We answer your questions about flowmeters, fittings, pressure/vacuum, and conductivity/resistivity/dissolved solid meters.
Q: What is the difference between correlated and direct reading rotameters?
A: A direct reading flowmeter indicates the flow rate on its scale in specific engineering units (e.g. ml/min or scfh). Direct reading scales are designed for a specific gas or liquid at a given temperature and pressure. While it is more convenient than a correlated flowmeter, a direct reading flowmeter is less accurate and limited in its applications.
A correlated flowmeter is scaled along either a 65mm or a 150mm length, from which a reading is taken. The reading is then compared to a correlation table for a specific gas or liquid. This will give the actual flow in engineering units. One correlated flowmeter can be used with a variety of fluids or gases.
Fittings
Q: I need a fitting for my tube? What type do I choose?
A: The two most common fitting types that we sell for tubing are barbed fittings or compression fittings. Barbed fittings are designed for softer tubing, such as silicone, Tygon®, Norprene, and peristaltic pump tubing. These fittings have a barb, which slips into the inner diameter of the tubing. The barb is larger than the ID, and because the tubing is flexible, a good seal is created. To size correctly, use the correct ID of the tubing; these are best for low pressure applications.
Compression fittings are designed to go over a rigid tubing, such as Polypropylene, PTFE/PFA, and HDPE (High Density Poly Ethylene) or LDPE tubing. The fitting works by compressing the tubing between a ferule and a locking nut. The fittings are sized by the outer diameter of the tubing.
Pressure/Vacuum
Q: What is the difference between absolute and gauge pressure. Which one is listed for this unit?
A: Absolute pressure is the pressure reading relative to an absolute vacuum—think of deep space (and then some). This is the true pressure of the fluid on the pressure instrument and is written as PSIA.
Gauge Pressure is relative to the atmospheric pressure (~14.7 PSI). This is the more common pressure for most applications. It is written as PSIG (and often just as PSI). For example, if your gauge pressure meter is reading 50 PSI, the absolute pressure would be 64.7 PSIA.
Pressure can be confusing due to the wide number of different scales commonly used. Each scale is best for a particular set or application. Other common scales are inches of mercury (“HG”) for low power vacuum, and millibar (mbar), Torr, or mTorr for high vacuum applications. Inches of Water (“H2O”) is often used as a scale for differential pressure as well.
Conductivity/Resistivity/Dissolved Solid
Q: What is the difference between TDS and conductivity?
A: The meter measures both by detecting the conductivity, and the meter applies a conversion factor to determine the TDS. Salts, minerals, and even dissolved gases contribute uniformly to the conductivity of a solution. This means that the conductivity can be used as an indicator of the amount of dissolved materials in a solution. TDS can be used fairly accurately when comparing the status of a single source, such as NaCl, but error occurs when trying to compare two different types of solutions. It is necessary to calibrate the meter using the same dissolved materials that are in the test solution.
Q: How does temperature affect conductivity readings?
A: The effect of temperature on conductivity readings depends on the solution being measured. The effect is greatest in low ionic strength (low conductivity) solutions. A general rule to follow is: there will be a 2% change (increase)/°C. This rule can be applied to most aqueous solutions. However, if you require a high degree of accuracy, you should consult a chart for the particular solution you are measuring. Organics have very different temperature curves. The temperature coefficient should be adjusted for accuracy in high purity water measurements.
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