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Sensors, switches and transducers
polarity reversal and short circuits, as is the RS485 digital interface. It
is independently scalable over wide ranges by generating the output
signal from the values calculated in the microprocessor. A turndown
by about factor 10, ie free assignment of the zero point and the end
point of the output signal to a measuring value start and end point, or
application-specific characteristic curves (eg filling level in horizontal
tanks) are just as feasible as a fully inverted output signal.
All the parameterisation you want
The key to application-specific parameterisation is the RS485 inter-
face. Basic information such as the serial number or the ‘as deliv-
ered’ state of the pressure measuring and temperature range can be
established via this interface. The current pressure measurements
can be output in different units. If the transmitter is used to record
derived pressure-dependent process variables, they can be output in
a mathematically correct way. These include the flow rate (pressure
drop at an aperture), filling level or level (pressure at base of tank or
in the sounding pipe at a defined depth), pressure difference (pres-
sure difference determined using two measuring cells) or gas density
(pressure at a given volume). In order to make optimum use of the
transmitter in controlled processes, a digitally adjustable deep-pass
filter is available for suppressing brief peaks.
The digital interface operates with an open master/slave protocol,
whereby data transmission is safeguarded with a CRC16 checksum.
Up to 128 transmitters can be interrogated in series during bus opera-
tion. Appropriate freeware is available for recording measurements
with graphics, as are a DLL and driver for LabView for simplifying
incorporation in other applications.
What does precision mean?
This article describes the properties of a series of high-precision
pressure transmitters. The primary standards available in national
reference laboratories for the pressure measuring value (pressure
scales) provide measuring uncertainty (‘accuracy’) of less than
±0,01%. In the calibration equipment of industrial pressure transmitter
production you usually find traceable, automatic primary standards
with measuring uncertainty of ±0,025%. They produce the refer-
ence values for calibrating the pressure sensors. This percentage
information relates to the relevant pressure measuring point and
designates the statistical deviation of many measurements at a given
pressure. Unless otherwise specially indicated, the error tolerance
band of pressure transmitters relates to the relevant end point of the
measuring range (FS = full scale). Some manufacturers specify the
non-linearity and hysteresis errors and the additional errors caused by
temperature changes separately. For the sake of reliability, Keller AG
uses the (total) error band method for a specified temperature range
for digitally compensated pressure transmitters. In the temperature
range between +10°C and +40°C the maximummeasuring uncertainty
(static conditions for temperature and pressure) is within ±0,05 % FS.
The tremendous quality improvement that results from digital com-
pensation is particularly evident in the low error band divergence of
±0,1 %FS with expansion of the compensated temperature range of
-10 °C to +80°C, ie from ΔT = 30 K to ΔT = 90 K.
Digital transmitter – optimised processes
There are many applications for producing accurate pressure meas-
urements. Typical use thereof is in water supply company level
measurements for monitoring the groundwater level or controlling
pumps. In this case, the digital pressure transmitter can be scaled
exactly in accordance with measuring point requirements. Ameasur-
ing range expansion by a factor of up to x10 can be realised with the
comparatively constant ambient temperature.
In chemical process engineering, high precision is required at
various measuring points for exact process control. Particularly in
the food industry a high degree of thermal repeat accuracy is re-
quired, ie reliable measurements after the performance of cleaning
cycles. A special gap-free measuring head is already available for
food production.
A
bbreviations
Keller 35x
precise
pressure
transmitter.
Keller 33x
precise pressure
transmitter.
DLL – Dynamic Link Library
EEPROM - Electronically Erasable Programmable Read Only Memory
TC – Thermocouple
TCN - Thermocouple zero point for temperature measurement.
43
September ‘12
Electricity+Control