These coolers/chillers are probably hard to come by … and if you do find one, it is most likely VERY EXPENSIVE. We can fix these. The usual failure mode is that your 951A (LoTempCo version) or 951C starts misbehaving and it seems to misbehave based on room temperature. It could also be impossible to zero or maintain a good zero. What is probably happening is your PMT (655168) is no longer being maintained at 6°C and that is creating a significant ‘dark current’ in the PMT. Or equally bad is that it is close to 6°C but the temperature is not stable, so getting a good zero on the instrument is fruitless.

Another failure symptom is scorched or burned wires associated with the chiller circuit. The chiller is fed by a high current DC power supply (roughly 14 VDC); when the Peltier chips go bad, more current is required to achieve the cooling required and ultimately, there is so much current that the wire limitations are exceeded (especially when you factor in Rosemount’s use of 18awg high current wire). Look at the J16 plug and the J15 plug on the Power Supply Board. Check the terminations on the terminal strip on the side of the high current power supply too.

The cooler is built with a metal can liner that surrounds the PMT; the metal liner is thermally connected to two Peltier chips (or elements); the Peltier chips are thermally connected to the aluminum fin housing that you see when you open the instrument. These components are all potted in Stryofoam insulation.

Check the continuity of the red and black wires going to the chiller unit. New Peltier chips will read about 2 ohms in one direction and -2 ohms in the other direction (the negative reading come from the battery effect of the Peltier chips; essentially these are a million thermocouples amassed together … and thermocouples are little batteries in the presence of heat).

Our repair consists of extracting the old Peltier cells, installing new ones, repotting everything, checking the feedback thermistor, and testing the final assembly.

Call us for more information.


Operation (excerpted from 400A manual): The ionization current generated by the burner is measured by an electrometer preamplifier located adjacent to the burner assembly. This small current is amplified and transformed into a signal voltage that is then further amplified by a post amplifier before being converted to a digital display suitable for direct data presentation. To cover the required dynamic range, the amplifier is provided with two gain ranges that differ by a factor of 100. Output voltage from the preamp is a precise function of ionization current.  The most sensitive gain range includes a trim adjustment so that inter-range correlation can be obtained over the entire signal span.

A buffer signal offering unity gain and noise filtration provide a low output impedance to drive the signal cable and post amplifier circuits on the main circuit board. Selection of the low or high range feedback resistors is made by relay K1 on the preamplifier board.  A variable offset current is injected into the summing node of the electrometer amplifier to compensate for background offset current. These currents influence the measurement procedure, and a variable voltage at the front panel allows the user to visually cancel these currents during the calibration procedure. Background current is due to unavoidable traces of carbonaceous material introduced into the burner flame by the fuel gas and air.

Operation comments by RIGAS: K1 is a N.O. relay (shelf state).  When open (de-energized), maximum feedback resistance is applied to U2 (first stage) thus resulting in maximum gain or high sensitivity.  When K1 is closed (energized by +5 VDC when range X100, X250, or X1000 is selected) then R17 is placed in parallel with R18 resulting in less feedback resistance and thus less gain or less sensitivity.

Jumper E1-E2-E3 should be in the E1-E2 position.  E2-E3 is a factory test position but could be used to determine the exact amount of amplifier offset or burner contamination since all Zero Compensation would be removed from the circuit.

Jumper E4-E5 should be in place.  This allows the polarizing voltage to be grounded out during lighting (when switch is set to “ignite”).

Typical Failures:

1. Glass encapsulated, high ohmage, precision resistors get dirty. Dirt conducts so the more dirt, the less ohmage.

2. Glass capacitors. Dirt conducts so more dirt changes capacitance.

3. Coax cable breaks down (signals get noisy)

4. Jumper wires get frayed, brittle, and break

5. Opamps (operational amplifiers) fail (use list of expected voltages here)

6. Purge / ignite switch fails

7. 3 VAC transformer fails

8. Interconnecting ribbon cable gets pinched and fails

9. Burner contact assembly fails (this is a ghost [‘looks’ like] a preamp board failure)

10. Burner collector ring connection fails (this is a ghost [‘looks’ like] a preamp board failure)

11. Burner temperature sensor fails (fuel solenoid won’t stay latched after lighting ‘pop’)

Other information:

RIGAS built a special resistor pack to simulate the ion current developed by burning hydrocarbons.  We have six 500 gigaohms resistors in series to mimic the very low ion current (3 x 10-11 amps) in the burner (remember that a 90 VDC polarizing voltage is applied at this end of the circuit). This helps us determine if noise is coming from the burner chamber proper or the preamp board.

Schematic 620424 (with RIGAS embedded notes)

List of expected voltages

Simplified electronic calibration


1. When the Zero pot is up near 10 (full CW) that is GOOD! That means that there is minimal contamination to overcome with a bias signal.

2. Failure to light is usually a fuel/air ratio problem (usually not enough fuel getting to the chamber)

3. Failure to light could be as simple a s blown glow-plug

4. The 400A is a PERCENTAGE readout analyzer and a TOTAL HYDROCARBON analyzer

the display reads a percentage of your calibration gas numbers

any hydrocarbon will read out on this analyzer.  If you calibrate with 20 ppm methane and inject 5 ppm of butane, you’ll get the same response

Click here for the 400A calculator spreadsheet

Normal maintenance:

1. Replace old jumper wires & their connectors

2. Replace coax

3. Clean resistors and capacitors

4. Replace DIP socket with gold plated DIP socket

5. Replace opamps with latest low-noise opamps

6. Test


Affected components:

1. 620428 Rosemount 400A Main Electronics Board (schematic 620429)

2. 620433 Rosemount 400A Isolated 4-20 maDC board (schematic 620434) … (item is now obsolete by OEM … replaced by RIGAS25C0007R0)

Situation: The analyzer appears to operate normally (mostly anyway). U13 seems to get very hot (so does U4 [voltage output buffer amplifier]) and there is a 620433 (V/I option board) installed. Also, when attempting to light the analyzer, the analog display will overrange and remain overranged until the power is cycled on the analyzer (TP-5 will be saturated at about 13 vdc).

Problem: someone has employed the E1-E2 and E3-E4 jumpers and this is causing a nasty feedback loop that U13 is trying to compensate for. When the analyzer goes upscale (it always spikes during startup) this causes the analog signal to spike which, in turn, causes the 4-20 maDC card to spike. The 4-20 signal being fed back to U13 has now locked it railed high.

Resolution: remove the 4-20 maDC board or remove the E1-E2 and E3-E4 jumpers.

Other info: U12 & U13 & U4 are µA714 opamps (a.k.a., uA714); they can be replaced with OP07 or OP77 or OP177.


Quarterly audits should be simple … thorough … but still as simple as possible. Unlike the Teledyne procedure (see below), our method is fully compliant with EPA Method 203, EPA Procedure 3, ASTM D 6216-98, and 40 CFR part 60.

This is a snippet of RIGAS’ procedure:

1. record the serial numbers
2. challenge the PLCF
3. note the Fault indicators
4. record current readings (including Dust Compensation)
5. record current calibration values
6. record calibration set points
7. record LED drive current
8. check purge air system (replace air filters as necessary [be careful not to disturb the dust, that will alter your EPA data])
9. align
10. perform a dust accumulation test (which cleans the optical surfaces as a PM measure)
11. manual calibration cycle check
12. record new Dust Compensation value
13. install Cal Kit Fixture
14. do NOT perform Background Set
15. do NOT perform Normal Set
16. do NOT remove Cal Kit Fixture
17. do NOT perform Zero Set
18. check Cal Zero value
19. do NOT manual calibration cycle check
20. analog output check
21. perform Calibration Error Test
22. record all EPA data from the data historian (or chart recorder)
23. clean the unit (outside)
24. complete the EPA compliant report

Please contact us if you’re looking for a vendor to do quality EPA audits, analyzer preventive maintenance, analyzer repair (on site or in our depot),parts, or telephone technical support. We also have Teledyne LightHawks to rent.


If asked how we would supply a sample source to a ‘pressure sensitive’ analyzer, we would tell you to control the source pressure and not throttle the source pressure/flow.

We’re in the “control the source pressure” camp and not of the “throttle the source flow” camp.  The Rosemount 400, 400A, 951A, 951C, 951E, NGA-FID1, NGA-HFID, and NGA-CLD series analyzers are sample pressure dependent analyzers, that is, this type of analyzer utilizes sample source pressure to create the motive force for sample to flow through a restrictor or capillary (amd in rare instances, through a mass flow controller).  In order for this type of analyzer to determine the % (percent) or PPM (parts per million) concentration of its sample, it requires a stable pressure at key pneumatic junctions within the analyzer itself.  Typically, this junction is near the sensing element and is usually a capillary tube or precision restrictor. To ensure that this pneumatic junction is controlled at some value, 3 psig for instance, a back pressure regulator (BPR) is employed.

This is how it is supposed to function: when sample is applied, the pressure within the analyzer builds until it reaches the control point (a.k.a., set point) of the BPR; if the pressure continues to build, the BPR bleeds the excess pressure off to a vent header or waste dump.  The bleed off is sometimes referred to as bypass flow.  This action by the BPR controls the pneumatic junction at a predefined value … as long as there is excess source pressure.

If we control the source pressure (i.e., sample pressure) then we can control the bypass rate as well, simply by changing the source pressure, that is, you control the delta-P (delta-pressure) between the two systems … and you get the benefit of great analyzer accuracy simply due to more accurate control of the key pneumatic junction.

Here are examples (assume that your analyzer runs at 3 psig internal sample pressure):


Example 1:

1. Source pressure = 3psig

2. Analyzer is happy (that is, the capillary is maintained perfectly at 3psig by either the internal backpressure regulator [BPR] or the external forward pressure regulator so the THC calibration is valid)

3. But there is no bypass flow so response time to process excursions is VERY poor.


Example 2:

1. Source pressure = 5 psig

2. Analyzer is happy (that is, the capillary is maintained perfectly at 3 psig by the internal backpressure regulator [BPR] so the THC or NOx calibration is valid)

3. and because there is a pressure delta between source pressure and control pressure, there is significant bypass flow (within the capabilities of the BPR) so response time to process excursions is good

4. let’s assume that this creates 1200 cc/min of sample bypass flow (this assumption used in the next example)


Example 3:

1. Source pressure = 4 psig

2. Analyzer is happy (that is, the capillary is maintained perfectly at 3 psig by the internal backpressure regulator [BPR] so the THC calibration is valid)

3. because there is a pressure delta between source pressure and control pressure, there is significant bypass flow (within the capabilities of the BPR) so response time is OK

4. we probably lost half of our bypass from the previous example so let’s assume that this creates 600 cc/min of sample bypass

5. We’re still OK.  No loss of accuracy. Response time to process excursions might be an issue on some systems.


Example 4:

1. Source pressure = 25 psig

2. Analyzer is NOT happy. The internal backpressure regulator [BPR] is bypassing at its maximum rate (with a nasty “singing” sound) and has lost control of the 3 psig at the sample capillary so the THC or NOx calibration is NOT valid).

3. But there is significant bypass flow so response time is good … it’s just not giving us valid readings!

4. All process readings will read much higher than normal, and thus, inaccurate.


This might be a good time to talk about regulators and back pressure regulators. It should be noted that as source pressure climbs from setpoint to the end of the control band of the regulator, there will be a slight upward creep of the control pressure value; this in turn affects the key pneumatic junction and the analyzer’s calibration curve. This has to do with the internal operation of the regulator.  In order to correct for something like this, you might have to invest in a digital controller and I/P module to control the dome loading of a specially designed regulator (one that controls pressure mechanically but will also accept an external pneumatic signal to bias its setpoint one way or another).

Some companies throttle the inlet and ‘hope’ that the capillary is maintained at the correct pressure.  You ‘could’ assume you have positive control of the capillary pressure by virtue of the fact that there is bypass flow.  But we’ve seen significant interaction between a throttled source and the internal control pressure (BPR); so much interaction that we don’t endorse this method of control.  It’s a method of control that will not yield the most stable calibrations or process readings.  So if you’re looking to get superior accuracy, control the source pressure and let the delta pressure (D/P) control the bypass rate.

Attempting to control the bypass rate at the back of the analyzer at the analyzer’s bypass exhaust port is very bad. This will essentially take the internal BPR’s control out of the equation … you won’t have any positive control of the pressure at the capillary head … so your calibrations will be at the whim of the source pressure (or source flow … which in turn creates a pressure).

We’ve always wanted to make YouTube video of this and put it out there to dispel all of the misinformation.   Watch for its release which will star Rachel Ward (Dead Men Don’t Wear Plaid) as Juliet.

Call us at 877-616-0600 if you want the verbal version!  (Just in case the written version doesn’t make any sense!)


We plan to write an article about this soon, but in the interim, here are a few bullet points:

1. this is also referred to as Method 9 (found in 40 CFR part 60)

2. there are many, many criterium required to perform a ‘legal’ visual observation

time of day

relative humidity


wind direction

sun position

weather conditions

status of your EPA VEO certification

distance to emission

angle of incidence

background contrast used

white smoke or black smoke

(will look up the other criteria and post it here)

OPM2000, OPM2000A READING 105%

Background: The 105% indication and reading is Rosemount’s way of showing an error message; it was presumed that everyone in the opacity business would recognize that there is no such thing as 105% opacity and that it would instantly mean ‘analyzer failure’ to anyone observing it on the monitor’s display. It was also an easy way to get the milliamp output signal to rail high at about 21 maDC.

Components affected: LCW (liquid crystal window), lamp (bulb), power supply (SLB, Stack LON Board), G-64 LON Board, interconnecting cabling, & temperature.

The fault alarm (105% opacity) can come from any of the following:

1. failing bulb/lamp or lamp power supply

2. failing LCWs or LCW power supply

VLTH [volts too high]

LMPF [lamp failure per software algorithm]

3. loss of Eshelon communications (LON)

4. failing wire harness (to lamp or LON communications)

5. failed calibration

6. corrupted software on the Stack LON Board

7. failing detector board (±15 vdc power comes from the SLB)


But not:

1. actual stack opacity conditions (high opacity)

2. misalignment

3. dust on barrier window and/or corner cube

4. steam that has changed phase to vapor


Call us to help you diagnose this. Please provide the following:

1. model number

2. age of LCWs

3. age of bulb/lamp

4. reference voltages (8) (under Cal, Reference Voltages)

5. current ‘run’ voltages (4) (under Data, Volts)

6. temperature


Your opacity analyzer normally runs fine, but then cold weather sets in and now the opacity readings are a bit flakey, spiky, noisy, weird, unusual, annoying, etc. Well, you should initially rule out the opacity analyzer … it’s not its fault! It’s probably physics! Or earth sciences! But it’s not sunspot activity.

What is most likely happening is:

1. Your stack has significant moisture in its effluent

2. Your purge air system for the opacity monitor draws makeup air from the very cold ambient air

3. When the very cold ambient air meets the very hot, moist stack effluent, the effluent temperature drops significantly



1. forces the relative humidity to approach 100%

2. then forces steam (water) molecules to coalesce (condense) … a phase change

3. then creates visible (water) vapor (some say ‘steam’ but it is really vapor)

4. if you can see it, so can the opacity analyzer’s stack light beam

So, how would you prove this before investing any money in the solution?  How about a very simple test:

1. Somehow stop the purge gas flow temporarily

block or partially block the blower’s (or blowers’) suction(s)

turn off the blower(s)

remove the feeder hose(s) to the injection ports

2. Observe your opacity readings for five to 30 minutes

did the spiking go away?

3. Re-employ the blower system

did the problem return?

If answers to questions 2 and 3 are “yes” then you have “Phase Change”

So, how do you fix it? Here are a few ideas:

1. if it only occurs once in a Blue Moon, just declare it on your quarterly reports and take it as “down time”

2. if it happens too often, your Regional EPA won’t enjoy seeing a high down time number

3. install ducting so that the opacity analyzer draws makeup air from a warmed room

4. install purge air preheaters (1500 watts per side minimum; 3000 watts per side for extreme conditions)

Call us at 877-616-0600 to discuss this in greater detail.


There are only two significant things to consider when installing LCW#1 in the OPM2000, OPM2000A, OPM2000R, or OPM2001: [1] over-tightening and [2] stand-offs.

When attaching the LCW to the transceiver’s mounting block, don’t over-torque or over-tighten the four (4) hold down screws. We always recommend “finger tight plus a skosh” (meaning finger tight plus just a tad (smidgen) more). The stack-up is normally in this order: aperture plate – stand-off – o’ring – backing plate (large holed) – LCW – top plate (small hole) – o’ring – screw head. An alternate stack-up can be in this order: aperture plate – stand-off – LCW – top plate (small hole) – o’ring – screw head.

The stand-offs are a factory modification and VERY important to the life span of the LCW. By putting the LCW 3/4″ away from the aperature plate, the beam has a chance to disperse and thus more of the actual LCW active surface area is utilized. This helps prevent “burning” the center out of the LCW (see picture). This is very critical in the OPM2001 as its high intensity beam from the 20 watt halogen lamp will cure the filler material and damage the LC event sites.

Stand-offs promote stability too because more liquid crystal event sites are being utilized if the beam is allowed to spread; as the LCW ages, LC sites tend to lock either open or closed, so if the beam is relying on 1000 events sites instead of 1,000,000 event sites, it will ‘seem” to become unstable sooner as event sites fail due to normal aging.


LCW#1: 40F0050R02


LCW holding screws: 4-40 socket head, black anodized screws typically

Stand-offs:  4-40, 3/4″, aluminum, hex barrel, male-female

(LC stands for liquid crystal)


Components affected: LCW, liquid crystal window, lamp, barrier window, alignment, bulb, & temperature.

High opacity can come from any of the following:

1. actual stack opacity conditions

2. misalignment

3. failing bulb/lamp or lamp power supply

4. failing LCWs or LCW power supply

5. dust on barrier window and/or corner cube

6. steam that has changed phase to vapor

Call us to help you diagnose this.

Please provide the following:

1. model number

2. age of LCWs

3. age of bulb/lamp

4. reference voltages (8)

5. current ‘run’ voltages (4)

6. temperature