1、 What is a two-wire explosion-proof pH meter?

✅ definition:

​Two wire explosion-proof pH meterIt is a type of adoptionTwo wires simultaneously transmit power and signalsAnd possessExplosion proof function (such as Ex d, Ex ia and other explosion-proof certifications)The online pH measuring instrument is mainly used forReal time monitoring and control of liquid pH value in flammable, explosive, and high-risk environments.

✅ Core features:

2、 Signal transmission method of two-wire explosion-proof pH meter

✅ 1. The meaning of two-wire system: power supply+signal multiplexing

In a two-wire system:

​Two wiresSimultaneously undertaking two functions:

​Power supply for instruments (transmitters)(Usually)​24V DC​​)

​Transmitting measurement signals(Usually standard)4-20mA current signal, corresponding to pH value)

✅ Namely:Power supply and signal share the same pair of wires, without occupying additional cable resourcesIt is very suitable for industrial sites with compact wiring and high explosion-proof requirements.

✅ 2. Signal type: 4-20mA industrial standard analog signal

​The correspondence between pH value and current signal (linear mapping)For example:

4 mA → pH 0 (or custom lower limit, such as pH 2)

20 mA → pH 14 (or custom upper limit, such as pH 12)

The intermediate value is output in a linear proportion, such as 12 mA → pH 7

​Why is it 4-20mA instead of 0-20mA​

​4mA as the 'live zero'If the signal is lost or disconnected, and the current is 0mA, the control system can recognize it asFault statusImprove system security.

​Signal transmission distanceStrong anti-interference ability of current signal, with a transmission distance of up toHundreds of meters or even thousands of kilometersAnd it is not significantly affected by line voltage drop.

✅ 3. Schematic diagram of signal transmission process (text version)

→ [two-wire pH transmitter] → [4~20mA signal] → [PLC/DCS/display instrument] ↑ [24V DC power supply]

Both power and signal are transmitted throughSame pair of two wiresConnect to the control room or PLC system;

The electrodes detected inside the transmitterPH potential signal (mV level)Convert toStandard 4-20mA current signalOutput.

3、 The anti-interference design principle of two-wire explosion-proof pH meter

The industrial site environment is complex, especially inExplosion proof areaDue to issues such as high temperature, high pressure, corrosion, electromagnetic interference, and line coupling, the two-wire explosion-proof pH meter is suitable forMultiple anti-interference and security measures must be taken in signal transmission and system design.

✅ 1. The advantage of using current signals (4~20mA) instead of voltage signals (core anti-interference design)

Comparison item	Voltage signal (such as 0~10V)	Current signal (4~20mA)
anti-interference	Weak, easily affected by line impedance and electromagnetic interference	​Strong, strong anti-interference ability of current loop​
transmission distance	Highly affected by voltage drop and short transmission distance	​Long transmission distance, up to several hundred meters​
Impact of Line Voltage Drop	Clearly affecting signal accuracy	​Almost unaffected(Because the receiving end detects current)
Explosion proof adaptability	Not suitable for intrinsic safety design	​More suitable for two-wire intrinsic safety explosion-proof systems​

✅ ​The 4-20mA current loop is naturally anti-interference and is one of the reliable analog signals in industrial process control. ​​

✅ 2. Two wire intrinsic safety (Ex ia/Ex ib) explosion-proof design principle

🔒 What is Intrinsic Safety?

Intrinsic safety refers toThe electrical sparks or thermal effects generated by the instrument during normal operation or malfunction are insufficient to ignite the surrounding flammable and explosive gases.

✅ Two wire explosion-proof pH meters are commonly used:

​Ex ia (intrinsic safety): In normal andTwo faultsSafe in all states and can be used forZone 0 (highest risk)​

​Ex ib (IB level intrinsic safety): In normal andA malfunctionSafe in state, used for​ Zone 1 ​​

Key points of explosion-proof design:

​​限制能量​All components in the circuit (including power supply, amplifier, electrode interface)Voltage, current, and power are strictly limited to certain safe valuesEnsure that explosive gases will not ignite;

​Intrinsic safety certificationThe transmitter as a whole has passed the explosion-proof certification of institutions such as ATEX, IECEx, NEPSI, UL, etc;

​Explosion proof/intrinsic safety isolation barrier (optional)Use safety barriers to isolate certain non intrinsic safety areas from intrinsic safety equipment.

✅ 3. Electrical and signal isolation design

​Signal isolationThe internal circuit adoptsOptoelectronic isolation, transformer isolationTo prevent external interference from entering the signal path;

​Floating designAvoiding ground loop interference, improving measurement accuracy and resistance to common mode interference;

​Shielding and filteringSignal cables usually come with a shielding layer, and there are internal components inside the instrumentEMI/RFI filtering circuitSuppress high-frequency interference.

✅ 4. Anti interference design of pH electrode and signal conditioning

​High impedance signal processingThe pH electrode output isHigh impedance mV signal (typically tens of mV, high impedance up to 10 ⁹ Ω)Extremely susceptible to interference, therefore:

AdoptingHigh input impedance amplifierTo avoid signal attenuation;

ByFilter, amplify, linearize circuitConvert the mV signal to a stable 4-20mA;

​Temperature compensationBuilt in temperature sensor, automatically compensates for changes in pH value with temperature, improving measurement reliability;

​Electrode diagnosisSome models can monitor electrode status (such as slope, zero drift) and warn of faults.

4、 Summary: Key points of "signal transmission and anti-interference" for two-wire explosion-proof pH meter