2.4 DUAL LEVEL SENSING

When two trip levels are desired, for

example for high-low limit sensing, the

electrode or probe set should have two

distinct tiers. A typical twin external

electrode is shown in Figure 2-3 (they are

connected together to the sense line);

typical internal twin electrodes are shown

in Figures 2-6, 2-7, 2-9, and 2-11. The

response of a properly constructed 2-tier

probe is shown in Figure 2-3.

Dual level electrodes should have an

approximately 3:1 surface area ratio or

more from T2 to T1; that is, the surface

area at T2 should be at least 3x the

surface area of the electrode at T1. There

is no penalty for making T2 excessively

large. The high ratio is required to

overcome the QT114's decreasing gain

with increasing Cx load (Figures 4-1, 4-2).

With internal dual-level probes where T1

and T2 are substantially separated, the

intervening connection between the two

levels should be more thickly insulated, for

example with a thick plastic spacer, and

any remaining internal gap inside the spacer should be filled

with silicone sealant or epoxy. This will help to prevent the

signal from rising much between the two levels, thus

preserving a crisp bi-level response like that shown in Figure

2-3.

2.5 GROUNDING CONSIDERATIONS

In all cases ground reference coupling to the fluid must be

made. In aqueous fluids, this can simply mean connecting

the metal vessel to circuit ground, or inserting a bare metal

element into the bottom of a plastic or glass vessel. The

degree of galvanic contact is not critical, so scale and

corrosion on the ground electrode are not of great concern

especially if the 'connection' to the fluid is substantial

enough.

If direct electrical contact to the fluid is not possible, a large

piece of external metal can be bonded to the outside of the

vessel and grounded. Once this is done, the signal should be

monitored while the vessel is touched by hand; if the

grounding is sufficient, the signal will not move or will move

only slightly.

Very large vessels, even if not grounded, often do not require

additional provision for grounding since the bottom surface

area and free-space capacitance of the tank may be

sufficient for ground return coupling.

In some cases (windshield washer tanks on cars for

example) there will exist a water path to a chassis-grounded

fitting somewhere downstream of the tank, or the water path

may be labyrinthine enough to provide enough capacitive

coupling to the grounded chassis even if it does not make

galvanic contact. In these cases no further provision for fluid

grounding is required. Simple experimentation will easily

determine whether the existing amount of parasitic coupling

to ground is enough to do the job.

In the case of coaxial probes, the ground connection is

inherent in the outer cylinder and no further ground

connection is required.

3 - PROCESSING &

CIRCUITRY

3.1 SLOSH FILTER

It is desirable to suppress rapid, multiple

detections of fluid level generated by the

surface movement of the fluid, for

example

in

a

moving

vehicle.

To

accomplish this, the QT114 incorporates a

detection

integration

counter

that

increments with each detection until a limit

is reached, after which point one of the

OUT lines is activated. If during a

detection ‘event’ the fluid level falls below

the electrode level (signal rises above a 'T'

point in signal counts), the counter

decrements back towards zero. Over a

long interval the up and down counts will

tend towards either zero or the limit, with

the result being a statistical function of the

number of detections vs. non-detections. If

on average there are more detections than

non-detections, the counter will eventually

make its way to the limit value and an

OUT line will activate.

Once a detection has been established,

the counter must find its way back to zero before the affected

OUT line goes inactive, via the same process. Although the

counter has a nominal reaction time of 15 seconds, in some

cases it may take several minutes before the outcome is

resolved depending on the violence of the fluid surface. If the

fluid surface is stable however, it will only require 15

seconds to change the state of an OUT line.

Both OUT1 and OUT2 have their own independent slosh

filters. Both are enabled or disabled in unison by strap

option, pin 4, 'FILT' as follows:

FILT = Gnd

Slosh filter off

FILT = Vcc

Slosh filter on

FILT strapping can be changed 'on the fly'.

3.2 CALIBRATION

Both the T1 and T2 trip point values are hardwired internally

as functions of counts of burst length. Sensitivity can be

altered relative to these trip points by altering electrode size,

geometry, degree of coupling to the fluid, and the value of

Cs. Selecting an appropriate value of Cs for a given

electrode geometry is essential for solid detection stability.

The QT114 employs dual threshold points set at 250 and

150 counts of acquisition signal. The signal travels in a

reverse direction: increasing Cx reduces the signal counts;

as a result, 250 counts of signal corresponds to the most

sensitive or ‘lower’ setting (T1), and 150 the least sensitive

'upper' setting (T2).

The baseline signal count when the electrodes are 'dry'

should begin at over 300 counts or more if possible. With a

small, weakly coupled electrode the baseline signal can be

trimmed to be closer to the 250 mark with a potentiometer to

provide a higher apparent gain by closing the gap between

the baseline and T1 (see below). The spread between T2

and T1 is fixed and cannot be separately trimmed.

Increasing Cs will increase the baseline counts, while

increasing Cx will decrease it. When optimally tuned, each

threshold point will be symmetrically bracketed by signal

- 6 -

Figure 2-12 A 2-tier spiral wire

probe with ground rod