Tag Archives: Analog

Water level measurements

Measuring liquid level is quite a common task.

There are numerous ways of doing this.

Probably the most known of them is the fuel level measurement in cars.
This post is about a tank water level gauges incorporating different types of sensors.
In principle the water level can be determined by measuring the water pressure at the bottom of the tank. The higher the pressure, the higher the water level.
wat1

Measuring water level with water pressure sensor can cause errors.

Unfortunately this method has some oversimplification. The environmental air barometric pressure hasn`t taken into account.

As weather continuously changes, so does the ambient air pressure. There is an average variation of 3 kPa, which corresponds to roughly 300 mmH2O measurement (even if the actual water level in the tank is unchanged).

This calls for measuring the barometric pressure. Instead of measurung the ambient air pressure, an alternative measurement method is used.

The ambient air pressure can be continuously taken into accout if a differential pressure sensor is used. One port of the gauge goes to the bottom of the water tank, while the other port stays open. This way the open port “monitors” every change in the ambient air pressure.
This is useful for measuring rainwater storage tanks.

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Measuring water level using differential pressure sensor

Due to the fact that integrated differential pressure sensors have some long term drift, optionally the measurement system can be completed with water level switches. One at the bottom and one at the top of the tank. These two switches create a so called auto tune option: they provide zero and full range levels, the differential pressure’s output value can be adjusted if either switch operates (changes state).

wat3

More accurate measurement using level switches

An alternative method is the floating ball water level measurement. Basically the angle of the floating ball’s stick is directly related to the water level. The angle can be measured with a potentiometer, but the casing must be watertight, which is difficult to achieve because of the moving elements.

wat4

Floating ball water level measurement with potentiometer

Another solution is to use an acceleration sensor. The sensor has to be mounted to the moving rod, so the angle of the rod can be calculated. Having the angle information the water level can be determined.

There can be additional level switches in the system to provide more accurate measurements, or to serve “underdraining” or “overfilling” signals.

wat5

Floating ball water level measurement with acceleration sensor

These techniques don’t provide highly accurate measurements, but are accurate enough for rainwater of greywater tanks level measurements.

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Programmable, 2 wire, relay controller

Although the previous posts described the two wire, 4-20 mA transmitters’ internal workings, sometimes a simpler action is adequate.

The first and pretty obvious example is the temperature switch (which is actually a bimetal). It doesn’t need any power supply, it makes (or breaks) a contact if the temperature reaches a certain threshold.

Another example is the float switch used in water tanks. it also makes (or breaks) a contact if the water level reaches the given level.

If the power they can deliver is insufficent, then these “sensing devices” usually operate a relay, and the relay’s contact switches on the higher power mechanism.

For example a reed switch based float switch (used in a water tank) can only switch a maximum of 0.5 A, but the water pump (operated by this float switch) needs multiple amps for its operation. In this case the float switch operates a relay, and the pump’s multiple amps flow through the relay’s contacts.

On the other hand there are more complex issues to be solved, as simple as can be.

Let’s see the following situation: There is a garden with an automatic sprinkler system. This system has a rainwater storage tank, a water pump, sprinklers and a soil humidity sensor. If the soil moisture falls below a certain level then the sprinklers begin to operate, and if the moisture rises to an other certain level the water pump stops.

It is a simple hystheresis function with adjustable low (turn on) and high (turn off) threshold.

Continue reading Programmable, 2 wire, relay controller

Universal, 4-20 mA, two wire industrial transmitter

This post unifies together the following posts:

Industrial, 4-20 mA current loop, measuring basics I.

Industrial, 4-20 mA current loop, measuring basics II.

Industrial, 4-20 mA current loop, measuring basics III.

Industrial, 4-20 mA current loop, measuring basics IV.

Internal workings of process controllers I

. Internal workings of process controllers II.

There are a lot of types of transmitters nowadays. Usually a separate one (a specific type) is needed for thermocouple measurement, an other one (an other type) is needed for level measurement…

The basic concept of this post is to present a way for a universally usable transmitter, which can accept almost any type of input (Pt100, pH probe, rotation, level, light…) and produce a configurable, standard 4-20 mA output.

Continue reading Universal, 4-20 mA, two wire industrial transmitter

Predictive digital filtering for first order systems

Today measurements must be fast, cheap and accurate at the same time.
These requirements are not easy to achieve.

This post describes a simple but very efficient software method to make a slow measurement faster.

Let’s look at a simple temperature measurement example.

The temperature sensor sits in a protective metal cover.

This sensor is at room tempeature, and we would like to measure the temperature of boiling water.

The example is very simple but it will clearly show the operation of the computing method.

When we put the sensor into the boiling water, we know that the measured value jumps from 20 °C to 100 °C.

but the sensor’s output rises slowly since the protective cover needs time to heat up.

This heat up phenomena acts as a first order filter function.

The first order filter function is described as:

f
where T(t) is the measured temperature at a given t time, T(final) is the final value (100 °C in this example), T(0) is the starting temperature (20 °C in this example) and Tau is the time constant of the filter.

It may seems a little complicated, but plotting a graph gives a very clear demonstration of how it works.

comp1

The problem here is that we know that the measured temperature momentarily rises as the sensor goes into the boiling water, but the measurement shows a slow rising to the final value.

Continue reading Predictive digital filtering for first order systems

Industrial, 4-20 mA current loop, measuring basics IV.

Continued from the previous post

Although the title says “basics”, here are some advanced circuits about the 4-20 mA current loop transmitters.

In the first example a USB connection is shown. Nowadays almost every transmitter incorporates a microcontroller. The transmitters can be programmed through their display, or through HART protocol, or via DIP switches.

Transmitters with HART communication are expensive, using displays also increases the cost, and using DIP switches provides a not so flexible way of configuration.

Some manufacturers use USB as a programming / configuration interface, but most of them are not loop powered, so they need a separate power supply, which increases the cabling costs.

The following solution shows a way of how to include the USB communication in the transmitter design, while the transmitter is powered from the loop.

Continue reading Industrial, 4-20 mA current loop, measuring basics IV.

Industrial, 4-20 mA current loop, measuring circuits basics III.

Continued from the previous post

There’s a major problem with the circuits shown earlier. They all have a capacitor between their output pins (marked C3 in the following picture).

4_20ma_2wire_transmitter_capacitorThe capacitor shown in the red circle

This capacitor is needed by the voltage regulator (LDO) and provides other features like filtering and stability, so it cannot be omitted.

Let’s see the following situation:

2-wire-transmitter_simple2 wire transmitter elements in the measuring loop at startup

We have a 24V power supply with a maximum of 2A output current, a 2 wire transmitter and a 4-20 mA analogue PLC input (with measureing resistance of 50 ohms).

The power supply is switched off, so the transmitter is de-energised (not working because the lack of power). At the time we switch on the power supply, the transmitter’s internal (output, C3) capacitor is beginning to charge and actually it is functioning like a short circuit. For a little time, the power supply’s output 24V falls across the transmitter’s output resistor (10 ohms) and the PLC’s input 50 ohms, which means 400 mA current.

Continue reading Industrial, 4-20 mA current loop, measuring circuits basics III.

Industrial, 4-20 mA current loop, measuring circuits basics II.

Continued from the previous post.

The same priciple is true for the followings, temperature to current transmitter. In this case the input voltage is propotional to the measured temperature, not the rotation.

4_20ma_2wire_transmitter_tempTemperature input, full analog, 2 wire, 4-20 mA loop powered transmitter

U3 is a low cost, NTC based, integrated temperature sensor.

The next schematic is a differential pressure to current transmitter.

4_20ma_2wire_transmitter_pressureDifferential pressure input, full analog, 2 wire, 4-20 mA loop powered transmitter

This circuit needs a little explanation.

Continue reading Industrial, 4-20 mA current loop, measuring circuits basics II.