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Tech Tips

Technical Tips/FAQ

What is the difference between differential, gauge pressure and absolute pressure?
Differential pressure is the difference in pressure between two sources. If one source is 5 psi and the other is 3 psi, the differential pressure is 2 psi. A 4-20ma differential pressure transmitter with a range of 0-4 psi would read half of the scale which would be 12 ma.

Gauge pressure is the pressure of a source relative to ambient pressure. It is similar to the differential pressure because it is taking the difference between two pressures, but one of the pressures is ambient pressure or the atmospheric pressure in the room. Many gauge pressure transmitters have a vent tube in the wire if the sensor is sealed or they may be vented inside the enclosure if it is like our transmitters. If the vent tube is plugged, that can affect the reading of the transmitter or switch.

Absolute pressure is similar to gauge pressure, except it references the reading to a perfect vacuum, so there is no vent tube to reference room pressure. At sea level, the reading of an absolute pressure transmitter that is open to the outdoor air is about 14.7 psi or 30 inches of mercury (Hg).

What is the difference between gauge pressure and vacuum?
Gauge pressure is a pressure that is greater than the surrounding room pressure. A vacuum is a pressure that is less than the surrounding room pressure. A pressure switch or transmitter with a high and low port can be used a a vacuum, gauge or differential pressure switch or transmitter. For gauge, it would be connected to the high port. For vacuum, it would be connected to the low port. For differential, the highest of the two pressure sources would be connected to the high port and the lowest would be connected to the low port.

What is "inches of water"?
Inches of water is a measurement of pressure based on the force one inch of water exerts on an object. There are 27.7" of water in one PSI. So a glass with 27.7 inches of water in it will exert 1 psi on the bottom of the glass.

What is a 4-20ma loop?
A 4-20 ma loop is the circuit a 4-20 ma transmitter makes with a 4-20 ma input device (such as a PLC or controller). There are two types of 4-20 transmitters. A 2 wire transmitter, which is by far the most popular transmitter type, requires a power supply (typically 24 vdc) to be put into the loop to power it. A 4 wire transmitter provides the power for the 4-20 loop, but also requires power to be fed to it (typically 24 vdc). A 2 wire transmitter allows the power supply to be placed anywhere in the loop and is typcially near the control panel where the PLC or controller resides since those devices require power as well. A 4-20ma transmitter can typically go 5,000 feet without signal degradation. That is what makes it the most popular output signal. See our rule of thumb for wiring a 4-20 loop for determining polarity connections at http://www.beccontrols.com/pdf/RULEOFTHUMBFORWIRING4-20loop.pdf.

What is deadband?
Our pressure switches have deadbands built in which is the difference in pressure between when the switch goes on with increasing pressure and where it goes back off with decreasing pressure. The deadband prevents the switch from oscillating on and off if the pressure it is sensing is at the switch setpoint. The deadband property is sometimes called hysteresis.

What is the purpose of a restrictor?
A restrictor on a pressure switch will prevent the switch from triggering on pressure transients or quick, short increases or decreases in pressure. A restrictor reduces the inlet orifice to the high port or the outlet from the low port. It limits the amount of airflow into the switch so transients will not cause nuisance switch triggering.

What is a ground loop when wiring a 4-20 transmitters?
A 4-20 ma ground loop is caused when the shield on a two conductor cable is grounded on both sides of the cable. Since the ground can be at slightly different potentials at different locations in a plant or building, a current can be created by connecting a conductor to the different potentials. This current, if it is fluctuating, can potentially induce a current in the two parallel wires that are carrying the signal which can in turn cause the signal to be innacurate or fluctuate. Only one side of the shield should be grounded. The other side should be left disconnected.

Can signal wires run alongside 120 vac or higher voltage wires?
A wire carrying an AC voltage creates a magnetic field that changes 60 times per second. The change or flux in a magnetic field can induce a current into wires that run parallel to it. It is the same principal that is used in transformers. If current is induced into signal wires, the signal will not be accurate and may fluctuate. It is best practice to run signal wires in separate conduit and wire trough from power wires. Running power wires perpendicular to signal wires will not induce current on the signal wires.

Can I fix my gauge isolator if it is not working?
Our gauge isolators are filled with hydraulic oil or other liquid between the isolator diaphragm and the gauge or other pressure sensing device. If the gauge that is mounted on the isolator is not reading pressure, it is possible the hydraulic oil or liquid has leaked out of the isolator which will not allow the pressure to be transmitted to the gauge. In that case, the gauge can be removed and the isolator can be refilled, then the gauge can be re-attached. The diaphragm can travel about 1.5", so when the gauge is screwed back on, the diaphragm should move to accomodate the amount of oil the gauge displaced. If the gauge reads a pressure after it is put back on the isolator, the diaphragm is likely fully extended and is holding pressure. Loosed the gauge and move the diaphragm up with a blunt object from the side opposite the gauge to push out some excess oil (bleed out the oil), then re-tighted the gauge. The diaphragm can be purchased from us separately as well if it is damaged. The three screws holding the isolator together can be removed to replace the diaphragm. They can then be re-tightened being careful not to strip the screws.

Can I put my own gauge on a gauge isolator?
Yes, see our instruction sheet on our gauge isolators at http://www.beccontrols.com/pdf/PROTECTOR-I-0603.pdf.

What is a zero and gain pot?
The zero and gain pot or potentiometer on our transmitters are used to calibrate the output signal.

Can I calibrate my pressure transmitter myself?
If you have a pressure source that matches the top end of the pressure range of the transmitter, you could re-calibrate your transmitter. For a standard pressure range that starts at 0: At zero pressure, change the zero pot until the output reads the bottom of the signal range (4-20ma would be 4 ma. 0-5 vdc would be 0 vdc). At the full scale pressure, change the gain pot until the output reads the top of the signal range (4-20ma would be 20ma, 0-5 vdc would be 5 vdc). For example, full scale pressure for a 0/5 psi transmitter would be 5 psi. Repeat the process starting with zero pressure. When the transmitter transmits the proper output at the zero pressure and full scale pressure without having to adjust either pot, the calibration is complete.

Can I calibrate my pressure switch myself?
If you have a pressure source that matches the pressure setpoint of the switch and a gauge to read the pressure, you could re-calibrate your switch setpoint. A switch with a two piece adjusting screw will allow you to turn the screw in or out. Turning it out all the way will bring the setpoint down to the lowest end of the range stated on the switch nameplate. Turning the screw in will increase the setpoint. Turning it out will decrease it. Increase the pressure very slowly. Watch the gauge to see what pressure the switch triggers. Adjust the screw in or out as needed, then repeat the process. See our switch instructions for more information on how our switch works at http://www.beccontrols.com/pdf/B-4021.pdf.

 

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