2-Wire 4-20mA Transmitter

This is a two-wire 4-20mA transmitter. Connection is via a 2-pin Molex connector (type: 22-27-2021). The transmitter board controls the current required by the 2-wires.

The analogue output is provided via a current loop and NOT a voltage loop, you should introduce a measuring resistor if you wish to convert the current into a voltage, to be absolutely clear, the resistor should be in-line with the supply line.

The measuring resistor is NOT across the two wires. Using Ohm’s law, you can then determine the current.

This system is favoured by many people because it only requires two wires, and can have very long cable lengths without any effect on the signal.

In the manual we explain how to choose the resistor: higher resistor values give you more resolution, but may cause too large a voltage drop.

You must supply a voltage (VDC in the range stated on the Technical Data Sheet) and the electrical current (mA – output on the same two wires) will vary according to the gas concentration the sensor is exposed to. Depending on how your transmitter is set-up your zero gas output will be 4 mA (less than 4 mA indicates a fault) and your full span will be 20 mA (over 20mA indicates over gassing).

Where transmitter boards are sold, we pre-calibrate our sensors (to customer requirements) on the boards. Sensors cannot be removed and/or replaced on the board without full re-calibration.

Calibration can be a difficult process, requiring target gas, correct tubing and suitable sensor hood - we do not supply these.

We caution re-calibration should only take place under strict safety procedures by a person with the appropriate technical knowledge.

Transmitter plus sensor accuracy depends on the range specified:

Zero (4mA) is calibrated ±0.02mA.  Full scale of specified range (20mA) is calibrated ±0.05mA.

In addition, there is gas concentration error at 20mA from gas bottle supply (either 1% or 2%, depending on the gas) and mixing error due to the mass flow controllers: this will also depend on the range specified.

There is also temperature error, but the Digital Transmitter includes on-board temperature correction.

Finally, there is sensor drift with time that is specified for the sensors on each sensor's data sheet.  Obviously service engineers can correct this when re-calibrating.

Any electronics drift will be minimal and is effectively ignored.

Rules for sampling chamber:

A) Minimise internal volume.

B) Build with materials that do not adsorb gases: PTFE, FEP, MFA are best; next best are HMWPE, PP, Acetal.

C) Ensure the airflow is across the face of the sensors, not directly onto them.

D) Use good quality tubing: we use FEP, but PTFE is also OK.  Maybe polyurethane if you want more flexible coupling.  Micro-polished SS316 is the best.

E) Purge your gas lines before testing: 30 minutes for NO2, 10 minutes for NO are rough times, but it will depend on your tubing length and type.