HEIGHT
Using an unpressurized container
Using the equation H = P/SG, the two examples in Figure 1 show how different media at different pressures may have the same liquid level height. The pressure sensor shown is Honeywell’s ABP Series, enhanced accuracy, digital or analog output, compensated/amplified, basic board mount pressure sensor. The MPR Series is an I 2 C or SPI digital output sensor.
Figure 1. Two Examples of Finding Liquid Level Height Using an Unpressurized Container
|Example 1:Water at 4°C
|Example 2:Ethyl alcohol at 20°C
|H = 8.5 inH2O/1.00
|H = 6.7 inH2O/0.789
|H = 8.5 in
|H = 8.5 in
Using a pressurized container
Instead of using a single ported sensor, as was used in measuring the liquid level height in an unpressurized container, the pressure sensor used here needs to be a dual-ported, liquid media compatible (wet/wet) device. The sensor is mounted at the bottom of the container. This positioning gives a liquid-coupled measurement. Although a small air bubble may be present due to trapped air between the container and sensor, this method, for the most part, directly measures the pressure which indicates the height of the liquid.
Example: What is the liquid level height of a container whereP1 = 9.0 inH20, P2 = 0.5 inH20, and the liquid is water at 4°C?Using the difference between P1 and P2 as P, the equation in Section 2.0becomes: H = (P1 – P2)/SG:H = (9.0 inH20 – 0.5 inH20)/1.00H = 8.5 in
Figure 2. Pressurized Container
Due to the relative unavailability of dual-ported sensors tolerant of liquid media on both ports, two single-ported gage or absolute pressure sensors have traditionally been used in this situation. This method not only carries the cost penalty of having to buy two sensors instead of one but may also double the measurement error. For this reason, ABP Series and TBP Series differential sensors are preferred.
SOLVING FOR SENSOR PRESSURE RANGE WHEN THE CONTAINER HEIGHT IS KNOWN
Example: What is the required pressure range of a sensor if the maximum height of the container is 25 inches, and the liquid is water at 4°C?Solving for P, the equation in Section 2.0 becomes: P = H x SG:P = 25 in x 1.00P = 25 inH20A 25 inH2O full-scale pressure sensor is used (1 psi ≈ 27.7 inH2O at 4°C), such as the ABP Series liquid media capable sensor ABPANT001PGAA5 or MPR Series MPRLS0001PG0000SAB as shown in Figure 3.
The ABPANT001PGAA5 sensor provides an analog output proportional to the applied pressure in the container, from 0.5 Vdc (no pressure applied) and 4.5 Vdc (1 psi applied). In this example, the MPRLS0001PG0000SAB the sensor provides an SPI digital output proportional to the applied pressure of approximately 9430370 counts at 16 inches of water from the 24 bit ASIC when the water level is at 16 in.
Figure 3. Known Container Height
REMOTE OR TOP MOUNTING
This method, as shown in Figure 4, allows the sensor to be remotely mounted using tubing to make the connection between the bottom of the container and the sensor. A single piece of tubing, or a piece of tubing connected to a tube, is used to run from the bottom of the container to the sensor.The advantage of this method is that the media has an air column between it and the pressure sensor, helping to isolate the sensor from harsh media. A possible concern is that if the tubing interface or sensor has even a small amount of mechanical leakage, it can have a significant impact on accuracy, which shows up as drift over time of the measurement.
Figure 4 . Remote or Top Mounting
USING A TBP SERIES UNAMPLIFIED SENSOR
A TBP Series, compensated/unamplified basic board mount pressure sensor with the liquid media option, may be used to measure liquids that are compatible with silicone. Figure 5 shows an example circuit that may be used to amplify the output.Figure 5. Example Output Amplification Circuit
Example: What is the required value of Rg if P = 27.8 inH2O maximum, the sensor used is 1 psi, and the liquid is water at 4°C? Equations used:Rg = 100000/(Gain-1)Gain = Span/SignalTo account for part-to-part sensor and amplifier offset variation, use a 100 kOhm potentiometer to calibrate Vout:Vout = 0.5 Vdc (no pressure applied)Resulting amplifier output:P at 0 inH2O = 0.5 VdcP at 27.8 inH2O = 4.5 VdcSpan = 4.5 Vdc – 0.5 VdcSpan = 4 VdcTBPDANS001PGUCV full scale output = 1.5mV/V, or 7.5 mV when using a 5 Vdc supply:Gain = 4 Vdc/0.0075 VdcGain = 533Rg = 100000 Ohm/(533 – 1)Rg = 100000 Ohm/532Rg = 188.0 Ohm
SEK002 Sensor Evaluation Kit (See Figure 6)
The MPR Series and digital versions of the ABP Series may be used in conjunction with the SEK002 Evaluation Kit (purchased from Honeywell authorized distributors). The SEK002 allows the user to obtain sensor readings without needing to develop any code.The SEK002 is plugged in as a shield board to an Arduino™ Uno Rev3 Microcontroller Board. Honeywell evaluation software downloaded to the user’s PC controls the Arduino Uno Rev3 to take sensor readings that are then displayed on the PC’s screen.
Figure 6. SEK002 Sensor Evaluation Kit (Shown with ABP Series Sensor and Arduino™ Uno Rev3)
MPR Series sensor on breakout board (See Figure 7)
Selected MPR Series sensors (catalog listing contains a “B” suffix) are available mounted on a breakout board which allows easier connection to the SEK002 Sensor Evaluation Kit.
Figure 7 . MPR Series Breakout Board
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