Digitrans Industrial Controls
Description of the Analog
The Digitrans Analog RF Probe was developed from the
Digitrans Digital RF Probe. The digital probe is used wherever precise
Brix values are required. It is particularly useful in high purity
solutions as found in refineries. The digital probe with its
microprocessor controller will give very accurate Brix readings (+/-
0.1Bx) provided care is taken with the calibration procedure. This can be
rather time consuming and requires a degree of skill in sampling technique
and care in entering the data. For those applications that do not require
a precise numerical Brix value a simpler probe was developed that would
give an output suitable for pan control but that would be easy to
calibrate. This development resulted in the analog probe.
PRINCIPLE OF OPERATION
The tuned circuit of an oscillator is coupled to the probe electrode
and changes in composition of the solution cause changes of the resonant
frequency of the tuned circuit. The shift in frequency is converted to an
analog voltage which controls the 4-20mA current loop. The analog probe
uses only a single measurement frequency of 18MHz. which responds to both
capacitance and conductivity changes. This makes its field of application
mainly in low purity solutions. It is not suitable for use in refineries.
The probe body is identical to the digital probe which is illustrated
in the accompanying pamphlet. The analog probe does not require an
additional controller, all the electronics being housed in the aluminium
box mounted at the head of the probe. The only visible difference between
the digital and analog probes is the connector. The digital probe has a
coax connector while the analog probe has a multi pin circular connector.
The analog probe is fitted to the process vessel by the same bronze
mounting socket as the digital probe which means that the two models may
be interchanged. The electronics has a very low power drain which enables
it to be powered from the 4-20mA loop. Thus only two wires are required to
connect the probe. No local display is provided.. The only two controls
that may be adjusted on the probe are zero and span. These are set by
screwdriver adjusted multiturn potentiometers inside the box. (See
calibration) The entire circuit is encapsulated in epoxy resulting in an
extremely rugged instrument.
The analog probe is used in many situations in raw sugar factories. It
can be used in syrup, batch pans and continuous pans
The probe is calibrated on the bench before being installed in the
process. Calibration is a simple two point adjustment of zero and span.
The two adjustments are non interactive. A calibrator, which is a switch
selectable array of resistors and capacitors, is clipped onto the probe
electrode to simulate the impedances corresponding to the high and low
brix ranges. The switch settings for the low brix value are read off a
chart provided for the appropriate substance.(massequite,syrup etc.) The
switches are set to these values and the zero potentiometer adjusted for
4mA. The switches are then set for the high brix value and the span
potentiometer is adjusted to give 20mA. The probe is then ready for
service. Calibrators are supplied as separate items.
Calibrator attached to probe.
SPECIFICATIONModels There is one standard length
model of 250mm wetted length. Special sizes may be supplied on request.
Probe diameter : 26mm
Probe body material : 316 stainless steel.
Connection into process : Adapter flange welded to
vessel. Bronze socket bolted to flange with O ring seal.
Probe body temperature : Max 120o C
Probe head temperature : Max 70 o C
Electrical connection : Four pin circular connector
Output : 4-20mA current loop into 500 Ohms
Power supply : Loop powered
Isolation : The probe circuit is isolated from
Protection class : IP 65.
Measuring range : 50 - 97 Brix. Smaller ranges
selected by zero and span adjustment.