SENSORS 101
Pressure Transducers: Halliburton uses a few different types of pressure transducers,
which can range from as low as 0-100psi all the way up to 0-15000psi. Pressure
transducers work on what is called a current loop or 4-20mA (mA = milli-amp) circuit.
For example if you have a 0-15000 psi transducer 0psi will represent 4mA and 15000psi
will represent 20mA or 100 % percent of the scaling. Below is an example for figuring
out the slope of a 0-15000psi transducer during calibration.
20mA minus 4mA gives you a range of 16mA.
Take the max pressure of your pressure transducer, which in this case is 15000 psi, and
divide it by 16.
15000psi / 16mA = 937.5psi per mA
The slope of this transducer will be 937.5psi per mA.
Typically we supply 12 or 24 volts DC to our sensors. Most of our sensors have an
operating range of 9 to 32 VDC. In a current loop configuration, it is only necessary to
have two wires going to the sensor. On all of our pressure transducers, we use four pin
receptacles in which pin C is your 12 or 24VDC input and pin B is your signal return
back to your Unipro II, FLECS, ACE CAN Module, or ARC Controller Card. Below is a
diagram for connecting a DVM (Digital Volt Meter) in series with a current loop to
measure mA.
Temperature Transducers: These sensors work in the same relationship as the
pressure transducers except that the unit of measurement is in Celsius or Fahrenheit. We
use these transducers on our engines to measure engine water temperature and pump oil
temperature. They also use the same four pin connector using pins B and C. The normal
operating range of our temperature transducers are 0 to 200 degrees C or 32 to 392
degrees F. Use the same formula as mentioned earlier to determine the slope. The
example below shows you how to determine the slope for a temperature transducer with
an operating range of 32 to 392 degrees Fahrenheit.
392 degrees F minus 32 degrees F gives you a range of 360 degrees F.
20mA minus 4mA = 16mA
360 degrees F / 16mA = 22.5 degrees F per mA
Below is another diagram showing how to connect your DVM in series to check mA.
pH Probes: We use these sensors to measure the pH of our process fluids on our
blenders. They are placed on the discharge and suction side of our blenders manifold.
These sensors also work on 4-20mA. Using two different buffer solutions you can
perform a two point calibration to calibrate the probes. The calibration processes for
these probes vary because we have both ACE and ARC blenders on the Stimulation
Vessels.
Level Sensors: Our blenders use K-TEK model MT2000 level indicators for our tub
leveling system. These are also referred to as Guided Wave Impulse Radar Sensors.
Once again these use a 4-20mA current loop. An empty tub would represent 4mA at 0%
and a full tub would represent 20mA at 100%. These K-TEK sensors use a very short
“pulse” of microwave energy that is applied to the probe. When this pulse of radar
energy travels the length of the probe and reaches fluid, part of the pulse is reflected back
towards the transmitter. A receiver measures the exact duration of time between the
transmitted and reflected signal. The device then analyzes the time elapsed between the
initial pulse and the reflected one. The electronics can then calculate the fluid level in the
tub. There are a few other level sensors that are used within Halliburton. One is a
magnetic level gauge and the other is a magnetostrictive (float level) sensor. Both also
work on 4-20mA. Click on the link below for a demonstration of the different types of
level sensors.
http://www.ktekcorp.com/dnn20/Default.aspx?tabid=57
Magnetic Pickups: In our applications we use both non-powered two pin mag pickups
and powered intrinsically safe three pin mag pickups with the two pin being the most
commonly used. We use magnetic pickups to determine flow rates and RPM’s.
Magnetic pickup sensors are devices which sense the rotation of the magnetic tips of flow
meter impellers or gear teeth on a transmission for example. A two pin magnetic pickup
typically produces a low voltage sine-wave signal whose frequency and amplitude is
proportional to the rotational velocity of the impellers and gear teeth. The three pin
magnetic pickup is needed for Class 1 areas. The output frequency of this device
produces a square pulse waveform (digital) which directly relates to the number of
impellers tips or gear teeth that have been sensed. These pulses can then be used to
determine both flow rate and RPM.
Turbine Flowmeter Rate Calculations: Hz X 60 / (Meter Factor X 42) = Barrel/Minute
Example: 125Hz X 60 / (55 X 42) = 3.24BPM