Dewpoint meter for compressed air application

Paul Boughton

Selecting the right dewpoint sensor and transmitter type to meet the stringent needs of your unique installation is a critical task. Steven Jiroutek guides you through the most essential questions you should answer when weighing your choices.

The first question to ask is whether you are you looking for a fixed or a portable dewpoint sensor and transmitter.Fixed mount transmitters usually require an external power source to operate and can range in complexity from a basic analogue only device to very sophisticated units with displays, alarm relays, datalogging, ethernet connections, and more.

The following set of questions will help narrow down products to choose from.

What do you need the instrument to do?

- Continuously monitor with a local display?

- Panel meter or integrated display?

- Send a signal somewhere for control or other purposes? What type of signal?

- Alarm at a high/low setpoint?

- Datalog?

Fixed mount units can be installed on supply-side OEM applications, for dryer monitoring or control, as well as on the demand-side for end user applications connecting to PLCs, dataloggers, and panel meter displays.

In terms of portable handheld devices, these battery-powered mobile devices of varying size and function are optimised for different applications. They are typically used for verifying fixed mount transmitters in the field or for spot-checking various points in a compressed air system (Fig. 1).

Dryer type

In most compressed air systems, the type of dryer will dictate the dewpoint operating range required from the dewpoint instrument or sensor type.

Currently there is no single instrument on the market that measures all dewpoint levels well. Sensing technologies are optimised to excel in specific ranges - very dry conditions, mid-range ambient levels, or high humidity condensing environments. Measuring dewpoint in each of these ranges presents its own unique set of challenges (Fig. 2).

Choosing the instrument with the right measurement range will impact both the short- and long-term accuracy and stability of the dewpoint measurement.

Desiccant dryers

Compressed air systems using a desiccant-type dryer can have dewpoints anywhere between -100 and -30°C (-148 2 to 22°F). However, most operate between -60 and -40°C (-76 to -40°F). It's important to identify what level you actually wish to control or monitor. If your dryer outputs air at -80°C (-112°F) dewpoints however your intended control setpoint is at -50°C (-58°F), this is where your accuracy and performance is critical. For these dry, low dewpoint conditions, products using Vaisala DRYCAP polymer sensor type technology are recommended for optimum performance.

Refrigerant dryers

In systems operating at higher levels, refrigerant dryers produce dewpoints between 2 and 4°C (35 and 40°F). For these mid-range measurements, sensors optimised for higher dewpoints are recommended, such as Vaisala HUMICAP sensor products as well as some of the DRYCAP sensor products.

Since application conditions such as pressure and temperature can affect the choice of sensor, it's a good idea to consult a specialist before making your final decision.

It is also important to know the pressure and temperature at the measurement location.

While the temperature of the compressed air will not have any affect on the dewpoint value, it may negatively impact the performance of the dewpoint sensor. If the air is too hot at the intended measurement point, choosing a cooler downstream location or use of a sampling system may be required.

Careful monitoring and management of pressure is critical to making a good dewpoint measurement. Since changes in pressure have a dramatic affect on dewpoint, it's important to know the pressure at each measurement location in order to compare dewpoint readings and baseline them to a single pressure value.

Refrigerant dryers

Will the measurement be made directly in the line or using a sample cell? Deciding how to install a dewpoint probe can be a difficult choice. Two options are usually considered - inserting the threaded probe directly into a T, or ball valve for an in-situ measurement or using a sample line and sample cell to isolate the measurement outside of the system. There are advantages and disadvantages associated with each method.

For in-the-line measurements, the benefits of in-situ measurements include a simplified and lower cost installation, and a potentially faster response time.

However, drawbacks include fluctuating line pressures (resulting in changing dewpoint values) and inability to isolate or remove the probe conveniently from the compressed air system when it's time for service or calibration.

Typically dewpoint probes come in two varieties, fixed and adjustable. Adjustable probes use a threaded compression fitting allowing for adjustable insertion depth into a line. Fixed probes have welded fittings that cannot be repositioned.

The alternative approach involves the use of a sample cell and stainless steel tubing connected to the main line at the point of interest.

A small amount of compressed air is allowed to flow past the sensor by opening a leak screw or valve. The flow rate through the cell must be carefully controlled in order to ensure minimal pressure drop from the process line which could cause erroneous readings.

The advantages of this approach are better control of air pressure minimising main line fluctuations, ability to isolate the sensor from the system, and option to cool and condition the air sample if necessary.

Contamination resistance

Finally, good dewpoint sensors have particular features that ensure a clean and dry compressed air system. These include: fast wet-to-dry response time; high resistance to contamination - immunity to compressor oil and other chemicals; complete recovery from saturated conditions; and long calibration intervals.

Steven Jiroutek is Regional Market Manager, Vaisala, Boston, USA.

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