Probe Port in Detail

The HYDROS Control system requires analog inputs for connecting pH, ORP, and other analog probes. Up until now, poor performance due to the lack of noise immunity, extremely high input current, and many design flaws have plagued these inputs with constant issues. The HYDROS Control system uses several techniques to achieve accurate, noise-resistant, and highly repeatable input readings from a pH probe.

Characteristics of a pH Circuit
A pH probe is a glass or plastic probe that is placed in a solution to measure the pH of that solution. In the aquarium industry, it is common to measure tank pH as well as the pH of some media reactors.
 
The probes produce a small DC voltage that is proportional to the pH of the solution. The volt reading is quite tiny (theoretically 59.16 mV per pH unit) and is bipolar (theoretically pH 7.0 produces 0 volts output). At first glance, it would appear that all you need is a voltmeter and a conversion chart. However, this is not the case.
Several factors make pH measurement tricky. The first is the source impedance of the pH probe, which typically runs in the 10 MΩ to 1000 MΩ. This extremely high source impedance requires that the input circuit have an even higher input impedance, on the order of 1 TΩ (that is 1 million ohms). This high impedance is very difficult to achieve and results in a circuit that is exceptionally subject to noise from surrounding electrical equipment.
 
Related to the input impedance is the input bias current. Because the internal source impedance of the probe is so high, the input bias current of the measurement input must be incredibly low; in the order of 100 fA (One Femptoamp is one quadrillionth of an amp). It is hard to conceive of how small of a current this is, but as a reference, one fA is equal to 62,420 electrons going by per second. If you were to put your thumb and index finger on the ends of a AAA battery, the current flowing through your fingers would be 300 million times the amount described here.
 
In the aquarium industry, pH probes connect using a BNC connector which makes things even worse. BNC connectors are designed for RF signals and not ultra-high impedance DC signals. A better connector would have 3 or 4 contacts and would allow for a balanced pair. However, since this connector has become a standard, it must be worked around.
Galvanic Isolation
Galvanic Isolation is an elegant term for circuitry that completely isolates the pH input circuit from all other electronics in the system. Galvanic Isolation provides two benefits. One is safety, though this is a minor advantage. The second and most important is noise immunity. Aquarium systems include lots of high power electronics such as pumps, heaters, and lights. Even the best of these devices leak some amount of electrical current into the water. The typical electrical cable used for a submersible pump or heater has a contact-to-water resistance of roughly 270 MΩ (million ohms). If you apply a 110V signal to that cable a current of 0.4 uA (micro amp, one millionth of an amp). While this current is tiny, it is roughly 41 million times the current coming from a pH probe. Since the cord is sitting in the same saltwater as the pH probe, this relatively huge signal can overwhelm the signal from the pH probe. Even if the cord insulation were perfect, the current flowing through the wire would still inductively create currents in the water.
 
Galvanic Isolation provides a useful answer to that problem. Since the entire pH circuit is completely isolated, the stray currents in the water can’t flow into it, significantly reducing the effect of interfering leakage currents. This concept can be challenging to understand, so perhaps a somewhat contrived example will help. Imagine that you are standing in the water on a beach. You want to drop a small rock in the water and measure how big of a wave it produces 6 inches from the point of impact. A measurement would be pretty easy to do with a ruler and a rock in perfectly still water. But now suppose there is a storm blowing up considerable waves in the ocean. You can always drop your stone in the water and try to measure the wave it produces, but the measurement will be overwhelmed by the enormous breakers in the ocean. But if you were to a put large metal bucket on the sand, the big waves would wash around it, and you could make your measurement in the bucket. The bucket acts as the Galvanic Isolation, preventing the stray currents from entering the measurement circuit.
 
Some aquarium equipment replaces Galvanic Isolation with a differential input amplifier. While this helps a lot, the common-mode rejection of such an amplifier is not usually adequate to remove the noise signals that are picked up in the water.

Characteristics of a pH Circuit

A pH probe is a glass or plastic probe that is placed in a solution to measure the pH of that solution. In the aquarium industry, it is common to measure tank pH as well as the pH of some media reactors.
 
The probes produce a small DC voltage that is proportional to the pH of the solution. The volt reading is quite tiny (theoretically 59.16 mV per pH unit) and is bipolar (theoretically pH 7.0 produces 0 volts output). At first glance, it would appear that all you need is a voltmeter and a conversion chart. However, this is not the case.
 
Several factors make pH measurement tricky. The first is the source impedance of the pH probe, which typically runs in the 10 MΩ to 1000 MΩ. This extremely high source impedance requires that the input circuit have an even higher input impedance, on the order of 1 TΩ (that is 1 million ohms). This high impedance is very difficult to achieve and results in a circuit that is exceptionally subject to noise from surrounding electrical equipment.
 
Related to the input impedance is the input bias current. Because the internal source impedance of the probe is so high, the input bias current of the measurement input must be incredibly low; in the order of 100 fA (One Femptoamp is one quadrillionth of an amp). It is hard to conceive of how small of a current this is, but as a reference, one fA is equal to 62,420 electrons going by per second. If you were to put your thumb and index finger on the ends of a AAA battery, the current flowing through your fingers would be 300 million times the amount described here.
 
In the aquarium industry, pH probes connect using a BNC connector which makes things even worse. BNC connectors are designed for RF signals and not ultra-high impedance DC signals. A better connector would have 3 or 4 contacts and would allow for a balanced pair. However, since this connector has become a standard, it must be worked around.

Galvanic Isolation

Galvanic Isolation is an elegant term for circuitry that completely isolates the pH input circuit from all other electronics in the system. Galvanic Isolation provides two benefits. One is safety, though this is a minor advantage. The second and most important is noise immunity. Aquarium systems include lots of high power electronics such as pumps, heaters, and lights. Even the best of these devices leak some amount of electrical current into the water. The typical electrical cable used for a submersible pump or heater has a contact-to-water resistance of roughly 270 MΩ (million ohms). If you apply a 110V signal to that cable a current of 0.4 uA (micro amp, one millionth of an amp). While this current is tiny, it is roughly 41 million times the current coming from a pH probe. Since the cord is sitting in the same saltwater as the pH probe, this relatively huge signal can overwhelm the signal from the pH probe. Even if the cord insulation were perfect, the current flowing through the wire would still inductively create currents in the water.
 
Galvanic Isolation provides a useful answer to that problem. Since the entire pH circuit is completely isolated, the stray currents in the water can’t flow into it, significantly reducing the effect of interfering leakage currents. This concept can be challenging to understand, so perhaps a somewhat contrived example will help. Imagine that you are standing in the water on a beach. You want to drop a small rock in the water and measure how big of a wave it produces 6 inches from the point of impact. A measurement would be pretty easy to do with a ruler and a rock in perfectly still water. But now suppose there is a storm blowing up considerable waves in the ocean. You can always drop your stone in the water and try to measure the wave it produces, but the measurement will be overwhelmed by the enormous breakers in the ocean. But if you were to a put large metal bucket on the sand, the big waves would wash around it, and you could make your measurement in the bucket. The bucket acts as the Galvanic Isolation, preventing the stray currents from entering the measurement circuit.
 
Some aquarium equipment replaces Galvanic Isolation with a differential input amplifier. While this helps a lot, the common-mode rejection of such an amplifier is not usually adequate to remove the noise signals that are picked up in the water.

Digital Galvanic Isolation

Galvanic Isolation is excellent; it helps significantly with noise reduction and shields the tiny pH signal from most of the noise coming from the tank. The very definition of Galvanic Isolation is that there is no electrical connection between the pH input circuit and the rest of the system. How do you transfer the reading obtained out to the system, with no connection to the rest of the system? The answer is an optocoupler (sometimes called an optoisolator) – a device equipped with an LED (light-emitting diode) at one end, a phototransistor or photodiode on the other end, and a transparent barrier in between. Thus the light produced by the LED can be sensed by the phototransistor on the other end.
In traditional Galvanic Isolation, the brightness of the LED corresponds directly to the voltage arriving from the pH probe. The following drawing may help to understand this. The signal from the pH probe arrives at the ultra-high impedance input amplifier and then travels to the LED within the optoisolator. From there, it goes to the A/D converter which converts the analog signal to a digital number. That number goes to the CPU for processing. (Some manufacturers use an A/D converter built into the CPU chip. While this is inexpensive, it is not usually a good idea because such built-in A/D converters are not generally high-quality linear devices).

Analog Galvanic Isolation

The problem with this traditional method is that the signal passing across the barrier is still analog. The LED and phototransistor tend to be reasonably non-linear, meaning the voltage coming out are not directly related to the volt reading coming in. Unfortunately, there are errors in the electrical-optical-electrical conversion. These errors directly affect pH measurement.
Here is a linearity curve for a typical optoisolator. If it were perfect, the curves would be straight lines.

Optocoupler linearity curve

HYDROS introduces a new method to overcome this difficulty; Digital Galvanic Isolation. The drawing below will help to clarify this. In this drawing, we moved the A/D converter to the other side of the Galvanic Isolation barrier, which converts the pH signal into a number BEFORE going through the optocoupler. The newly converted digital number is unaffected by the nonlinearity of the optocoupler. The measurement was already collected. This process is very similar to the difference between an analog audio storage system such as a cassette/8-track vs. a digital CD. The signal stored on the 8-track tape is analog and is affected by any nonlinearity in the system. The signal stored on a CD is digital and unaffected.
Part of the reason that this technique is not widely used is that this makes it impossible to use the A/D converter built into the CPU chip. An external A/D converter is needed, which adds cost and complexity.

The HYDROS pH circuit

The result of this is a pH input circuit with noise immunity and repeatability unparalleled in the aquarium industry.

The background provided in the earlier parts of this document prepares the reader for a discussion of the HYDROS pH circuit. The salient features of the HYDROS pH circuit are as follows:
  • The input amplifier is an ultra-low noise differential input circuit with an input bias current of less than 100 fA. Meaning there is no significant loading on the pH probe.
    • This differential input amplifier has a DC Common Mode Rejection Ratio > 80 dB. This means that a factor of 10,000 to 1 immediately reduces any noise signal present on the input.
  • The signal is then delivered to a very hard low pass filter to remove any AC components because the signal is DC while the noise is mostly AC.
  • After the filter, the signal moves to a very high-quality A/D converter which is entirely monotonic across its entire input voltage range.
  • The numeric output from the A/D converter transfers to an optocoupler which provides 2500 volts of UL recognized galvanic Isolation.