Teledyne Hastings Instruments Blog

Using thermal mass flow instruments by Teledyne Hastings is an easy way to quickly and accurately measure gas flow. And in some cases, a mass flow instrument may be calibrated for one gas, but then the user may want to use the instrument in another gas. In this blog, we will show how to use GCFs (Gas Conversion Factors) when using flow instruments in different gases.

Before we get into GCFs, let’s quickly review the operation of one of our flow sensors. Below, we show a diagram of the 200 Series flow sensor. In this sensor, gas flows through a capillary tube which is heated in the middle to a temperature which is approximately 130°C. Two thermocouples, one upstream (TC-1) and one downstream (TC-2), measure the temperature. The temperature difference between the two thermocouples is proportional to the heat flow through the capillary tube. The heat flow, in turn, is proportional to the mass flow times the specific heat Cp of the gas. So, to first order, if we want to use a thermal mass flow meter that has been set up for one gas, and use it with another gas, we will multiply the output of the meter by the ratio of the specific heats. GCF ~ Cp1 / Cp2

There are a couple of things we need to point out. First, the ratio shown above is a simple approximation and does not tell the whole story. Next, the best GCFs are those that have been measured experimentally. However, in the case of dangerous gases, we use the best thermodynamic data available.

Here is a table of some common GCFs.

Next, we will discuss how we apply GCFs in practice. Let’s take an example of a flow meter that is calibrated for nitrogen. If we wanted to use the flowmeter in argon, we would take the output and multiply by the GCF for Argon.

Here is another example; suppose we have a meter that is calibrated in helium and we want to use it in hydrogen. You would start by dividing the output by the GCF for helium (think of it as converting to the nitrogen equivalent), and then multiplying by the GCF for hydrogen.

Remember, always use the appropriate set of GCFs for the flow series that you are using. In other words, if you are using our Digital 300 Series, don’t apply GCFs from a 200 Series manual – they are not the same. And certainly don’t use non-Teledyne table of GCFs for use with Teledyne flow products. They might get you in the ballpark, but they will not be your best conversion.

One other quick note about applying GCFs. Our line of flow power supplies, the THCD-101 (single channel) and the THCD-401 (four channel), can be used to quickly scale the analog input which is equivalent to applying a conversion factor. Let’s take another look at the Argon example. If we used the THCD-101 power supply with the nitrogen flow meter as shown below, at the nominal full scale of the flow meter, we will have a 5 VDC signal. If we want to use this same meter and power supply with Argon, we just need to “tell” the THCD-101 what value to display when it receives 5 VDC. So, if our flow meter was calibrated for nitrogen to give 5 VDC at 250 sccm, then the same flow meter will give 5 VDC in argon at 350 sccm. (250 * 1.4 = 350). So, we would then range the THCD-101 for 350 sccm. This can be done from the front panel or via the internal webserver.   

Now let’s make things a little more interesting and discuss a flow controller example. Analog flow controllers work by receiving a command signal (usually 0-5 VDC, or 4-20 mA) and then they adjust their control valve such that the flow, and thus the analog signal output, matches the command signal input. (You can think of it like the cruise control in your car – you tell it you want to go 78 miles per hour, and then the engine does what it needs to do to maintain that speed). In the case of a 0-5 VDC flow controller, a 5-volt setpoint command is instructing the flow controller to set the flow to 100% of full scale. The relationship between flow rate and command signal is linear, so if the user wanted to control at 25% of full scale, then they would send a 1.25 VDC command signal (0.25 * 5 VDC = 1.25 VDC).

Now, suppose we had an HFC-202 flow controller (200 Series) that was calibrated for 200 sccm of methane and we wanted to use it to control the flow of argon. What voltage level would we need on the command signal to have a flow rate of 100 sccm of argon? Let’s first determine the full-scale flow rate (5 VDC) when using argon:

Flow (Ar) = Flow (CH4)/GCF (CH4) * GCF (Ar) = (200 sccm / 0.77) * 1.401 = 363.9

So, a 5 VDC command signal will give us 363.9 sccm of argon. If we want 100 sccm, we would send:

Command Voltage = 100 sccm (5 VDC / 363.9 sccm) = 1.374 VDC.

Now, one important note about using flow controllers in different gases. Just because we can apply GCFs does not mean that a flow controller’s valve will work properly when switching from one gas to another. As an extreme example, a flow controller valve that has an orifice sized to handle hydrogen will have a hard time handling significant flows of large polyatomic molecules like C2H6.

Teledyne flow products are easy to install and use. And our application engineers are standing by to help. We can be reached by email (hastings_instruments@teledyne.com), by phone 757-723-6531, or via LiveChat on our website www.teledyne-hi.com or by clicking the contact us button below.

There are many benefits for having a Flow Service Plan for your Mass Flow Meters/Controllers.  This blog touches on just a few of them.

Maintaining calibration on measurement instrumentation is essential to minimizing uncertainties and ensuring accurate readings. Hastings Instruments offers its flow calibra­tion services featuring deeply discounted pricing. The Flow Service Plan allows the user to integrate high-quality calibrations into metrology schedules for Hastings’ 200 Series, 300 Series, and digital 300 series flow meters and controllers. The Flow Service Plan may be purchased for either new or recently reconditioned instruments.

Each instrument under the plan is eligible for three calibrations anytime within a 36-month period. Under the discount Flow Service Plan, the user purchases two calibrations and receives a third at no cost. At the time of purchase, the user may specify a calibration interval; Hastings Service will track the unit’s history and provide advance notice (four weeks) of the next scheduled calibration.

The calibration and service department will clean, recalibrate, and ship the instrument back to the user within 5 working days or less per instrument.  The Flow Service Plan will improve up-time at the user’s facility while ensuring compliance to metrology requirements. All calibration performed at Teledyne Hastings is traceable to the National Institute of Standards and Technology (NIST).  In addition to this, calibrations are compliant to ISO 17025 requirements. 

Hastings offers a complete service department dedicated to recalibration, repair, and service for all of our mass flow and vacuum products.

If you have any questions or would like a Flow Service Plan quoted for your new or recently purchased mass flow instrument, click the button below

or contact your local Teledyne Hastings representative or the factory (757-723-6531 or HASTINGS_INSTRUMENTS@teledyne.com)

We are occasionally asked for the turndown ratios of our flow meters and flow controllers.  There are varying perceptions as to what this term actually means.

The turndown ratio of a Mass Flow Meter (MFM) or Mass Flow Controller (MFC) defines the usable range for which it can operate while maintaining its published accuracy.  It can be expressed using the following formula:

A flow meter with a large turndown ratio will have a large operating range.  This can also be indicative of the flow meter’s cost.  For example, variable area flow meters (rotameters) typically have lower turndown ratios compared to thermal mass flow meters.

Most analog mass flow meters have an accuracy of ± 1% of Full Scale (FS) and have resolution better than 1%.  The usable range is from 1% to 100%.  They will have a turndown ratio of 100/1 or more commonly expressed as 100:1.  Digital flow meters will have an even greater turndown ratio due to their higher accuracy.

Most analog mass flow controllers also have an accuracy of ± 1% FS.  However, they typically have an automatic valve shut circuit that closes the valve at flow rates below 2% of FS.  This is to ensure full valve closure in the event of a small zero offset.  The usable range is from 2% to 100%.  Since measurement is not possible below 2%, these will have a turndown ratio of 100/2 = 50/1 or 50:1.

 For more information on Turndown Ratio or our Flow Meters, please contact Wayne Lewey 

This is part two of a two-part blog on Thermal Mass Flow sensors.  In part one, we described the desired characteristics of a thermal mass flow sensor.  In part two, we will discuss the operation of the 300 series flow sensor (Patent #6,125,695) and how its design addresses the desired traits.

In our previous blog, we showed a cutaway of a thermal mass flow meter.  Now let’s take an inside look at the 300 series flow sensor:

When gas is flowing through the bypass shunt, a small pressure drop is developed which will direct a fraction of the flow through the arced / semi-circular capillary tubing in the flow sensor. On the outside of the capillary tube, there are two resistive wire coils which are tightly wound and in excellent thermal contact with the tube. These two identical windings are referred to as:

• Upstream Heater Coil (1)
• Downstream Heater Coil (2)

Associated with each of the two heated coils is an ambient coil. The ambient coil is in excellent thermal contact with the aluminum ambient block.  Aluminum has a very high thermal conductivity which ensures that both ends of the sensor tube and the two ambient coils (3 and 4) will be at the same temperature.

Two identical Wheatstone resistance bridges are formed from the two pair of coils (see image on right).

The circuit shown in the image on the right is designed to ensure that the heated coils (upstream and downstream) are maintained at a constant temperature (ΔT) above the corresponding ambient coils.

Next, we calculate the power (W) required to maintain ΔT by:

This power will be calculated for both the upstream bridge and the downstream bridge. It can be shown that:

So, by maintaining both heaters at the same ΔT above ambient, the mass flow rate is directly proportioned to the difference in power (W) between the two bridges. For example, when no flow is passing through the capillary sensor tube, the power needed to maintain ΔT will be the same (i.e. ṁ = 0)

As gas flow increases in the tube, heat is transferred from the upstream heater to the gas stream.  This will force the upstream circuit to use more power to maintain ΔT. In turn, the gas will transfer heat to the downstream heater which will cause the downstream circuit to use less power to maintain ΔT.

Now, here is the best part: the mass flow rate is directly proportional to the power difference. In other words, LINEARITY!

In our previous blog, we discussed how excellent linearity leads to improved accuracy. And, not only does the 300 series sensor give excellent linearity, the circuit shown on the right reacts very fast to changing flow. Thus, the 300 series has excellent responsive time.

One last note, we have designed the 300 series to use relatively large diameter tubing.  This larger tubing allows flow meters to be designed with lower pressure drop than many mass flow meters on the market.  

Visit our website for more information on Teledyne Hastings 300 series Flow Meters.

Teledyne Hastings' Thermal Mass Flow Sensors are used worldwide.  Download our application note on High Throughput Leak Detection to learn about improving lead testing precision and throughput and how to reduce testing time.  

Be sure to visit our website for additional information on Teledyne Hastings Mass Flow Controllers and Mass Flow Meters

Last month, we passed our ISO 9001 surveillance audit.  It has been over twenty years since we first obtained ISO and we wanted to take a step back and review some significant accomplishments.  

Teledyne Hastings Instruments rich history and customer centric vision continues to support, influence and grow with those who depend on quality process control and automation.

That's why we wanted to take a moment and celebrate a milestone with our core clients and those considering a Teledyne Hastings Instruments Flow instrument or Vacuum Gauge for the first time.

Teledyne Hastings Instruments' has been providing quality thermal mass flow instruments and vacuum meters and controllers for applications ranging from academic research to space exploration for over 70 years.  Let us work with you to find the best solution for your process.

OEM, custom applications, lead time crunch, just curious: