Application note
Analogue transmitter installation
A transmitter is calibrated to a specific measurement range. The measurement range is shown on the calibration label on the transmitter. Normally calibration is stated as:
“Minimum measurement value” to “Maximum measurement value”.
The analogue input module converts the transmitter signal (0-20mA / 4-20mA / 0-100mV) to a real process value in SI engineering units, based on ZeroPoint and Span.
ZeroPoint
contains the value to be shown for the minimum transmitter signal.
Minimum transmitter signal is 0mA, 4mA or 0V, depending on the channel and transmitter type.
Span
contains the measurement range of the transmitter.
Span is calculated as: Maximum measurement – Minimum measurement.
Direct transmitter signal:
If the transmitter measures the absolute process value, the installation is very simple:
ZeroPoint is loaded with the Minimum measurement value.
Span is loaded with the difference between the Maximum and Minimum measurement range. (Maximum – Minimum).
| Transmitter calibration label: | ZeroPoint: | Span: |
|---|---|---|
| 0 to 10,000 Litres | 0 | 10,000 |
| 0 to 10,000 Litres | 1.5 | 6.5 |
| 4 to 9 pH | 4 | 5 |
| –600 to –100 mBar | -600 | 500 |
| –500 to +500 mBar | -500 | 1000 |
Scaled transmitter signal:
If the transmitter does not measure the absolute process variable, the data from the label cannot be used directly, e.g. a tank content, measured by a pressure transmitter adjusted to a certain water column of X meters. In this case, the tank content depends on the tank diameter, bottom cone shape, density of the product etc. ZeroPoint and Span must therefore be adjusted to reflect the real process variable.
This is achieved in three steps:
1. Initiate ZeroPoint and Span to the expected values
2. With no load on the transmitter; adjust ZeroPoint until AnalogIn shows the correct value
3. Put a known load on the transmitter: adjust Span until AnalogIn shows the correct value
Raw transmitter signal:
If the transmitter signal is to be used in a calculation that cannot be performed by the analogue module, e.g. a non-linear system, one of the following set-ups can be used.
To read out the raw signal in mA or mV the following constants must be used:
| Signal: | ZeroPoint: | Span: | Unit |
|---|---|---|---|
| 0-20mA | 0 | 20 | mA |
| 4-20mA | 4 | 16 | mA |
| 0-100mV | 0 | 100 | mV |
To read out the signal in %, use the following constant: (Same constant for all signal types)
| ZeroPoint: | Span: | Unit |
|---|---|---|
| 0 | 1 | 0 -1 |
| 0 | 100 | 0-100 (%) |
| 0 | 1000 | 0-1000 (‰) |
Example 1:
A pH transmitter is calibrated to measure 4pH to 9pH.
ZeroPoint is set to 4 and Span is set to 5.
Example 2:
Tank contents measurement with a pressure transmitter adjusted to x meter of water column, is used as a tank contents transmitter:
Installation and adjustment:
1. Install the transmitter in the empty tank.
2. Insert 0 in ZeroPoint and insert the best guess of maximum signal into Span e.g. 100,000 for a 100,000 litre tank.3.
3. With the tank empty: Adjust the ZeroPoint constant until AnalogIn reads 0.
4. Fill a known amount of product into the tank. (Best accuracy is achieved with as full tank as possible).
5. Adjust Span, until AnalogIn reads out the correct value.
Related Topics
AnalogIn calculation principle
Transmitter error
compensation
Signal filter
PD 640 Application Notes (mA)
PD 642 Application Notes (mV / thermocouple)
Analogue modules
